Chemistry Notes - vtupro

FUELS
The discussion in this chapter relates to fossil fuels or chemical fuels
Definition
Fuel is a carbonaceous combustible substance, which on combustion liberates a large
amount of energy in the form of heat.
Classification
On the basis of occurrence, fuels are classified as primary and secondary fuels
Primary fuels occur in nature and are used without processing.
Secondary fuels are obtained by chemical processing of primary fuels.
On the basis of physical state, fuels are classified as solid, liquid and gaseous fuels
Primary
Fuels
Secondary
Solid Liquid Gaseous Solid Liquid Gaseous
E.g. Coal Crude oil Natural gas Charcoal Petrol Coal gas
Wood (Petroleum) Coke Diesel Water gas
Calorific Value
The quality of a fuel is determined by the amount of energy released per unit mass or
volume referred to as calorific value.
Definition
Calorific value of a fuel is the amount of heat liberated when a unit mass or a unit
volume of the fuel is burnt completely in air or oxygen.
Fuels generally contain hydrogen in addition to carbon. During combustion, the
hydrogen is converted to steam.
In the determination of calorific value of the fuel if the products of combustion are
cooled to ambient temperature (room temperature), the latent heat of steam is also
included. This is referred to as gross calorific value (GCV) or higher calorific value.
In practice, the products of combustion are allowed to escape and the amount of
heat realized is lesser than the GCV (since the latent heat of vaporization is not
released). This is net calorific value (NCV) or lower calorific value.
GCV = NCV + latent heat of steam
Gross Calorific value is the amount of heat liberated when a unit mass or a unit
volume of the fuel is burnt completely in air or oxygen and the products of
combustion are cooled to ambient temperature.
Net Calorific value is the amount of heat liberated when a unit mass or a unit
volume of the fuel is burnt completely in air or oxygen and the products of
combustion are allowed to escape.
1

Determination of Calorific Value of a Solid Fuel Using Bomb Calorimeter
Sample
Stirrer
Construction
The bomb calorimeter (shown in the fig.) consists of an outer cylindrical steel vessel
(bomb) with an airtight screw and an inlet for oxygen.
The bomb has a platinum crucible with a loop of wire. The ends of the wire project
out and can be connected to a source of electric current.
The bomb is immersed in a rectangular vessel (calorimeter) containing water, which
is continuously stirred.
A Beckmann thermometer is introduced into the calorimeter.
Working
A known mass of the fuel is made into a pellet and taken in the crucible.
Oxygen is passed through the bomb.
A known mass of water is taken in the calorimeter and is closed with the lid.
The initial temperature of water is noted.
The ends of the wire are connected to an electric source so as to ignite the fuel.
The heat released is absorbed by water. The temperature of water rises.
The final temperature is noted.
Calculation
Let
m = mass of fuel
W = mass of water
w = water equivalent of calorimeter
t1 = initial temperature of water
t2 = final temperature of water
Oxygen
A
B
Wires for ignition
Thermometer
Lid
2

s = specific heat of water
GCV ( solid fuel) = (W+w) (t2-t1) s
m
If the fuel contains x% hydrogen, NCV of the fuel is calculated as follows
2 atoms of hydrogen produce one molecule of water
2g of hydrogen produce 18 g of water
x g of hydrogen produce 9 g of water
x % hydrogen 9 x g of water = 0.09 x g of water
100
NCV = GCV - latent heat of steam formed
= GCV - 0.09 x latent heat of steam
Latent heat of steam = 2454 kJ kg -1
1 calorie = 4.187 kJ kg -1
The calorific value of a liquid fuel can be determined using bomb calorimeter.
Formulae for Solving Numerical Problems:
GCV (solid fuel) = (W+w) (t2-t1) s
m
NCV (solid fuel) = GCV - latent heat
= G.C.V. - (0.09 % of H) latent heat
GCV( gaseous fuel) = W s (t2- t1)
V
NCV ( gaseous fuel) = GCV – latent heat
= G.C.V. – amount of water collected latent heat
V
= G.C.V. – v latent heat
V
(1 cm 3 of water 1 g of water)
Numerical Problems
Problem 1: Calculate the gross calorific value and net calorific value of a sample of coal 0.
5g of which when burnt in a bomb calorimeter, raised the temperature of 1000g of water
from 293K to 301.6K. The water equivalent of calorimeter is 350 g. The specific heat of
water is 4.187 kJ kg -1 , latent heat of steam is 2457.2kJkg -1 . The coal sample contains 93%
carbon, 5% hydrogen and 2% ash.
m = mass of the fuel = 0.5 g
W = mass of water taken = 1000 g
w = water equivalent of calorimeter = 350 g
t1 = initial temperature of water = 293 K
t2 = final temperature of water = 296.4 K
s = specific heat of water = 4.187 kJ kg -1 K -1
GCV (solid fuel) = (W+w) (t2-t1) s
3

m
= (1000 +350) g (296.4 -293)K 4.187 kJ kg -1 K -1
0.5g
= 1350 g 3.4 K 4.187 kJ kg -1 K -1
0. 5g
= 3 8437 kJ kg -1
NCV (solid fuel) = GCV - latent heat
= G.C.V. - (0.09 % of H) latent heat
= 38437 kJ kg -1 - (0.09 5) 1105.7 kJ kg -1
= 38437 kJ kg -1 – 1106 kJ kg -1
= 37331 kJ kg -1
Problem 2: Calculate the gross calorific value and net calorific value of a gaseous fuel,
0.012 g of which when burnt raised the temperature of 3.5kg of water by 8.2K. Specific
heat of water is 4.2 kJ kg -1 K -1. Latent heat of steam is 2.45 kJ kg -1 . The volume of water
collected is 6.5 cm 3 . Latent heat of steam is 2457.2kJ kg -1
V = volume of the gas burnt = 0.015 g
W = mass of water = 3.5 kg
t2- t1 = rise in temperature = 15.6 K
s = specific heat of water = 4.2kJ kg -1 K -1
v = volume of water collected = 6.5 cm 3
GCV( gaseous fuel) = W s (t2- t1)
V
= 3.5 kg 4.2 kJkg -1 K -1 15.6 K
0.012m 3
= 11073 kJm -3
NCV( gaseous fuel) = GCV – latent heat
= G.C.V. - amount of water collected latent heat
V
= 11073 kJm -3 – 6.5 10 -3 kg 2457.2kJkg -1
(1 cm 3 of water 1 g of water)
0.012
= 11073 kJm -3 – 6.5 10 -3 kg 2457.2kJkg -1
0.015
= 11073 kJm -3 – 1065 kJm -3
= 10008 kJm -3
Cracking of Petroleum
Heavy oil is a major fraction of petroleum. It is converted to petrol by cracking.
Definition:
Cracking is the breaking down of high boiling high molecular mass petroleum
fractions (heavy oil) into smaller fragments.
Fluidized Bed Catalytic Cracking
Heavy oil is cracked using zeolite (Y type) catalyst with a rare earth oxide.
4

Heavy oil is heated to about 300°C in a preheater and passed through a riser column
(shown in fig.) into a reactor.
Exhaust gases Cracked vapours into
fractionating column
600°C 500°C
Regenerator Reactor
P=1-5 kg cm -2
Crude
Oil Spent catalyst
Oil + Catalyst Spent catalyst
+ hot air Hot air
Preheater Pump
The reactor contains finely powdered catalyst maintained at about 500°C.
The heavy oil undergoes cracking.
The cracked product is fractionated to give petrol.
Regeneration of Catalyst
After some time, the catalyst gets deactivated in the reactor due to the deposition of
carbon and oil on its surface.
Steam is passed through the riser column into the reactor.
The deactivated catalyst is forced into a regenerator along with hot air.
The regenerator temperature will be maintained at about 600°C.
Air oxidizes C to CO2 and steam removes the oil.
The regenerated catalyst is sent again to reactor with fresh oil.
Reformation of Petrol
Reformation is a process of bringing about structural changes in the hydrocarbons
with the primary objective of improving the octane number of petrol.
The changes in structure could be isomerization, cyclization, aromatization or
polymerization.
Isomerization straight chain hydrocarbons are converted to branched
hydrocarbons
CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH3 CH3 - CH - CH2 - CH2 - CH2 - CH3
n- heptane
CH3
methyl hexane
5

Cyclization straight chain hydrocarbons are converted to cyclic compounds
CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH3 - CH3
n- heptane methyl cyclohexane
Aromatization cyclic compounds are dehydrogenated.
- CH3 - CH3
methyl cyclohexane toluene
There are two types of reformation:
1. Thermal reformation: Thermal reformation is carried out by heating the gasoline to
500-600°C at a pressure of 85 atmospheres in a reactor. The conditions are controlled by
quenching the hot vapours with cold spray of oil to avoid the formation of gases.
2. Catalytic reformation: Catalytic reformation is carried out by passing the petrol
through Pt (0.75%) supported on alumina at about 500 0 C and 50 kg cm -2 pressure.
Knocking in IC Engines
The power output and efficiency of an IC engine depends on the Compression ratio
which is the ratio of the volume of the cylinder at the end of the suction stroke to the
volume of the cylinder at the end of the compression stroke.
Compression ratio =
Volume of (Fuel + air )in the cylinder at end of suction stroke
Volume of (Fuel + air ) in cylinder at end of compression stroke
Under ideal conditions, in an IC engine the petrol-air mixture drawn into the
cylinder of the engine undergoes compression and then ignited.
The hydrocarbons in petrol undergo complete combustion and the flame propagates
smoothly.
Sometimes, due to deposits of carbon on the walls of the cylinder the hydrocarbons
in petrol form peroxy compounds.
The accumulated peroxides decompose suddenly and burst into flames producing
shock waves.
The shock wave hits the walls of the engine and the piston with a rattling sound.
This is knocking.
The reactions that take place in an IC engine are given below (taking ethane as an
example for the hydrocarbon present in petrol):
6

Under ideal conditions
C2H6 + 7/2 O2 2 CO2 + 3H2O (Normal combustion)
Under knocking conditions (Explosive combustion)
C2H6 + O2 CH3 –O-O- CH3
(Dimethyl peroxide)
CH3 –O-O- CH3 CH3CHO + H2O
CH3CHO + 3/2 O2 HCHO + CO2 + H2O
HCHO + O2 H2O + CO2
Note that the overall reaction is the same under both the conditions. One molecule
of ethane reacts with 7/2 molecules of oxygen forming carbon dioxide and water
with the release of energy.
Under ideal conditions, the energy is released at a uniform rate.
Under knocking conditions, the energy is released slowly at first followed by a lag
(formation of peroxides) and finally the energy is released at a very fast rate
(decomposition of peroxides).
Ill effects of knocking
1. Decreases life of engine
2. Causes Piston wrap
3. Consumption of fuel is more
Octane Number
The resistance to knocking offered by petrols is expressed in terms of an arbitrary
scale called octane number
Isooctane has least tendency to knock and n-heptane has more tendency to knock.
The octane value of isooctane is arbitrarily taken as 100 and that of n – heptane as
zero.
Octane number is the percentage by volume of isooctane present in a standard
mixture of isooctane and n – heptane, which has the same knocking characteristic as
the petrol under test.
Different standard mixtures ( 90:10; 80:20, 75:25 etc) of isooctane and n–heptane
are prepared and the compression ratio of each of these is determined under
standard conditions.
The compression ratio of the fuel under test is determined under the same
conditions.
Suppose the compression ratio of the fuel is same as that of 80 :20 mixture, the
octane number of the fuel is 80.
Cetane Number:
The resistance to knocking offered by diesels is expressed in terms of an arbitrary
scale called cetane number
Cetane (hexadecane) has least tendency to knock and - methyl naphthalene has
more tendency to knock. The cetane value of Cetane is arbitrarily taken as 100 and
that of - methyl naphthalene as zero.
7

It is the percentage by volume of cetane present in a mixture of cetane and -
methyl naphthalene which has the same knocking characteristic as the diesel under
test.
Prevention of Knocking
Addition of tetraethyl lead (TEL) to Petrol:
Tetraethyl Lead decomposes the peroxides formed and prevents knocking. In the
process, lead gets deposited on the inner walls of the engines and at spark plugs. Hence
dichloroethane and dibromoethane are added along with tetraethyl lead. These convert
the lead into lead halides, which are volatile and escape with exhaust gases.
The release of lead compounds pollutes the atmosphere.
Catalytic converters (rhodium catalyst) are used in IC engines to convert CO in the
exhaust to CO2. Tetraethyl Lead used as anti knocking agent poisons the catalyst
and hence leaded petrol is not advisable in such IC engines.
Nowadays usage of leaded petrol is phased out completely due to pollution caused
by the lead present in it.
Addition of MTBE:
Methyl tertiary butyl ether (MTBE) is added to petrol (unleaded petrol) to boost its
octane number. The oxygen present in ether group of MTBE brings about complete
combustion of petrol preventing peroxide formation and hence knocking is prevented.
MTBE can be used as antiknocking agent in IC engines with catalytic converter.
Power Alcohol:
This is alcohol-blended petrol.
Gasohol is a blend of 10 – 85% of absolute ethanol and 90 – 15% of petrol by
volume and is used as a fuel in the United States. Absolute alcohol is used in the
preparation of Power alcohol to prevent phase separation.
Alcohol contains higher percentage of oxygen than MTBE and hence brings about
complete oxidation of petrol more effectively.
Therefore power alcohol has better antiknocking characteristics than unleaded
petrol.
Advantages of power alcohol
power output is high
does not release CO, causes less pollution.
alcohol is obtained from molasses, a agricultural product and hence renewable.
biodegradable.
Bergius process
In the Bergius process, lignite is hydrogenated to give liquid hydrocarbons for use as
synthetic petrol. Hydrogen is obtained by the reaction of water gas on coal or by partial
oxidation of natural gas. Lignite is ground to a fine dust. The dust is mixed with heavy oil
and made into a paste. Iron oxide or nickel catalyst is added. The mixture is pumped into a
reactor maintained at about 500 – 550 0 C and a pressure of about 250 atmospheres .
Hydrogen gas is passed through the reactor. Lignite gets hydrogenated and a mixture of
hydrocarbons is obtained. This mixture is passed through a fractionating column to get
petrol.
Lignite
dust +
Heavy
Oil
Paste
Crude Oil Vapours
500 -550°C
250 atm
Cooler
Gasoline
Catalyst Water
Out
Kerosene
8

FISCHER TROPSCH PROCESS
Water in
Heavy
oil
Compressor Reactor H2 gas Fractionating Cracking
Column
Gasoline
A schematic diagram of Bergius process
Crude Oil Vapours Cooler
CO+ H2 Water Gasoline
Out
Catalyst
H2 Kerosene
200-300°C
Water in
Heavy Oil
Pre-Heater
Compressor Catalytic Fractionating Cracking
Converter
reactor
Column
Gasoline
A schematic diagram of Fischer – Tropsch process
In the Fischer – Tropsch process, water gas ( a mixture of CO and H2) is mixed with half of
its volume of hydrogen and passed over a catalyst such as cobalt mixed with oxides of
magnesium and thorium at about 230 – 300 0 C under a pressure of about 200 atmospheres.
The product consists of a mixture of hydrocarbons and is fractionated to yield petrol and
other liquid fuels.
(2n+1) H2 + 2n CO CnH2n+2 + nH2O
2n H2 + n CO CnH2n + nH2O
2n H2 + n CO CnH2n+1 OH + (n-1)H2O
------------------------------------
9

ELECTRODE POTENTIALS AND CELLS
Single electrode potential (E): the potential that is developed when an element is in
equilibrium with its ionic solution is called single electrode potential. It is denoted by E
Eg. Cu rod in Cu 2+ ions (CuSO4 solution)
Zn rod in Zn 2+ ions (ZnSO4 solution)
Origin of single electrode potential:
When a metal is placed in a solution containing its own ions exhibit two types of tendencies
(1) the metal shows the tendency to go into the solution as metal ions by using electrons
is called oxidation or dissolution reaction
M M n+ + ne
Zn Zn 2+ + 2e
This is observed when the metal ion concentration is low in the solution
(2) the metal ions in the solution shows the tendency to get deposited on the metal
surface as metal atom by gaining electrons is called reduction or deposition reaction
M n+ + ne M
Cu 2+ + 2e Cu
This is observed when the metal ion concentration is high in the solution
After some time the dissolution and deposition reactions attain a state of equilibrium
M M n+ + ne
10

If the dissolution reaction is faster than deposition reaction, the metal goes into the
solution as metal ions with the liberation of electrons. These electrons accumulate on the
electrode surface as a layer of –ve charge. The –vely charged electrode surface attracts a
layer of +vely charged ions at the interface developing an electrical double layer
(Helmholtz double layer) at the metal solution interface.
The difference in potential across the EDL is called single electrode potential.
Similarly, if the deposition reaction is faster than the dissolution reaction, the +vely
charged metal ions get deposited on the metal surface by consuming electrons. As a result
the electrode surface develops a layer of +ve charges which attracts a layer of –ve charged
ions at the interface developing an EDL. The difference in potential across the EDL is
called single electrode potential
.
HDL
+ + + +
+ +
+ +
+
+ +
+
+ +
+
+ +
electron
GCL
+ Positive
metal ion
(a)
Potential
Standard electrode potential (E o ): the single electrode potential developed when a metal is
dipped in a solution of its own ions of 1 molar concentration at 298 K is called as Standard
electrode potential and is represented as E o .
Derivation of Nernst’s equation
Single electrode potential can be expressed in the form of Nernst’s equation.
Nernst’s equation is a thermodynamic equation, which relates the change in free energy
(G) and cell potential with concentration (M n+ )
Potential
ri
Distance from metal solution interface
(b) (c)
11

The maximum work is given as the decrease in the free energy change
Wmax = -G (1)
But, Wmax = no. of coulombs x energy available
Wmax = nF x E (2)
From equation 1 and 2,
G = -nEF and,
G o = -nE o F where, G o is the std. Free energy change and
E o is the std. Electrode potential
For a metal ion-metal electrode (redox reaction) of the type,
M n+ + ne M (3)
Substituting the values for G and G o in equation 3 we get,
-nEF = -nE o F + RT ln [M] (4)
[M n+ ]
Divide the above equation by –nF,
E = E o + RT ln [M]
nF M n+ ]
where, R is called gas constant = 8.314 JK -1 mol -1
T is the temperature = 298 K
F is the Faraday constant = 96,500 Coulomb mol -1
N is the number of electrons
[M] is 1
Substituting the above constants converting the natural log into log10 in equation 4, we get,
E = E o - 0.0591 ln [1]
n [M n+ ]
E = E o + 0.0591 log10 [M n+ ]
n
E = E o + 0.0591 log10 [reactants]
n [products]
E = E o + 0.0591 log10 [species at the cathode]
n [species at the anode]
In the Nernst equation, E is directly proportional to T and M n+ ,
(i) As the temperature is increased E is increased
(ii) As the metal ion concentration is increased E is increased
Electrochemical Cell: It is a device used to transform chemical energy of a spontaneous
reaction into electrical energy or to bring about non-spontaneous chemical change using
electrical energy from an external source.
Electrochemical Cell
12

Galvanic or Voltaic Cell Electrolytic Cell
Primary Secondary Concentration
Cell Cell Cell
1. Galvanic Cell: It is an electrochemical cell in which chemical energy is converted into electrical
energy
(i) Primary Cell: In these cells the cell reaction is not completely reversible hence
discarded.
Eg. Dry cell, Daniel Cell etc.,
(ii) Secondary Cell: In these cells the cell reaction is completely reversible hence can be
recharged. Eg. Ni-Cd cell, Pb-acid accumulator etc.,
(iii) Concentration Cell: discussed later.
2. Electrolytic Cell: It is an electrochemical cell in which electrical energy is applied from an external
source to bring about a non-spontaneous chemical change. Eg. Deposition and extraction process
Galvanic Cell (Daniell Cell)
A Daniell Cell is formed when Zn rod is dipped in 1M solution of Zn salt
(ZnSO4) and Cu rod is dipped in 1M solution of Cu salt (CuSO4) and these two
electrodes are connected to voltmeter externally by a wire and internally by means of a
salt bridge. The electrode at which oxidation takes place is called as anode (where Zn is
oxidized to Zn 2+ ) and the electrode where reduction takes place is called as cathode
(where Cu 2+ is oxidized to Cu). Each electrode is regarded as a half cell.
Zn
anode
e
Zn 2+
NO 3 -
NO 3 -
Zn
half-cell
Voltmeter
The half cell reactions are
At anode Zn Zn 2+ + 2e
(Oxidation)
(Negative electrode)
At cathode Cu 2+ + 2e Cu
(Reduction)
e
Cu 2+
NO 3 -
NO 3 -
Cu
half-cell
Cu
cathode
13

(Positive electrode)
Net Cell reaction Zn + Cu 2+
Cu
e
[Zn 2+ ] = M 1
Voltmeter
e
[Zn 2+ ] = M 2
Zn 2+ + Cu
EMF of the cell: the force which causes the flow of the electrons from one electrode to
another and thus results in the flow of current is called electromotive force.
It is represented as EMF of a cell or Ecell
It is expressed in volts
It can be calculated as Ecell = Ecathode – Eanode
Representations, Notations and conventions of a cell:
Representations: let us consider a Daniell Cell. It is obtained by coupling Zn half cell
and Cu half cell through a salt bridge. It is represented as,
Zn / Zn 2+ (1M) // Cu 2+ (1M) / Cu
Notations
(1) A single vertical line indicates the phase boundary between the metal and the
solution of its own ions.
(2) The double vertical line represents the salt bridge
(3) The direction of the arrow indicates the direction of flow of the electrons in the
external circuit.
Conventions
(1) The electrode where oxidation takes place is called anode (-ve electrode) and is written at left side
(2) The electrode where reduction takes place is called cathode (+ve electrode) and is written at right
side
(3) The anode is written as metal first and then electrolyte. The anode if half cell is written as
Zn / Zn 2+ (1M)
(4) The cathode is written as electrolyte first and then metal. The anode if half cell is
written as
Cu 2+ (1M) / Cu
(5) Electrode potential is always referred to as reduction potential and is represented as
EMn+/M.
(6) Ecell can be calculated using Ecell = Ecathode - Eanode
Concentration Cells
A concentration cell is an electrochemical cell where similar electrodes are
in contact with solutions of the same electrolytes but of different concentrations.
For example: two Zn electrodes are in contact with ZnSO4 solution of M1 and M2
molar concentration
Cu
14

The cell is represented as
The half cell reactions are,
Zn / Zn 2+ (M1) // Zn 2+ (M2) / Zn
At anode Zn ZnM1 2+ + 2e
(Oxidation)
(Negative electrode)
At cathode Zn M2 2+ + 2e Zn
(Reduction)
(Positive electrode)
Net Cell reaction Zn M2 2+
ZnM1 2+
Hence, it is nearly the change in concentration as a result of current flow
The EMF of concentration cell is,
Ecell = Ecathode - Eanode
Ecathode = E o + 0.0591/n log M2
Eanode = E o + 0.0591/n log M1
Ecell = E o + 0.0591/n log M2 - E o + 0.0591/n log M1
Ecell = 0.0591 log M2
n M1
Cases:
(1) When the two solutions are of same concentrations,
log M2 = 0. Hence, no current flows
M1
(2) For a spontaneous reaction to occur E should be +ve. It is
possible only when M2/M1 > 1, i.e. M2 > M1
i.e. the more concentrated solution becomes cathode and lesser
concentrated solution becomes anode.
Measurement of single electrode potential
The potential of a given electrode is measured using the SHE whose potential is taken as
zero at all temperatures and is used as reference electrode for potential measurements.
Construction of cell assembly: the electrode whose potential has to be measured is coupled with
SHE through a salt bridge. The EMF of the cell assembly is determined using an electronic
Voltmeter
voltmeter. Eg. Let us measure the potential 0.76V of Zn electrode. It is coupled with SHE through a salt
0.34V
bridge
(-)
Zn
1M Zn 2+
Zn/Zn 2+
1M H +
SHE
(+)
H 2
(-)
1M H +
SHE
1M Cu 2+
2+
(+)
Cu
15

Assigning the sign on the electrode: the anode and the cathode of the cell can be identified by
connecting the electrodes to proper terminals (Ecell = +ve or the pointer should deflect within the
scale). For the above couple Zn is connected to –ve terminal (anode) and the SHE to the +ve
terminal. The potential measured was 0.76V
E 0 cell = E 0 cathode – E 0 anode
E 0 cell = E 0 SHE - E 0 Zn2+/Zn
0.76V = 0 - E 0 Zn2+/Zn
E 0 Zn2+/Zn = -0.76V
The cell is represented as Zn / Zn 2+ (aq) // H + (1M) / H2 gas / Pt
Types of single electrodes
(1) Metal – Metal ion electrode
The metal is in contact with its own ionic solution
Eg. Zn 2+ / Zn, Cu 2+ / Cu
(2) Gas electrode: The gas is in contact with an inert metal dipped in an ionic solution of
the gas
molecules.
Eg. Hydrogen electrode.
(3) Metal – Metal insoluble salt electrode:
This consists of a metal and a sparingly soluble salt of this metal and a solution of a
soluble salt of the same anion.
Eg. Calomel electrode, silver-silver chloride electrode.
(4) Ion –Selective electrode or membrane electrode
In this electrode, a membrane is in contact with a solution with which it can excange
ions
Eg. Glass electrode, solid state electrode.
Reference electrodes:
Reference electrodes are the electrodes with known constant electrode
potential with reference to these the electrode potential of any other electrode can be
measured.
Primary reference electrodes used are SHE whose potential is taken as zero at all
temperatures
Disadvantages:
(1) very difficult to set up the electrode
(2) Electrode becomes inactive in presence of impurity present in hydrogen gas.
To overcome some of these drawbacks secondary reference electrodes are used
(calomel, Ag/AgCl electrodes)
Calomel Electrode
The calomel electrode consists of a glass tube with a side tube. Mercury is
placed at the bottom of the tube. A paste of calomel (Hg2Cl2) and Hg is placed over a
pool of Hg. The remaining part of the tube is filled with sat KCl solution. Electrical
16

connection is made through a Pt wire dipped in the Hg at the bottom of the tube. The
side tube is filled with sat solution of KCl solution and it acts as a salt bridge.
The electrode is represented as Hg / Hg2Cl2 (sat) / KCl (sat)
The half cell reaction is ½ Hg2Cl2 + e Hg + Cl -
Saturated
KCl
The forward reaction is a reduction reaction and the backward reaction is an oxidation reaction.
Hence, calomel electrode can act as both anode and cathode depending on the nature of other
electrode coupled. As calomel electrode is reversible w.r.t Cl - ions the potential of the electrode
depends on the concentration of KCl used.
Saturated KCl; Ecalomel = 0.2422V
1M KCl; Ecalomel = 0.280 V
1M KCl; Ecalomel = 0.334 V
Advantages:
(1) It is simple to construct
(2) Cell potential is reproducible and is stable over a long period of time]
(3) Cell potential does not vary with temperature.
Ag / AgCl electrode
Construction: It is prepared by coating a thin layer of Ag on Pt wire by electrolysis in
AgCN (argentocyanide) solution, the silver is then partly converted into AgCl by
making it an anode in KCl solution and passing a low current density for 30 minutes.
The wire is placed in sat KCl with 1 to 2 drops of 1M AgNO3. A definite potential is
developed. The electrode is represented as Ag / AgCl (salt) / KCl (sat)
The half cell reaction is AgCl + e Ag + Cl-
The forward reaction is a reduction reaction and the backward reaction is an oxidation reaction.
Hence, Ag/AgCl electrode can act as both anode and cathode depending on the nature of other
electrode coupled. As Ag/AgCl electrode is reversible w.r.t Cl- ions the potential of the electrode
depends on the concentration of KCl used.
Saturated KCl; EAg / AgCl = 0.290V
0.1M KCl; EAg / AgCl = 0.199 V
Advantages:
Pt wire
Salt bridge
Electrical contact
Saturated
KCl
Calomel
paste
Ag wire
Saturated
KCl
Hg +
Hg2Cl2
Paste
Fritted
disc
Micro hole
Solid
KCl
Agar
Porous
plug
Hg
disc
or asbestos
thread
Calomel electrode (a) with salt bridge (b) compact form (c) Ag-AgCl
17

(1) Used as reference electrode to determine the uniformity of potential in ship hulls and pipe lines
protected by sacrificial corrosion
(2) Used as reference electrode for submerged oil pipelines, can operate upto a depth of 300 meters
and measures the potential to a precision of ±1mV
(3) Used as reference electrodes in ion – selective electrodes
Ion Selective electrodes
Several electrochemical systems are used as interface between chemical
systems and electronic devices that display, record and manipulate data. This depends
on the electrodes that can selectively detect and quantitatively measure a particular
chemical species. These electrodes are called as Ion Selective electrodes or membrane
electrodes.
Principle: This electrode selectively responds to specific ion in the solution and the
potential developed at the electrode is the function of the concentration of that ion in the
solution. The electrode generally consists of a membrane which is capable of
exchanging the specific ions with the solution with which it is in contact.
Eg. Glass electrode
The membrane potential is given by the equation
Membrane
Reference Solution to Internal standard Reference
electrode 1 be analyzed solution electrode 2
[M n+ ]=c 1 [M n+ ]=c 2
External reference electrode Ion selective membrane electrode
EM n+ = 0.0591 log C1 (1) C1 > C2
n C2 external > internal
solution solution
Ecell = EM n+ - Ereference
Ecell = EM n+ -
Ecell = 0.0591 log C1 - 0.0591 log C2
n n
Ecell = constant + 0.0591 log C1 where, H + = 1 and C1= H +
Ecell = L - 0.0591pH (2)
Hence, the cell potential depends on selectivity of the membrane for the ion M n+ . the response of
the membrane to changes in the concentration of M n+ is determined by equation 1 and not the
concentration of other ions present. Hence, the membrane should be selective to the ions to be
analyzed
Construction: a glass electrode consists of a long glass tube with a thin walled bulb made of low M.P
and high electrical conductance. (Corning glass 22% Na2O, 6% CaO and 72% SiO2. the glass is
sensitive to H+ ions upto pH 9. the bulb contains 0.1M HCl and a Ag-AgCl electrode (internal
reference electrode) is immersed in a solution and connected by Pt wire for external contact.
The electrode is represented as
18

Ag / AgCl (salt) / 0.1M HCl / glass
The membrane undergoes an ion exchange reaction, the Na + ions on the glass are exchanged for H +
Ions
H + + Na + Gl - Na + + H + Gl -
The boundary potential established due to the above reaction is due to glass electrode potential EG.
EG = Eb + EAg-AgCl + E assymitric potential
EG = L -0.0591 pH + EAg-AgCl + E assymitric potential
EG = L / - 0.0591 pH
where Eb is the boundary potential given by
equation1
The EG of the solution is calculated by using the solution of known pH. Once the EG of the solution
is known the pH of the unknown solution can be found out as L / is constant. {The glass electrode is
immersed in the solution whose pH has to be calculated with a reference electrode (calomel) through
a salt bridge.}
Advantages:
(1) Glass electrode can be employed in the presence of strong oxidizing and strong reducing
substances ands metal ions
(2) Not poisoned easily
(3) Used in un-buffered solution and can be adopted for measurements with small quantities of
solutions
(4) Simple to operate and can be used in portable instruments
Disadvantages:
(1) Electrode can be used upto pH = 13 but becomes sensitive to Na+ ions above pH = 9
(2) Cannot be used in highly acidic solutions of pH < 1.
19

(3) Cannot be used to measure pure alcohol and some organic solvents.
Introduction
BATTERY TECHNOLOGY
A battery is a portable energy source with three basic components-an anode (the
negative electrode, a cathode (the positive electrode), and an electrolyte. As current is
drawn from the battery, electrons start to flow from the anode through the electrolyte, to
the cathode.
20

It is a device which enables the energy liberated in a chemical reaction to be converted
directly into electricity.
The term battery originally implied a group of cells in a series or parallel arrangement, but
now it is either a single cell or group of cells.
Examples: It ranges from small button cells used in electric watches to the lead acid
batteries used for starting, lighting and ignition in vehicles with internal combustion
engines.
The batteries are of great importance based on the ability of some electrochemical systems
to store electrical energy supplied by the external source. Such batteries may be used for
emergency power supplies, for driving electric vehicles, etc.
For the commercial exploitation, it is important that a battery should provide a higher
energy, power density along with long shelf life, low cost and compatible rechargeable
units.
Battery Characteristics
A cell may be characterised in terms of
its available capacity
its available energy and
the power it can deliver.
Voltage. The voltage of a battery depends on the free energy change ( Nernst Equation)in
the overall cell reaction and hence on the choice of electrode systems. The overpotential
and the cell resistance affect the voltage. To derive maximum voltage from the cell, the
difference in the electrode potentials must be high, the electrode reactions must be fast so
as to minimize the overpotential and the internal resistance of the cell must be low.
Ecell = (EC – EA) - A - C - iRcell
where EC and EA are the electrode (reduction) potentials of cathode and the anode
respectively, A and C are the overpotentials at the anode and the cathode respectively and
iRcell is the voltage drop. The cell voltage thus depends on the
difference in the electrode potentials.
The electrode reactions should be so chosen as to ensure that the active mass at the
positive electrode reduces readily and that at the negative electrode increases easily leading
to an overall reaction with a high negative free energy change. The cell should be designed
to minimize voltage drop. This can be achieved by keeping the electrodes close to each
other and also by using an electrolyte of high conductivity.
Capacity
It is defined as the quantity of electrical charge measured in Ampere hour (Ah), capable
of being provided by a battery during discharge. (One Ah = current of one Ampere
flowing for one hour).
The theoretical capacity may be calculated using the relation, QT = x (nF), where x (x =
w/M) is the theoretical number of moles of the electroactive material associated with
the complete discharge of the cell.
The practical capacity (Qp) is the actual number of coulombs (or Ah) of electrical
charge delivered, it is always lower than the theoretical capacity.
21

A plot of V against t at a fixed current discharge is shown in Fig. The variation of the
battery voltage during discharge is shown by the flatness of the curve. The length of the
flat portion of the curve is a measure of the capacity of the battery; longer the flat portion of
the curve better is the capacity. Such a characteristic is one of the primary requirements of
a battery.
Electricity storage capacity is usually expressed with an Ampere-hour (Ah) rating,
which means the amount of electrical current that the battery will deliver over a given
number of hours at its normal voltage and at a temperature of 25 O C. For example: A
battery rated at 60 Ah, should produce 3 amperes for 20 hours (Example: 60 Ah/3A =
20 hrs, based on a 20 hour discharge). Obviously, higher the Ah rating, the better the
battery.
Voltage
A measure of the force or "push" given by the electrons in an electrical circuit. It may
also be defined as a measure of electrical potential. One volt produces one amp of
current when acting against a resistance of one ohm.
Voltage of a battery may be calculated using the Nernst equation (cf. Electrochemical
energy systems).
Current
An electric current, which is a flow of charge, occurs when there is a potential
difference.
For a current to flow it requires a complete circuit.
Current (I) is measured in amperes (A), and is the amount of charge flowing per second.
current : I = q / t, with units of A = C / s
Energy
Voltage
TIME
E min cell
Energy is defined as the capacity to do work . It is expressed in terms of Joules or
calories.
The theoretical energy for one mole of the reaction may be calculated using -G =
nFEcell and practical energy is the actual amount of energy delivered for one mole of the
reaction.
Energy efficiency is defined as the ratio of useful energy output to the total energy
input (during charging).
Energy density The ratio of the energy available from a cell or a battery to its weight (or
volume) is referred to as energy density. It is expressed as
For example, if a battery to be used to operate a toy car, the energy stored in the battery is transformed
into mechanical energy which exerts a force on the mechanism that turns the wheels and makes the car to
move. This continues until the stored energy (i.e. charge) is used up completely. In its uncharged
condition the battery no longer has the capacity to do work.
22

i t E ave cell
Energy density =
W
where t is the time taken at the fixed current i to reach an average voltage, E ave cell. Energy
density is determined by determining the capacity and recording the average voltage
(voltage averaged during the discharge) and the total weight (or volume) of the battery. It
depends on the cell voltage. Requirement for batteries include a continuous energy density
above a certain value or a very high energy density for a short period.
Power
The level of discharge current drawn from a cell is determined principally by the
external load resistance.
The power (P) delivered is given by the product of the current flowing and the
associated cell voltage, is expressed in Watts (W).
As more and more current is drawn from a cell, the power initially rises, it reaches a
maximum and then drops as the cell voltage falls due to polarisation effects.
Power density * is a measure of how much power can be extracted from a battery per
unit of battery weight or volume. It is expressed in W/Kg. It is convenient parameter to
compare the performance of different battery systems using this parameter.
(v) Electricity storage density Electricity storage density is a measure of the charge per
unit weight stored in the battery, i.e., it is the capacity per unit weight. The weight indicates
the weight of the complete battery and includes the weights of all its components . A high
storage density indicates a good battery design and appropriate selection of electrode
reactions. For instance, use of lithium (lightest metal) is preferred to use of zinc as zinc has
lesser weight.
Cycle life
Cycle is a single charge and discharge of a rechargeable battery, and the number of
cycles a battery provides before it is to be discarded is called cycle life. If the capacity
of a battery falls below 60% to 80%, it should be discarded.
Shelf life
The period of time a battery can be stored without significant deterioration.
Aging is subject to storage temperature and state of charge. While primary batteries
have a shelf life up to 10 years, lithium- based batteries are can be used for 2 to 3 years,
nickel – based batteries are efficient for 5 years, etc.
23

Classification of Batteries:
Batteries are classified as primary (non-rechargeable), secondary (rechargable) and reserve
(inactive until activated):
Primary batteries Secondary batteries Reserve batteries
A primary battery is one whose
useful life is ended when its
reactants have been consumed
completely during discharge.
It is non-rechargeable.
Primary batteries are often
relatively inexpensive; they are
used in long-term operation
with minimal current drain.
Example: Dry cell.
Classical Batteries:
A secondary battery can
be recharged after
discharge under specified
conditions.
It behaves as an
electrochemical energy
storage unit.
The energy derived from
the external current is
stored as chemical
energy.
Example: Lead acid
battery.
Reserve batteries are special
purpose primary batteries designed
for emergency use and also for
long term storage.
The electrolyte is usually stored
separately from the electrodes
which remain in a dry inactive
state.
The battery is only activated when
it is needed by introducing the
electrolyte into the active part of
the cell.
Hence deterioration of the active
materials during storage can be
avoided and also eliminates the
loss of capacity due to self
discharge until the battery is put
into use.
Example: Magnesium-water
activated batteries, zinc-silver
oxide batteries, etc.
Zn-MnO2 battery:
Construction: Fig-1
The Zn-MnO2 battery consists of a zinc container as anode, and graphite rod as cathode.
The electrodes are separated by the electrolyte mixture i.e., graphitised manganese
dioxide and a paste of ammonium chloride and zinc chloride in water.
The MnO2 is mixed with graphite powder to increase the conductivity.
The cell representation is: ZnZnCl2(aq),NH4Cl(aq)MnO2(s),C(s)
The electrode reactions are: At anode: ZnZn +2 + 2e -
At cathode: MnO2 + H2O+ 2e - Mn2O3+2OH -
Net cell reaction: Zn + MnO2 + H2OMn2O3 +Zn +2 + 2OH -
Certain chemical reactions are not directly involved in the electrode reactions and hence do
not contribute to the EMF of the cell. These reactions are called secondary reactions.
24

The secondary reactions involved in the Zn-MnO2 cell are:
2NH4Cl +2OH - 2NH3+2Cl - + 2H2O
Zn +2 +2NH3+2Cl - [Zn(NH3)2]Cl2
The above secondary reactions are irreversible and hence the cell cannot be recharged.
The potential of the dry cell is 1.5V.
Applications: Used in portable electronic devices, viz. radios, transistors, tape
recorders, flash lights etc. where only small amount of current is required.
Limitations: When current is drawn rapidly from the cell, the products are deposited on
the electrodes resulting in a drop in the cell voltage, the cell capacity is low, the acidic
medium in the cell decreases the shelf life.
Lead-acid battery
Construction:
Lead-acid battery consist of (in the charged state) electrodes viz. lead metal (Pb) and
oxidized lead (PbO2) in the form of plates as anode and cathode respectively (or) the
electrodes may be lead grids containing spongy lead in one of the grid (as anode) and the
other containing lead dioxide (as cathode). The electrode pairs are separated by porous
partitions and are dipped in an electrolyte of about 37 % H2SO4. In the discharged state
both electrodes turn into lead sulfate and the electrolyte is consumed during the process.
The chemical reactions are (charged to discharged):
Anode: PbPb +2 + 2e -
Pb +2 +SO4 -2 PbSO4
Pb + SO4 -2 PbSO4+ 2e -
Cathode: PbO2+4H + + 2e - Pb +2 +2H2O
Pb +2 +SO4 -2 PbSO4
PbO2+ 4H + + SO4 -2 +2e - PbSO4+2H2O
The net cell reaction is: Pb + PbO2+ 2H2SO4 2PbSO4+2H2O
0 0.
0591
The potential of the cell is given by: Ecell = E log[ H SO ]
cell 2 4
n
From the above equation, it is evident that the potential of the lead acid battery depends
on the concentration of the electrolyte at the given temperature.
During charging the above cell reaction is reversed and sulphuric acid is regenerated.
2PbSO4+2H2O Pb + PbO2+ 2H2SO4
The OCV is 2.1V.
Applications
The lead acid battery is preferred for hospital equipment, telephone exchanges,
emergency lighting and UPS systems. It is also used in automobiles to start the engine.
Advantages
Economical for larger power applications where weight is of little concern.
Inexpensive in terms of cost, Low maintenance and simple to manufacture.
The self-discharge rate is lowest among the rechargeable battery systems.
Limitations
1. The lead acid battery has the lowest energy density, making it unsuitable for
handheld devices that demand compact size.
2. The performance of the battery at low temperatures is poor.
3. The electrolyte is extremely corrosive.
4. Overcharging may generate oxygen and hydrogen gases andmay lead to
explosion.
25

5. Low energy density.
6. The electrolyte and the lead content can cause environmental damage
(environmental concerns regarding spillage in case of an accident).
Nickel-Cadmium Battery
Rechargeable nickel-cadmium battery is a type of alkaline storage battery. In this cell the
electrodes containing the active materials undergo changes in the oxidation state.
Construction:
The Nickel-cadmium battery consists of nickel oxyhydroxide (NiOOH) as the charged
active material in the positive plate (cathode), together with up to 5% of Co(OH)2, Ba(OH)2
to improve the cell capacity and cycle life, 20% of graphite to increase the electronic
conductivity. Cadmium metal (Cd) is the charged active material in the negative plate
(anode), along with up to 25% of iron and small quantities of nickel and graphite to prevent
agglomeration.
During discharge, the charged nickel oxyhydroxide goes to a lower valence state, i.e.
Ni(OH)2, by accepting electrons from the external circuit and cadmium is oxidized to
cadmium hydroxide Cd(OH)2, and releases electrons to the external circuit.
The electrodes are isolated from each other by a porous separator, usually non-woven
fabric or nylon or polypropylene. This separator material in addition to isolating the
plates, contains the aqueous solution of potassium hydroxide with one to two percent of
lithium hydroxide as an electrolyte through which the chemical reaction take place.
During recharging of the battery, the reactions are reversed, thus returning the cell to
the original voltage and capacity.
The chemical reaction which occurs in a Nickel Cadmium battery is:
At anode: Cd Cd +2 +2e -
Cd +2 + 2OH - At cathode:
Cd(OH)2
2NiO(OH) + 2H2O + 2e - 2Ni(OH)2 + 2OH -
The net cell reaction is: 2 NiO(OH) + Cd + 2 H2O
2 Ni(OH)2 + Cd(OH)2
The above reaction goes from left to right when the battery is being discharged and from
right to left when it is being recharged. The alkaline electrolyte (commonly KOH) is not
consumed in this reaction.
The open circuit voltage is 1.35V
Uses
These cells are used in military and aerospace applications
These cells are used in
Advantages
Possess good load performance and allows recharging even at low temperatures.
Long shelf life, simple for storage and transportation. Good low temperature
performance.
It is the lowest cost battery in terms of cost per cycle.
Available in a wide range of sizes, high number of charge/discharge cycles.
Limitations
Relatively low energy density, low capacity when compared to other rechargeable
systems.
The lithium hydroxide is usually added to minimise the coagulation of the NiO(OH) and to prolong the
service life by making the cell more resistant to electrical abuse. For low temperature applications, more
concentrated KOH solutions are used (without LiOH, which increases electrolyte resistance).
26

It is environmentally unfriendly, since the Ni-Cd cell contains toxic metals. Has
relatively high self-discharge and need to be recharged after storage.
Modern Batteries
Zinc-air battery
Metal/air batteries consist of a reactive anode and air electrode as an inexhaustible cathode
reactant. The zinc-air, electrochemical system can be more formally defined as
zinc/potassium hydroxide/oxygen, but commonly known as “zinc-air” cell. It “breathes”
oxygen from the air for use as the cathode reactant. The limitless supply of air enables the
zinc-air cell to offer many advantages compared to other batteries. Zinc-air delivers the
highest energy density of any commercially available battery system, and at a low operating
cost.
Construction: cf. Fig:2
It consists of an oxygen reduction cathode (or air cathode) and anode containing zinc
gel.
The anode consists of anode can (i.e. nickel plated steel). A nylon insulator which
surrounds the can insulates the negative terminal from the positive terminal. The anode
mixture is prepared by mixing a zinc powder-electrolyte mix with a gelling agent.
The cathode assembly consists of the cathode can and the air electrode. The cathode can
is made of nickel-plated steel, and contains multiple air holes punched at the bottom to
provide air access to the cathode.
The cathode material is laminated with a Teflon layer on one side and a porous
separating membrane on the other. The separating membrane is placed directly over the
holes to ensure uniform air distribution across the air electrode. The Teflon layer allows
oxygen, to diffuse into and out of the cell, and also provides resistance to leakage. The
separator acts as an ion conductor between the electrodes and as an insulator to prevent
internal short-circuiting.
The alkaline electrolyte employed is an aqueous solution of potassium hydroxide with a
small amount of zinc oxide to prevent self-discharge of the anode. Potassium hydroxide
provides good ionic conductance between the anode and cathode to permit efficient
discharge of the cell.
The nominal open circuit voltage for a zinc air cell is 1.4 Volts. The operating voltage is
between 1.25 and 1volts.
Electrode reactions are:
At anode: ZnZn +2 +2e -
Zn +2 +2OH - Zn(OH)2
Zn(OH)2ZnO+H2O
At cathode: ½ O2 +H2O+2e - 2OH -
Net cell reaction Zn + ½ O2ZnO
Advantages
Very high capacity for its size.
Constant voltage output.
It can be used in medium current applications.
Environmentally safe.
High energy density and low operating cost.
Disadvantages
It can be used only if the battery compartment is vented to the atmosphere.
The cells are hygroscopic.
Actual performance of the cell depends on the relative humidity.
27

They can not be used in watches, as they require atmospheric oxygen to function,
and they may emit water which is corrosive to metal parts.
Uses
Used in hearing aids.
They are also well suited for use in telecommunication devices such as pagers and
wireless headsets.
Zinc-air batteries are often used to power a number of medical devices, such as
patient monitors and recorders, nerve and muscle stimulators, and drug infusion
pumps.
FUEL CELLS
These are galvanic cells in which electrical energy is obtained by the combustion of
fuels. Here, the fuels are supplied from outside and do not form integral part of the cell.
These do not store energy. Electrical energy can be obtained continuously as long as the
fuels are supplied and the products are removed simultaneously. In these aspects fuel
cells differ from conventional electrochemical cells
Advantages of fuel cells:
Power output is high.
Do not pollute the atmosphere
Electrical energy can be obtained continuously.
Hydrogen – oxygen fuel cell
Anode
H2
e
1.23 V
e
Cathode
O2
Porous graphite
electrode coated with
platinum electrocatalyst
28

It has an anodic compartment and cathodic compartments. Both contain
graphite electrodes impregnated with Pt-Ru-Co.
Hydrogen is bubbled through the anodic compartment
Oxygen is bubbled through the cathodic compartment.
Electrolyte is concentrated KOH solution
Reactions:
At anode H2 + 2OH ⇌ 2H2O + 2e
At cathode 1/2 O2 + H2O + 2e ⇌ 2OH
Water is formed as the product, which dilutes the KOH, and hence the electrolyte is
kept hot and also the cell is provided with a wick, which helps in maintaining the
water balance.
Uses: in space vehicles.
Methanol – oxygen fuel cell
O 2
Cathode
Membrane
Excess O 2
and water
Cathode + Anode -
Anode
It consists of anodic and cathodic compartments.
Both the compartments contain platinum electrode.
Methanol containing H2SO4 is passed through anodic compartment.
CO 2
CO 2
H2SO4
CH3OH +
H2SO4
electrolyte
29

Oxygen is passed through cathodic compartment.
Electrolyte consists of sulphuric acid.
A membrane is provided which prevents the diffusion of methanol into the
cathode.
Reactions:
At anode: CH3OH + H2O CO2 + 6H + + 6e
At cathode: 3/2O2 + 6H + + 6e 3H2O
Advantages:
Methanol has low carbon content
The OH group is easily oxidisable
Methanol is highly soluble in water.
Uses: in military applications.
Alkaline fuel cells:
These operate at 80 0 C.
At anode: hydrogen
At cathode: oxygen
Electrolyte: alkali
Advantages:
Hydrogen and oxygen are cheap.
Since the electrolyte is an alkali, any type of electrode can be used.
When started at room temperature has low efficiency but on operation gets
warmed up and gives optimum efficiency.
Phosphoric acid fuel cell
These operate at 200 0 C.
At anode: hydrogen or pure LPG
At cathode: air
Electrolyte: conc. Phosphoric acid adsorbed on a solid..
Electrodes are made of Teflon.
Uses: in supplying light and heat in buildings.
Molten carbonate fuel cell
These operate at 600 0 C.
At anode: hydrogen
At cathode: oxygen
Electrolyte: LiAlO2 + Li2CO3 + K2CO3
Reactions
At anode H2 + CO3 2 CO2 + H2O + 2e
At cathode 1/2 O2 + CO2 + 2e CO3 2
Nickel electrodes with a small amount of Cr are used.
Solid polymer electrolyte cell
These operate up to 200 0 C
Anode: hydrogen
Cathode: oxygen
30

Electrolyte: ion exchange membrane such as Nafion R
Anode and cathode are made of platinum electrodes.
Uses: in space vehicles
Solid oxide fuel cells
These operate at 1000 0 C
Anode: Ni on ZrO2
Cathode: strontium doped LaMnO2
Electrolyte: ZrO2 – Y2O3
Advantage: does not corrode
Uses: In locomotives since large amount of heat is evolved.
Biochemical Fuel Cells
These operate at 0 – 40 0 C
These convert chemical energy into electrical energy using bioorganisms.
An example is a biochemical fuel cell which the oxidation of glucose in the
presence of FAD as the enzyme and methylene blue (MB) as intermediate.
The active material at anode consists of glucose , FAD and MB and the cathode
consists of a metal such as Mg.
C6H12O6 + FAD C6H10O6 + FADH2
FADH2 + MB FAD + MBH2
MBH2 MB +2H + +2e
C6H12O6 C6H10O6 + 2H + +2e at anode
Mg 2+ + 2e Mg at cathode
Questions
1. What are fuel cells? How are they different from conventional electrochemical cell?
2. Explain with a neat sketch construction and working of hydrogen – oxygen fuel cell.
3. Explain with a neat sketch construction and working of methanol – oxygen fuel
cell.
4. Write notes on
a. Solid oxide fuel cells
b. Molten carbonate fuel cells
c. Solid polymer electrolyte fuel cells
d. Biochemical fuel cells
e. Alkaline fuel cells
Phosphoric acid fuel cell
31

Corrosion Science
Corrosion is the destruction of metals or alloys by the environment through electrochemical
or chemical reactions.
Electrochemical theory of corrosion
According to electrochemical theory of corrosion. When a metal such as iron is exposed to
atmosphere,
minute galvanic cells are formed on the surface of the metal.
Oxidation takes place at anode i.e. electrons are released at anode
Fe Fe 2+ + 2e
Reduction takes place at cathode i.e. electrons are accepted at cathode
If the medium is aerated and neutral
2H2O + O2 + 4e 4OH
If the medium is deaerated and neutral
2H2O + 2e H2 + 2OH
If the medium is deaerated and acidic
2H + + 2e H2
the metal ions formed at anode and the OH formed at cathode react to form the
hydroxide, which is corrosion product.
Fe 2+ + 2OH Fe(OH)2 O2 Fe2O3 ( rust)
Types of corrosion
1. Differential metal corrosion
2. Differential aeration corrosion
3. Stress corrosion
32

Differential metal corrosion: This type of corrosion occurs when two different metal are
in contact with each other. One of he metals acts as anode and the other as cathode. The
former corrodes. This happens due to the difference in the potential at the two electrodes.
The metal, which is placed higher in the electrochemical series, acts as anode and
undergoes corrosion. Thus, when Fe is in contact with Zn. Zn corrodes whereas if Fe is in
contact with Cu, Fe corrodes.
When screws and nuts are made of different metals, this type of corrosion takes place.
Differential aeration corrosion: this type of corrosion occurs when different parts of a
metal are exposed to different concentrations of oxygen. Thus when an iron is half
immersed in water, the part immersed in water is less aerated and acts as anode. The part
which is above the surface water is more aerated and acts as cathode. Thus corrosion
begins at the bottom portion of the rod.
When equipments are placed on flat base, this type of corrosion takes place. Hence
equipments are placed on legs.
Corrosion in barbed wires and corrosion at the joints in cross wires are other examples
Water line corrosion
In steel water tanks, the bottom portion is less aerated. Corrosion begins at this portion and
moves slowly upwards until the entire tank corrodes.
Pitting corrosion
When dust settles on the surface of a metal, the portion of the metal below the dust is less
aerated than the rest of the metal. The dust covered portion acts as anode. In a corrosive
environment, this portion undergoes corrosion, forming a pit.
Stress corrosion
When a metal rod is under stress (such as bending), the bent portion acts as anode due to
slight displacement of atoms in this region. The remaining portion of the metal acts as
cathode and hence the stressed portion undergoes corrosion when the environment is
favourable for corrosion of the metal. Thus iron rod under stress undergoes corrosion in the
presence of alkali and stressed brass undergoes corrosion in the presence of ammonia.
Thus for stress corrosion, (1) the metal should be stressed such as due to welding, riveting
and (2) the presence of specific corrosive environment for the metal is necessary.
Factors that affect the rate of corrosion
Nature of corrosion product or passivity
If the corrosion product is non-porous, non conducting and non stoichiometric, the
corrosion rate decreases because the product forms a protective coating over the surface of
the metal and prevents further corrosion. E.g., aluminium
If the corrosion product is porous, conducting and non-stoichiometric, corrosion proceeds
uninhibited. E.g., iron.
Electrode potential
If two different metals are in contact corrosion takes place due to potential difference. Thus
if the two metals are placed closer in the electrochemical series, the potential difference is
less and the corrosion is faster. If the two metals are placed farther apart in the
electrochemical series, the potential difference is high and corrosion rate is high.
33

Thus iron corrodes faster when in contact with than when in contact with
Anodic and cathodic areas
Smaller the anodic area and larger the cathodic area, faster is the corrosion. A large
cathodic area requires more electrons and since this has to be supplied by a small anodic
area, the corrosion will be fast. Thus when tin is coated on iron, (iron is anode and tin is
cathode), even if a pinhole is formed, results in small anodic area and large cathodic area.
Therefore corrosion will be intense. If zinc is coated on iron, zinc being anodic to iron,
even if the coating peels off at certain places, corrosion would not be intense.
pH
In general, lower the pH i.e., more acidic the conditions, higher are the rate of corrosion. If
the pH is > 10 i.e., in highly alkaline medium, corrosion practically ceases. If the pH is
between 3 and 10, corrosion takes place in the presence of oxygen. If the pH is less than 3,
corrosion is intense.
Temperature
As the temperature increases, conductance and diffusion of the medium increases and
hence corrosion becomes intense. High temperature decreases passivity and increases
corrosion rate.
Nature of the metal
Corrosion also depends on the position of the metal in the galvanic series. A metal which
is placed higher in the galvanic series has a lower electrode potential , is more reactive and
hence corrodes easily.
When two metals are in contact, the difference in their potentials greatly influences the rate
of corrosion. Higher the difference in the electrode potentials, faster is the corrosion.
Physical state of the metal
The rate of corrosion also depends on the physical state of the metal which includes grains
size, stress etc. If the grain size is small, the surface contact is more, and the corrosion rate
is high. A stressed metal (due to bending, riveting, welding etc.) undergoes corrosion faster
because the stressed portion is anodic with respect to the remaining portion of the metal
Hydrogen overvoltage
cathodic reaction involves reduction. If the medium is acidic
there are two competing reactions at the anode
The H + ions can accept electrons and get liberated as hydrogen gas
The metal can accept electrons and form m A metal with low hydrogen over voltage
(OV) is more susceptible to
corrosion, when the cathodic reaction involves hydrogen evolution.
The reduction in the over voltage of the corroding metal/alloy,
accelerates the corrosion rate.
Example: when Zn metal in contact with 1N H2SO4, it undergoes corrosion
by the evolution of hydrogen gas. The rate of the reaction is very slow,
because its O.V. is high (~0.7V). If a few drops of Cu solution is added
the rate of corrosion increases since, Cu gets deposited on Zn forming
minute cathodes, where the hydrogen OV value is only 0.33V.
34

Anodic and cathodic polarizations
Larger the difference in the potentials at anode and cathode faster is the corrosion. As
corrosion current flows, due to some irreversible reactions, the potentials at the anode and
cathode vary. This is called polarization. Polarization decreases potential difference and
corrosion rate decreases.
Anodic polarization: if the anode alone undergoes polarization, corrosion rate depends on
lllllanodic polarization. A plot of current density against potential shows that anode
polarization curve is steeper.
Cathodic polarization: if the cathode alone undergoes polarization, corrosion rate depends
on cathodic polarization. A plot of current density against potential shows that cathode
polarization curve is steeper.
Corrosion control
Design of the equipment
Corrosion can be controlled by proper design of the equipment.
Avoiding use of different metals in contact can control differential metal corrosion.
If two different metals have to be used, the metals chosen should be placed closer to each
other in the electrochemical series.
Anodic material should be as large as possible.
The two metals should be separated by an insulator such as wood.
There should be no gap between the metals.
The equipment should be mounted on legs and not on blocks. This would prevent
differential aeration corrosion. Sharp corners should be avoided.
Metal coatings
Ec
E curr
Ea
Galvanizing
Coating of iron with zinc (anodic to iron) is called galvanizing.
The base metal is first degreased with organic solvents and then treated with sulphuric acid
to remove any oxide that may be present on the surface. The metal is washed with water to
remove the acid.
The metal is treated with a solution of a zinc chloride and ammonium chloride. This acts
as flux.
The metal is finally dipped in molten zinc at 450 0 C when zinc gets coated on the metal. It
is rolled to remove the excess zinc from the surface.
Cathodic coating
Current density
icorr
Ec
Ecorr
Ea
Current density
icorr
Ec
Ecorr
Ea
Current density icorr
35

In this method of metal coating, the base metal is coated with a metal, which is cathodic to
it. Tinning of iron is an example.
The base metal iron is degreased, treated with dilute sulphuric acid and washed with water.
It is immersed in a solution of Zinc chloride and ammonium chloride. It is dipped in
molten tin and finally in palm oil to remove the excess tin. Tinning is employed for cans
used for storing food. This is because tinning prevents corrosion, is nontoxic and is more
economical than electroplating.
Anodizing
Metals like aluminium are made anodes and dipped in a sulphuric or chromic acid. When
current is passed, protective metal oxides are formed on their surface. This is called
anodizing
The object to be protected (anode) and copper rod (cathode) are dipped in a solution of
chromic or sulphuric acid. When a potential is applied, a layer of oxide is formed on the
surface of the metal preventing it from further corrosion.
Cathodic protection
In this method corrosion of a metal is controlled by supplying electrons externally so that
the metal acts as cathode. This can e achieved by two methods
Sacrificial anode method: In this method, the base metal is combines with a metal, which is
more anodic than the metal itself. E.g., iron is combined with zinc. Zinc provides electrons
and thus iron becomes cathode so that zinc corrodes and iron is protected. However, zinc
has to be replaced from time to time. Ship hulls and buried pipes are protected from
corrosion by this method.
Impressed voltage or impressed current method:
In this method, electrons are supplied to the metal by an external D.C. source. The negative
terminal of the source is connected to the metal. Water tanks and oil pipe lines re protected
from corrosion by this method.
Anodic protection
This method of corrosion control is applicable for metals like aluminium, which form a
protective oxide layer on their surface.
The metal (anode) is kept in an oxidizing medium. Calomel electrode and a platinum
electrode are used as cathodes. A current generator and a potentiostat are arranged as
shown. Initially the potential is increased passivating potential. Later it is increased to flade
potential when passivation is complete. Now the potential is again maintained at
passivation potential.
A
C
IP
CURRENT
B
Ep
Icrit
Passive
Active
Reference electrode
Potentiostat
In Out
E
Oxidizing medium
Pt
Metal
36

Corrosion inhibitors
These are chemicals, which inhibit anodic and cathodic reactions.
Anodic inhibitors
These inhibit anodic reactions by forming a protective coating on the surface of the metal.
These are oxidizing agents such as chromates and molybdates. These form protective layer
of oxides on the surface and thus prevent corrosion of the metal.
Phosphates, borates are non-oxidizing inhibitors. These form insoluble metal phosphates,
borates and deposit as protective coating on the surface of the metal. They are used in IC
engines, internal surface of pipelines.
Cathodic inhibitors:
These inhibit cathodic reactions and prevent formation of hydroxides and hydrogen.
Amines, mercaptans and thiourea are used in acidic medium. They form a protective layer
on the surface of the cathode.
Sulphates of magnesium, nickel etc. are used in neutral and alkaline medium these react
with the hydroxyl ions liberated at the cathode and deposit as insoluble hydroxides on the
surface of the cathode. Thus further cathodic reaction is prevented.
--------
37

Metal finishing
1. What is metal finishing ? Give the Technological importance of metal finishing
The term metal finishing is referred to all surface processes, which are carried out to bring
about modification of surface by depositing another metal or polymer. These processes
include, electroplating, electroless plating, surface conversion coatings, etc.
Technological importance of metal finishing
Metal finishing is carried out to impart one or more of the following properties to the metal
surface;
1. Corrosion resistance,
2. Electrical and thermal conductivity,
3. Impact resistance,
4. Abrasion and wear resistance,
5. High strength,
6. Hardness
7. Optical reflectivity.
2.Explain the following terms
1. Polarization
i) Polarization
ii) Decomposition potential
iii) Over voltage
The electrode potential of a single cell is given b the Nernst equation
E = E o + (0.0591/n) log [M n+ ]
Where E o is standard reduction potential of M, n is the number of electrons involved and
[M n+ ] is concentration of metal ions. When there is a passage of current, the concentration
of metal ions decreases at the vicinity of electrode surface due to reduction of metal ions
into metal atoms.
M n+ + n e -
M
This leads to change in electrode potential and shift in equilibrium. However, the
equilibrium reestablished due to the diffusion of metal ions from the bulk of electrolyte to
1
38

the surface. The diffusion arises due to concentration gradient between electrode and
electrolyte. If the diffusion rates are slow, the electrode potential changes and the electrode
in such a situation is said to be polarized. Hence the polarization can be defined as a
process where there is a change in electrode potential due to in adequate supply of species
from the bulk of electrolyte to electrode surface. Polarization of electrode should be less for
an efficient electrolysis.
Polarization depends on
2. Decomposition voltage
1. Nature and size of the electrode
2 Conductivity and concentration of the electrolyte
3. Temperature
4. Products of electrolysis
5. Rate of stirring
Decomposition potential is defined as the minimum potential or voltage required to bring
about continuous electrolysis. If a potential is applied between two platinum electrodes
dipped in 1 M solution of H2SO4 and the potential is increased, the current increases slowly
and gradually. When the voltage reaches a certain value, the current suddenly increases
rapidly with a slight increase in the voltage; this voltage is referred as decomposition
potential. The sudden increase in current is due to continuous electrolysis, which takes
place with evolution of H2 and O2 when dilute acids or alkalies are electrolyzed.
The decomposition potential of electrolytes can be determined using a cell as shown:
A B
V
D
Ammeter
Cell
2
The cell consists of two platinum electrodes immersed in electrolyte. The voltage between
the electrodes is varied by moving the contact maker D along the wire AB. With increase in
the potential, the current also increases slowly. If the electrolyte is dilute acid or alkali,
I
V
ED
39

when the potential is increased to 1.7 V, the current increases suddenly due continuous
electrolysis (which is accompanied by evolution of H2 or O2). Hence the potential of 1.7 V
is said to be the decomposition potential for dilute acids or alkalies. Potential and current
graph are shown in fig, where ED is decomposition potential.
ED = ECathode - EAnode
3. Over voltage or over potential
In general, for continuous electrolysis to take place, the applied potential should be slightly
greater than the decomposition potential. However, in a few electrolytes a potential greater
than the decomposition potential required to bring about electrolysis. The excess potential
is referred as over potential or over voltage. It is represented as .
The over potential can be defined as the excess potential that has to be applied above the
theoretically calculated decomposition potential to bring about the continuous electrolysis.
Over potential depends on
Hydrogen over voltage
= Experimental potential - Theoretical decomposition potential
ED = E Cathode - E Anode +
1. Nature of the electrode
2. Nature of the material deposited
3. Current density
4. Temperature
5. Rate of stirring.
The voltage required for the liberation of hydrogen at an electrode is referred as hydrogen
over potential. This is high compared to the over voltage for metals because liberation of
hydrogen takes place in 3 steps whereas metal deposition takes place in 1 step.
3
Though the decomposition potential for Zinc salt is higher than that for liberation of
hydrogen, electrodeposition of zinc is possible because the over voltage for hydrogen is
higher.
H3O + H + + H2O
H + + e - H
H + H
Electroplating
H2
The process of deposition of a metal over another metal or alloy or polymer or composite
using electrical energy is called electroplating.
M n+ + ne - M
40

Fundamentals of electroplating
The process of electroplating is carried out by electrolyzing a suitable salt solution of metal
being plated. The electrolyte consists of a salt solution of the metal being plated and it
should be highly concentrated and conducting. Additives are added to the electrolyte to
improve the quality of deposit. The cathode is the substrate on which electroplating is
MA (Electrolyte) M n+ (aq) + A n-
M M
carried out and anode is the pure metal to be deposited on the substrate. On electrolysis, the
n+ + ne- at anode
metal at anode undergoes oxidation and metal ions are formed. The metal ions undergo
reduction at the cathode and get deposited over the substrate.
In some cases, the anode is made up of inert electrode like platinum. In such cases, the
metal ions from the electrolyte get deposited on the object. These type of electrodes
involves liberation of oxygen. Under acidic conditions, liberation of hydrogen takes place
at cathode as a side reaction.
M n+ + ne - M at cathode
Mechanism of electroplating
Formation of deposit over the substrate takes place sequentially as follows-
1. Diffusion of metal ions from bulk of the electrolyte to the electrode surface (Cathode).
This step is referred as mass transport process.
2. Electron transfer to form ad-atoms. The ad-atoms are formed due to reduction of metal
ions to metal atoms by gaining electrons.
3. Diffusion of ad-atoms over the electrode (object) to a most favourable and stable
positions. This step is called surface diffusion process.
4
Generally, overall electrodeposition takes place in two distinct phases. In first phase,
formation of nuclei with few ad-atoms takes place and in second phase, the deposit over the
nuclei grows to give deposit in macroscopic thickness.
3. Explain the following factors (Plating variables) on the nature of electro deposit
The nature of electrodeposit is affected by several factors which include current density,
concentration and conductivity of plating bath, pH of plating bath, temperature and
throwing power, presence of additives such as levelers, brightners, structure modifiers or
stress relievers, wetting agents etc.
41

1. Current density
Current density the current density of electrodeposit is defined as the current per unit area
generally expressed in milliamperes per square centimeter (mA cm -2 ).
At low current density, the rate of surface diffusion of ad-atoms at cathode is more
compared to electron transfer and hence, the ad-atoms find more favourable or stable
positions, which leads to good deposit.
At higher current density, the rate of surface diffusion of ad-atoms at cathode is slow
compared to electron transfer and hence, the ad-atoms may not find favourable positions
and leads to poor deposit. Further at high current densities, hydrogen evolution takes place
and the deposit becomes powdery and contains hydroxides and oxides of the metal.
An optimum current density is preferred for good deposit.
2. Plating bath
The plating bath contains salt solution of metal to be plated and other electrolytes,
complexing agents and additives.
The main constituent of plating bath is salt solution of metal. The concentration and
conductivity of metal ions should be high. In order to improve the conductivity, other
electrolytes having high conductivity are added.
In some cases, complexing agents are added to get complex ions, which gives deposits with
improved qualities. Complexing agents are added (i) when the metal is reactive with object,
(ii) when anode becomes passive-to increase the dissolution of metal at anode, (iii) to
reduce the potential for electrodeposit and (iv) to improve the throwing power of
electrolyte. The electrolyte should be highly conducing and should have high concentration.
Most commonly used complexing agents are cyanides, hydroxides, chlorides and
sulphonates. 5
3. Organic additives
Some additives such as brightners, levelers, structure modifiers or stress relievers and
wetting agents are used to improve the deposit.
4. Brighteners: These are added to get fine-grained deposit. When the deposit has a size
less than the wavelength of the incident light, the light gets reflected giving brightness to
the surface. Brighteners help deposition parallel to the surface and inhibit deposition
perpendicular to the surface.
E.g., thiourea, sulphonates, compounds with CN, NCS, or CO groups.
5. Levelers: These substances help in deposition with uniform or an even thickness. The
levelers get deposited in the regions where rapid deposition takes place and makes
uncovered surface available for deposit.
42

E.g., sulphonates (sodium allylsulphonate)
6. Structure modifiers or stress relievers:
These substances are added to relieve stress due to lattice defects. These modify structure or
surface properties and relieve stress otherwise cracks may develop.
E.g., Saccharin.
7. Wetting agents:
These are added to release the gas bubbles (hydrogen) that may be formed during
electrodeposition. Otherwise, the gas bubbles get trapped in the metal and get released later
on, which makes coating with pits or pores.
E.g., sodium lauryl sulphate
8. pH:
At low pH, i.e., acidic conditions, liberation of hydrogen gas takes place which gives burnt
deposit (poor deposit). At high pH, i.e., basic conditions, formation of hydroxide coating
tales place. Therefore the pH is maintained between 4 and 8 by adding buffer.
9. Temperature:
At lower temperatures, the rate of diffusion of species is low and rate of deposit is slow. A
good deposit with fast rate takes place slightly at elevated temperatures due to high
diffusion rates of ions. However, at very high temperatures, liberation of hydrogen takes
place at cathode, which leads to poor deposit. Also, at high temperatures corrosion of
equipment and decomposition of additives takes place. A moderate temperature of 35 to 60 o
is maintained during electroplating. 6
10. Throwing power
Throwing power is the capacity of an electrolyte to give a uniform and even deposit
throughout the objective irrespective of the size and shape of the object. Throwing power
can be improved by (i) using highly conducting electrolyte (ii) placing the anodes at
optimum position (iii) addition of complexing agents and additives.
4. Explain the Electroplating process
1. Plating tank: The process is carried out in a rectangular tank made of wood or steel. The
interior walls of the tank are coated with ceramic or polymeric material to provide thermal
insulation.
2. Heating equipment: Many electroplating processes operate at moderate temperature.
The electrolytic bath is heated using electrical coils. The tank is also provided with a stirrer
and a thermometer.
43

3. Electrolytic bath: The electrolyte is a solution of the slat of the metal to be deposited. It
should be highly conducting and its concentration should be high. Additives are added to
improve the throwing power.
4. Electrical equipment: the electroplating process requires a D.C voltage, which is
generally provided using a DC rectifier or a motor generator.
5. Anode: The anode is made of the metal to be deposited. The anode material is generally
taken in the form of powder or granules or pellets in polythene bags. The polythene bag
serves as filter for separating impurities.
6. Cathode: The cathode is object to be coated. The metal to be coated is pretreated before
giving coating. Some times cathode jigs are used to hang several objects for coating. Above
the tank or vat, runs a bus bar from which anodes and cathodes are hung.
The pretreated cathode and anode are hung from the bus bar and dipped in the electrolyte
placed in the vat at appropriate positions. A suitable voltage is applied. Plating is complete
in a few seconds to half an hour.
Anode metal filled
in polythene bags
Cathode jigs to
hang object
Cathode
Anode
5. Explain the pretreatment of electrode surface (surface preparation )
Electrolytic bath
a. Removal of organic impurities by vapour degreasing process: In this process
First the metal surface is degreased by treating with organic solvents such a
7
Heating
equipment
44

trichloro ethylene, methylene chloride. In the manufacture of PCBs and other
electronic objects, the degreasing is done by treating with 1,1,1-tri chloroethane.
b. Alkaline cleaning : The above organic impurities such as oil, grease , oxide film
and nonmetallic substance can be removed by treating with sodium hydroxide
c. Pickling, removal of oxide film: The excess alkali present on the surface, and also
oxide film is removed by treating with 10 % sulphuric acid. Effective removal of
oxide film is achieved by immersing in sulphuric acid and making surface anodic
(100 mA cm -2 ).
d. Polishing: Mechanical or electropolishing are applied to get a polished surface.
Mechanical polishing involves, grinding surface with silicon carbide wheels and
electropolishing involves, anodic dissolution of surface of metal.
e. Rinsing and drying: Finally, the metal is washed with water preferably under hot
condition and dried before electroplating.
6. Discuss the electroplating of chromium . Why chromium anode is not used in
electroplating of chromium ?
A number of theories have been proposed to explain the mechanism of chromium (VI)
plating. All the theories are based on the following multiple reactions.
Main reactions :
CrO3 + H2O H2CrO4 CrO4 2- + 2 H +
H2CrO4 H2Cr2O4 + H2O Cr2O7 2- + 2H + + H2O
Cat.(SO4 2- )
Cr2O7 2- + 14 H + + 6 e 2Cr 3+ + 7H2O
Cr 3+ + 3e Cr
2Cr 3+ + 3O2
pbO2
2CrO3 + 6e
Plating conditions Chromium
Plating bath CrO3 + H2SO4 (100: 1)
Current density 100-200mA Cm-2
8
45

Temperature 45oC
Anode Pb-Sn Coated with lead dioxide
Cathode Pretreated object
Applications Decorative
In chromium plating , sulphate ion provided byH2SO4 is believed to act as a catalyst . Cr is
present in the hexavalent state [ Cr (VI) ] as CrO3 in the bath solution. This is converted
into trivalent Cr(III) by a complex anodic reaction in the presence of sulphate ions. The
chromium formed is coated as elemental chromium on the cathode surface . The amount of
Cr(III) ions , however should be restricted in order to obtain satisfactory deposit .
Insoluble anodes like Pb-Sb or Pb-Sn alloys covered with a layer PbO2 which oxidize
Cr(III) to Cr(VI) are used thus controlling the Cr(III) concentration. Chromium anodes are
therefore not used in Cr plating for the following reasons .
(i) Chromium metal passivates strongly in acid sulphate medium and
(ii) Chromium anode gives rise to Cr (III) ions on dissolution. In the presence of large
concentration of Cr (III) ions, a black Cr deposit is obtained.
7. Discuss the electroplating of gold and mention it’s applications
In electronic industries , gold plating is applied to the following three specific classes
of components.
i) Semiconductors ii) Printed / etched circuits iii) Contacts / connectors
Gold plating baths that are actually used in the electronic industries may be conveniently
classified on the basis of pH range.
Alkaline cyanide bath pH over 10.0
Neutral cyanide bath pH 6.0 to 9.0
Acid cyanide bath pH 3.5 to 5.0
Gold cyanide is unstable and precipitates below a pH of 3.5
Alkaline cyanide and neutral cyanide baths are employed to semiconductors while acid
cyanide bath is preferred for printed circuits and connectors
Typical bath formulations are given below
9
46

Plating Bath
solution g/L
Alkaline Cyanide Bath Neutral Cyanide Bath Acid Cyanide Bath
Potassium gold cyanide 8-20
Dipotassium phosphate 22-45
Potassium cyanide 15-30
Potassium gold cyanide 8-20
Monopotassium phosphate 80
Potassium citrate 70
Potassium gold cyanide 8-16
Citric acid 90
pH 12 6.0 – 8.0 6.0 – 8.0
3.8-4.3
Temperature 49-71oC 71 21-49oC
Current density 3-5 ASF 1-3 ASF 100-400 ASF
Anode Stainless steel Platinum clad columbium Platinum clad columbium
Cathode Article Article Article
8. What is electroless plating ? What are advantages of electroless over
electroplating ?
The process of deposition of a metal from its salt solution on to a catalytically active
surface of substrate in presence of suitable reducing agent without using electrical energy is
termed as electroless plating. The reducing agent reduces metallic ions to metal, which get
plated over catalytic active surface. Elctroless plating can be represented schematically as
shown below-
The important aspect of electroless plating is preparation of catalytically active surface.
This is achieved by-
1. Acid treatment
Metal ions + Reducing agent + catalytic active surface (object)
Metal over catalytic active surface + oxidized product
2. Electroplating- a thin layer of metal (same as that going to plated by electroless plating)
is deposited by electroplating.
3. Treating with stannous chloride and palladium chloride alternatively over non-
conducting surfaces.
10
47

Advantages of electroless plating
1. Use of electrical power and electrical contacts can be eliminated.
2. Semiconductors, insulators (like plastics) can also be plated.
3. Have good throwing irrespective of size and shape.
4. Produces hard coatings than conventional coatings
5. Deposits have unique chemical and mechanical properties.
6. Hydrogen gas does not trapped.
7. No levelers are required to add.
Composition of electroless plating bath
1. Source of metal ions: the metal, which gets deposits on the object, is taken in the form
of its salt solution usually chlorides and sulphates of metals.
2 Reducing agents: The driving force of electroless plating is use of suitable reducing
agent, which reduces metal ions to metal on the object. E.g., sodium hypophosphite,
formaldehyde, hydrazine, sodium hydroboride.
3. Complexing agent: In order to improve the quality of electroless plating and throwing
power of electrolyte, some complexing agents are used. E.g., citrate, tartrate, and succinate.
4. Exaltant: Use of complexing agents some times reduces the rate of deposit. In order to
improve the rate of electroless plating some additives are added and which are referred as
exaltant. E.g., anions such as succinate, glycinate and fluoride.
5. Stabilizers: In order to prevent the decomposition of electro less plating bath
constituents, some stabilizers are used. E.g., Lead, Calcium, Thallium and Thiourea.
6. Buffers: Maintaining pH is one of the most important one in electro less plating. This
can be achieved by using suitable buffers.
9. What is the difference between electroplating and electro less plating ?
Property Electroplating Electro less plating
Driving force Electrical energy Auto catalytic red ox reaction
Site of cathode
reaction
Substrate or object Substrate with catalytically
active surface
11
48

Anode Separate anode Substrate. No separate anode
Reactant Metal Reducing agent
Throwing power Low Good.
Application Plating through holes impossible Plating through holes possible
10. Discuss the electro less plating of copper with any two applications
Pretreatment and activation of surface: The object is degreased by washing with organic
solvents or alkali. Then it is treated with dilute solution of sulphuric acid and washed with
water. Some metal like Fe, Ni, Ag, Au, Pt, and Rh can be plated directly without activation.
Non-metallic materials are activated by treating with stannous chloride and palladium
chloride alternatively. During this treatment, the palladium gets coated as a thin film, which
provides the catalytic active surface for electro less plating.
Plating bath:
source of metal : CuSO4 solution
Reducing agent: formaldehyde
Exaltant and complexing agent: EDTA
Buffer: Rochelle salt
pH: 11
Temperature: 25 0 C
Reactions
Cu 2+ + 2e - Cu
2HCHO + 4OH 2HCOO + 2H2O + H2 + 2e-
Overall reaction: Cu 2+ + 2HCHO + 4OH Cu + 2HCOO + 2H2O + H2
Applications: In manufacture of PCBs.
The polymer sheet is first electroplated with copper it is etched at points where holes are to
be drilled. Holes are drilled and electro less plating is carried out. This gives plating of
12
49

copper through the holes so that there will be continuity of electrical connection above and
below the sheet.
11. Discuss the electro less plating of nickel with any two applications
Pretreatment and activation of surface: The object is degreased by washing with organic
solvents or alkali. Then it is treated with dilute solution of sulphuric acid and washed with
water. Some metal like Al, Fe, Brass and Cu can be plated directly without activation,
whereas, stainless steel is activated by dipping in hot solution of stannous chloride
containing 1:1 sulphuric acid. Non-metallic materials are activated by treating with
stannous chloride and palladium chloride alternatively.
Plating bath:
Source of metal: NiCl2 solution
Reducing agent: sodium hypophosphite
Exaltant and complexing: sodium succinate
Buffer: sodium acetate
pH: 4.5
Temperature: 95 0 C
Reactions:
Ni 2+ + 2e - Ni
H2PO2 + H2O H2PO3 + 2H + + 2e -
Overall reaction: Ni 2+ + H2PO2 + H2O Ni + H2PO3 + 2H +
Applications: Since, the deposits formed on the object are harder than conventional
coatings and hence used in domestic, electronic and automotive industries. Due to good
throwing power, it used to deposit metal on irregular shaped objects, tubes and hollow
cylinders, threads etc. Another important application is Ni-electroless plating on aluminium
develops solderability surface.
13
50

**********************************************************
14
51

HIGH POLYMERS
Polymers are long chain compounds of high molecular mass formed by the covalent linkage
of a number of repeating units called monomers.
Classification of polymers
I. Based on occurrence, polymers may be classified as
a. Natural polymers: e.g., cellulose, silk, proteins, starch, natural rubber.
b. Synthetic polymers: e.g., polythene, PVC, nylon, bakelite, terylene, teflon.
II. Based on the linkage of monomers, polymers may be classified as
a. Linear polymers: the monomers are linked to form a straight chain. Each monomer
molecule is linked to only two other monomer molecules.e.g. polythene
– CH2 – CH2 – CH2 – CH2 – CH2 – CH2 – CH2 – CH2
b. Branched polymers: the monomers are linked to form a straight chain with
branches. E.g., polythene manufactured under high pressure
– CH2 – CH – CH2 – CH2 – CH2 – CH – CH2 – CH2
CH2 CH2
CH2 CH2
c. Cross linked polymers: the monomers are linked to form a three dimensional cross
linked polymer. E.g., Bakelite
OH OH OH OH
CH2 CH2 CH2
CH2 CH2
CH2 CH2 CH2
OH OH OH OH
III. Based on the method of polymerization polymers can be classified as
a. Addition Polymers: These are formed by addition polymerization i.e., by the self
addition of monomers without the loss of any part of the molecule. The molecular
mass of the polymer is an integral multiple of the molecular mass of the monomer.
The integer is called the degree of Polymerization. Degree of polymerization is
defined as the number of times the monomer repeats itself in a polymer molecule.
The monomers contain multiple bonds and are usually alkenes or substituted
alkenes. E.g. polyethylene.
n CH2 = CH2 ( CH2- CH2) n
b. Condensation polymers: These are formed by condensation polymerization i.e., by
the condensation reaction between two or more monomer molecules with the
52

elimination of simple molecules like water, ammonia, hydrogen chloride etc. The
monomers contain functional groups such as amino, carboxylic acid, hydroxyl
group. E.g., nylon
nHOOC CH COOH nH N CH NH OC
CH CONH CH NH )
2
2 2
2 2
2
n
4
adipic acid hexamethylene diamine nylon – 6,6
6
( 4
6
c. Based on stereoregularity polymers may be classified as
a. Isotactic polymers: In these polymers, the substituents (R) are present on the same
side (either below of above the plane) of the C – C chain.
– CH2 – CH – CH2 – CH – C H2 – C H – (below the plane of C – C chain)
R R R
(OR)
R R R
– CH2 – CH – CH2 – CH – C H2 – C H – (above the plane of C – C chain)
b. Syndiotactic polymers: In these polymers, the substituents alternate regularly above
and below the C – C chain
R R
– CH2 – CH – CH2 – CH – C H2 – C H –
R
c. Atactic polymers: In these polymers there is a random distribution of substituents
R R R
– CH2 – CH – CH2 – CH – C H2 – C H – C H2 – C H –
R
Techniques of polymerization
There are four techniques industrially employed for the manufacture of Polymers.
(i) Bulk or mass polymerization
(ii) Solution polymerization
(iii) Suspension polymerization
(iv) Emulsion polymerization
(i) Bulk polymerization.
In bulk or mass polymerization, the monomer is taken in the liquid state and a small amount
of the initiator is dissolved in it to form a homogeneous mixture and is stirred. Initiation of
polymerization is effected by either heating or exposing to radiation. Chain transfer agent
whenever used to control molecular weight is also dissolved into the reaction mixture. It is
kept under agitation for proper mass and heat transfer. As the reaction proceeds, the
viscosity of the medium increases and mixing becomes difficult. This leads to broad
molecular weight distribution.
Advantages: The product obtained is pure and does not require any physical process of
isolation. The method is simple.
53

Disadvantages: As the viscosity increases, stirring becomes difficult. Heat dissipation
becomes inefficient leading to localized heating. This can result in decolourization of the
polymer. Thermal degradation and branching or cross linking may also take place.
This technique is used in making polystyrene, polymethylmethacrylate and PVC
(ii) Solution polymerization.
In this technique, the monomer and an initiator which is soluble in the monomer are taken
and dissolved in an inert solvent. Ionic or coordination catalyst if used can either be
dissolved or suspended. The solution is heated in a reaction vessel with constant agitation.
The solvent reduces the viscosity of the reactant mixture and hence better heat dissipation
can be achieved. The polymer is isolated by filtration or by distilling of the solvent.
Advantages: Agitation and termination is easy, better control over molecular weight
distribution.
Disadvantages: The polymer formed is not pure and the product has to be isolated by
filtration or distillation of the solvent. Removal of final trace of solvent is difficult. High
molecular mass polymers are not obtained.
Polyacrylonitrile and polyisobutylene are obtained by this technique.
(iii) Suspension polymerization.
Only water insoluble monomers can be polymerized by this technique. The monomer is
suspended in water in the form of fine droplets which are stabilized by adding surfactants
(gelatin, sodium polyacrylates). The size of the monomer droplets formed depends on the
monomer to water ratio, the type and concentration of the stabilizing agents and the type
and speed of agitation employed. The initiator used will be soluble in monomer droplets.
The reaction mixture is heated with continuous stirring. The reaction progresses within each
droplet to form the polymer.
Advantages: The reaction goes to completion since it progresses independently in each
droplet, the polymer molecules have uniform chain length, the polymer is obtained in the
form of beads or pearls, isolation of the polymer is easy as it just needs filtration of the
beads and washing the surfactant with water and exothermicity is well controlled.
Disadvantages: It is applicable only for water insoluble monomers and it is difficult to
control particle size.
Beads of polystyrene (used for making polystyrene foams), styrene divinyl benzene beads
are manufactured by this technique.
(iv) Emulsion polymerization.
In this technique the monomer is dispersed in the aqueous phase not as discreet droplets but as
a uniform emulsion. Emulsions are dispersions of oil and water. These are unstable and are
stabilized by the addition of emulsifiers. Emulsifiers contain a long chain with a nonpolar group at
one end (hydrophobic) and a polar group at the other end (hydrophilic). The emulsifier links the oil
and water through its end groups thus stabilizing the emulsion. The emulsifier may be anionic,
cationic or neutral. Water soluble initiators like persulfate or hydrogen peroxide are used to initiate
the polymerization. The mixture is vigorously agitated and is heated. The initiator slowly gets
diffused into organic phase, so the initiation starts where the monomer is placed in a spherical
structure known as micelle in colloidal form. As the polymerization progresses (Fig d), the number
of monomers within the micelle decreases. They are replenished by the medium. This continues till
the polymer becomes very large, the micelle bursts and the polymer comes out into the medium. In
the medium the polymerization is terminated by combination. The pure polymer is isolated from the
emulsion by the addition of a de-emulsifier.
A concentration beyond which only micelle formation is possible is known as critical micelle
concentration (CMC). At concentrations of surfactant beyond cmc, the surfactant molecules
aggregate to form micelles where the nonpolar groups collect together facing each other with polar
ends directing away as shown in Fig. b. This results in the flocking of monomers within the
micelle. Every surfactant is having different CMC value. If the amount of the surfactant added is
below a CMC, the surfactant molecules are freely dispersed in the medium and the polymers that
are formed under this condition will have smaller molecular masses (Fig a).
Flocking
of monomer
molecules
in a micelle
54

(a) (b)
Surfactant molecule
Hydrocarbon chain Polar end
(nonpolar end)
(c) (d)
Emulsion polymerization process (a)Free surfactant molecules below cmc (b) Aggregation of surfactant molecules to
form micelles above cmc (c) Diffusion of initiator (d) Polymer chain growth
Advantages of emulsion polymerization: The rate of polymerization is high;
Polymerization takes place within the micelle to form large molecular weight polymers
because termination by combination is unlikely within the micelle. Polymers with a fairly
narrow molecular mass distribution and chain length can be obtained, heat transfer is
efficient and hence viscosity build up is low.
Disadvantages: Polymer needs purification, it is difficult to remove the entrapped
coagulants, emulsifiers, and more chemicals are required.
Emulsion polymerization is used in the manufacture of a number of industrial polymers
such as polymers of chloroprene, vinyl chloride, vinyl acetate, butadiene, acrylates and
methacrylates.
MECHANISM OF ADDITION POLYMERIZATION
Addition polymerization (polymerization of simple unsaturated molecules) involves a chain
reaction which can be explained on the basis of free radical mechanism and ion-chain
mechanism. The free radical mechanism of the chain reaction essentially involves four
stages namely (i) Generation of free radicals (ii) initiation (iii) propagation and (iv)
termination.
(i) Generation of free radicals:
55

Free radicals are generated by thermal or photochemical decomposition of initiators.
Initiators are thermally unstable compounds, when exposed to heat or radiation produce
free radicals which are highly reactive.
Example, decomposition of dibenzoyl peroxide
or hv
(C6H5COO)2 2C6H5COO • 2CO2 + 2 •C6H5 (or R•)
Free radicals
(ii) Initiation:
The free radicals attacks the double bond of unsaturated monomer forming a new free
radical.
R• + CH2=CH2 R-CH2 -C•H2
(iii) Propagation:
The new free radical attacks another molecule of monomer and a chain reaction is set up as
shown below.
R-CH2 -C•H2 + CH2 =CH2 R-CH2-CH 2-CH2 -C•H2
or in general,
R-[CH2-CH2 ]x-1-CH2 -C•H2 + CH2=CH2 R-(CH2-CH2)x-CH2 -C•H2
R-[CH2-CH2 ]y-1-CH2 -C•H2 + CH2=CH2 R-(CH2-CH2)y-CH2 -C•H2
(iv) Termination
Two free radicals combine by coupling or by disproportionation and the polymerization
terminates.
a. Termination by coupling
R - (CH2-CH2)x - CH2 -C•H2 + C•H2 - CH2 - (CH2 - CH2)y R coupling
R (CH2 -CH2)x –CH2 –CH2 –CH2-CH2 -(CH2- CH2)y R
Polymer
b. Termination by disproportionation
R-(CH2 -CH2)x - CH2 -C•H2 + R-(CH2 -CH2 )y - CH2 -C•H2
R (CH2 - CH2)x -CH2 –CH3 + R - (CH2CH2)y -CH=CH2
Glass transition temperature (Tg):
The temperature, below which a polymer is hard and brittle and above which it is
soft is called glass transition temperature.
The hard and brittle state is known as the glassy state and the soft flexible state is
known as rubbery state or visco-elastic state. On further heating, the polymer
becomes highly viscous and starts flowing. This state is termed as visco-fluid state.
The transition is taking place at its flow temperature (Tf).
Glassy state Rubbery or visco-elastic state Visco-fluid state
(Brittle plastics) (Tough plastics and rubbers) (Polymer melts)
56

Factors that affect glass transition temperature:
1. Molecular geometry: Linear polymers having straight chain of single bonds
are highly flexible and have a lower Tg than polymer which have substituents or
branches. Bulkier the substituent, higher is the Tg. This is because the
substituents or branches resist rotation of the polymer chain.
-CH2 -CH2-CH2-CH2-CH2-CH2- free rotation of C chain possible
Polyethylene
-CH2- CH-CH2 -CH-CH2- CH-CH2- free rotation hindered
Polystyrene
Tg of polyethylene (linear polymer) is -110 0 C whereas Tg of polystyrene with bulky
substituent) is 100 0 C
2. Rigid polymers having cross-linked structures have high Tg. The three
dimensional structure hinders rotation about the main C – C chain of the polymer.
3. Polymers with electrostatic attraction (such as hydrogen bonds) between the
polymer molecules offer resistance to molecular rotation and hence have a high Tg.
Tg of polypropylene (containing no polar groups) is –18 0 C whereas Tg of nylon 6, 6
containing Hydrogen bonding is 57 0 C.
4. Isotactic polymers with substituents on one side of the C – C chain have lower Tg
than syndiotactic polymers which have substituents alternately above and below the
C – C chain because of the resistance offered by the substituents from both the sides.
R R R
– CH2 – CH – CH2 – CH – C H2 – C H – isotactic polymer
R R
– CH2 – CH – CH2 – CH – C H2 – C H – syndiotactic polymer.
R
5. Molecular weight: In general, as the molecular mass increases Tg increases since high
molecular mass restricts rotation about the C - C chain. But beyond degree of
polymerization of 250, the increase in Tg is not significant.
6. Additives: A polymer with plasticizer has a lower Tg because of the flexibility offered by
the plasticizer. PVC has a Tg of 80 0 C. But addition of diisooctyl phthalate (a plasticizer)
reduces the Tg to 20 0 C.
Determination of glass transition temperature.
Glass transition temperature can be determined by studying the variation in properties (for
example, change in volume and Young’s modulus) at different temperatures. An abrupt
change in the property is observed at Tg. By plotting a graph, the temperature can be
determined.
57

Commercially
useful range
Tg Tg
T T
Significance of glass transition temperature: .
Tg value is an important parameter of polymers. It decides whether the polymer at a
particular temperature behaves like rubber and plastic. Tg value measures the flexibility of
the polymer. It helps in predicting the response of a polymer, its response to mechanical
stress and also in predicting the coefficient of thermal expansion, heat capacity, refractive
index, modulus of elasticity and electrical properties of a polymer helps in deciding the use
of polymer over a range of temperature. It helps in determining the suitability of a polymer
for a particular application. It is useful in choosing appropriate temperature range for
processing operations such as molding, extrusion of polymer, etc.
RELATION BETWEEN STRUCTURE AND PROPERTY OF POLYMERS
(i) Strength.
Polymers with high molecular mass are tough, have high tensile strength, and high heat
resistance. They have high Tg. Cross-linked ones are stronger than linear polymers. Tensile
and impact strengths increase with molecular mass up to a certain extent and then become
constant. As the molecular mass of a polymer increases, its melt viscosity increases
gradually in the beginning and later shows a steep rise at high molecular masses. Low melt
viscosity and high tensile and impact strengths are desirable properties for a polymer to be
commercially useful.
Commercially
Useful range
200 2000 200 2000
DEGREE OF POLYMERIZATON DEGREE OF POLYMERIZATON
(a) (b)
From the above figure it can be observed that for a polymer to be economically useful, the
degree of polymerization should be between 200 and 2000.
Crystallinity: For a polymer to be crystalline,
(a) it should have a closely packed structure or
58

(b) it should have a symmetrical structure.
If a polymer has a linear structure without substituents or branches, the molecules can
come close together and hence will be crystalline. e.g. polyethylene is highly crystalline
because the atoms along the chain permit closer approach . On the other hand, polyvinyl
acetate is less crystalline as the branching here prevents closer approach.
-CH 2-CH 2-CH 2-CH 2-CH 2-CH 2- -CH 2 - CH - CH 2 - CH-
Polyethylene O O
CO CO
CH 3 CH 3
Polyvinyl acetate
Isotactic polystyrene syndiotactic polystyrene
C6H5 C6H5 C6H5 C6H5 C6H5 H C6H5 H
| | | | | | | |
….C - CH2 – C – CH2 – C – CH2 – C – CH2 -… …. -C- CH2 – C – CH2 – C – CH2 – C – CH2-
| | | | | | | |
H H H H H C6H5 H C6H5
Isotactic polystyrene (having substituents on only one side of the chain) is more crystalline
than syndiotactic polystyrene because the latter has phenyl group on both sides of the C –
C chain and does not permit the polymer molecules to come close on both sides.
A polymer with electrostatic attraction such as hydrogen bonding between the molecular
chains permits close packing of the molecules and hence is more crystalline. Example:
nylon
CH2 CH2
C=O H - N
H – N C=O
CH2 H2C
(CH2)2 O- CO- CO-O-(CH2)2
Nylon (Polyamide)
Polyethylene terephthalate is more crystalline than polyethylene phthalate because in the
former the high symmetry of para substituted ring permits close approach whereas in the
latter the ortho-substituted ring leads to irregular structure .
Polyethylene terephthalate
(CH2)2 O- CO CO-O-(CH2)2
Polyethylene phthalate
59

Crystalline polymers have high tensile strength, high impact resistance. Closely packed
structure hinders permeability of reagents and hence crystalline polymers have low
solubility and high chemical resistance. Crystalline polymers are translucent.
(iii ) Elasticity. A polymer is said to be elastic if it can be stretched by the application of an
external force and if it regains its original shape and size on withdrawal of the force. The
polymer chains should not snap on prolonged stretching. Breaking takes place when the
chains slip past each other and get separated on application of an external force.
For a polymer to be elastic (i) cross links should introduced at suitable molecular
positions, (ii) bulky side groups such as aromatic and cyclic structures should be avoided on
the chain and (iii) non-polar groups should be introduced on the chain so that the chains
do not separate on stretching.
(iv)Chemical resistance and nature of polymer chain
Polymers having polar groups such as - OH or -COOH groups are usually attacked by
polar solvents such as water or alcohols. Polymers with non-polar groups such as -CH3
and -C6H5 dissolve in non-polar solvents such as petrol, benzene and carbon tetrachloride.
Crystalline polymers have closely packed structure and hence have high chemical
resistance. But if the polymer contains ester group (e.g., cellulose acetate), it will be
attacked by alkalies because the ester group undergoes saponification. Strong acids and
alkalies attack polymers with groups such as -NHCO- and -NHCOO- (polyamides
(nylon) and polyurethanes).
Increase in crystallinity, cross linking and molecular weight results in the increase of
chemical resistance. Polythene and PVC are resistant to acids and alkalies.
(iv) Elastic deformation (rheology) of polymers: Rheology is a measure of deformation
caused in a polymer on the application of load on the polymer material. In general,
deformation increases as the load increases and the polymer regains its original size and
shape instantaneously when the load is withdrawn. The deformation depends upon the
crystallinity, cross linking and also on the glass transition temperature of the polymer.
Based on rheology, polymers can be classified as (i) soft and tough (vulcanized rubber), (ii)
soft and weak (natural rubber), (iii) hard and tough (polythene), (iv) hard and strong,
(nylon) and (v) hard and brittle (polystyrene). Highly cross linked elastomers (e.g.
vulcanized rubber) are amorphous and exhibit soft and tough behaviour above Tg . Hard
and strong character against stress is shown by fibres such as nylon and hard and brittle
character is shown by polystyrene below their glass transition temperature.
Compounding of Resins to Plastic:
The conversion of resin into plastic with addition of some compounds called additives is
known as compounding. The objectives of compounding are; 1. Improve the properties of
resin, 2. Make fabrication easy, 3. Introduce new properties into the fabricated article, and
4. Make the polymer product more pleasing and colourful.
This is done by mixing the resin with some additives like fillers, plasticizes, etc.
Resin: The basic component of a plastic is the resin. It binds all other ingredients. The
resin gives the desired properties like plasticity and electrical insulating properties to the
plastic. Examples of resins include polyethylene, polystyrene, and pvc.
Additives:
Plasticizers: These are organic compounds when added increases polymer flexibility,
lowers viscosity and Tg, helps to convert glassy polymer to soft and flexible material. They
enhance plasticity and reduce surface cracking. They should be inert and miscible with
polymers. They should have high boiling point and non volatile. Ex: Phosphate ester,
diisooctyl phthalate and tricresyl phosphate.
60

Fillers: These are added to increase the bulk of plastics without altering its salient features.
They improve the thermal and mechanical stability, non combustibility, water resistance
and electrical insulation. They are added in the form of powders (mica, carbon black,
graphite, barium sulphate, MgO, saw dust, etc.) fibres (cotton, asbestos) or sheets (paper,
cloth).
Hardeners: Hardeners enable the conversion of resin into solid state by forming a polymer
having a three dimensional structure. Ex: quartz
Accelerators: Accelerators are used in the case of thermosetting plastics with the object of
accelerating the polymerization of fusible resin into the cross linked infusible form. They
increase the necessary rate of solidification at a lower temperature.
Ex: hydrogen peroxide, benzoyl peroxide and acetyl sulphuric acid.
Stabilizers: Stabilizers enable the plastic to retain its properties for a long time and
improve their thermal stability Ex: Alkyl phenols, naphthols, white lead, lead chromate,
lead silicate and stearates of lead, cadmium and barium.
Lubricants: Lubricants make moulding easier and impart glassy finish to the product. They
prevent plastic material from sticking to fabricating equipment.
Ex: wax, soap and oils.
Dyes and pigments: These are added to impart desired color to the plastics for improving
its appearance.
Ex: TiO2 – white color, Lead – yellow color, Zinc chromate – green color
POLYMERS BASED ON ADDITION POLYMERIZATION
(a) Polymethylmethacrylate, Plexi glass
Polymethylmethacrylate [PMMA] also known as plexiglass is made by subjecting
methylmethacrylate, CH2=CCH3 COOCH3 to emulsion polymerization at 60-70 0 C in
presence of hydrogen peroxide
CH3 CH3
emulsion polymerization |
n CH2=C CH2-C
H2O2, 60-70 0 C |
COOCH3 COOCH3 n
Properties: It is highly transparent, softens at 120 0 C, hard solid, resistant to organic
solvents, having good mechanical strength and less scratch resistant.
Applications: Plexiglass is used for glazing airplanes and automobiles, and in the chemical
equipments (as lining material), sign boards, lenses for automobile lighting, and domestic
appliances.
(d) Teflon or polytetrafluoroethylene (PTFE).
Polymethylmethacrylate
It is obtained by the emulsion polymerization of tetrafluoroethylene under pressure and
heat, by using either H2O2, or Ammonium persulphate as initiator.
61

n(CF2 = CF2) ( CF2- CF2)n
Properties: It is having excellent electrical properties, highly resistant to corrosive acids;
density is 2.1 to 2.4 g/cm 3 . It is a highly crystalline solid.
Applications: PTFE is used for wire and cable insulation, insulation for motors, generators,
in the manufacture of seals and gaskets and in non stick cookware. It is used in bearings
and pads for railway bogies, as an insulator in high voltage power transmission.
POLYMERS BASED ON CONDENSATION POLYMERIZATION
(a) Phenol-formaldehyde resin:
It is produced by the polycondensation of phenol (P) and formaldehyde (F). When an
aqueous solution of formaldehyde (75% stoichiometric quantity) reacts with a large excess
of phenol (P/F ratio > 1), heated in the presence of an acid catalyst, a linear polymer called
Novolac is formed.
OH OH OH
| | |
n + ¾ n HCHO acid catalyst - CH2- -CH2- -CH2-
Phenol (Excess) Formaldehyde Polymerization Novolac resin
When a large amount of aqueous formaldehyde reacts with phenol in the presence of an
alkali catalyst, a cross-linked thermosetting resin called resol resin is obtained. Phenol,
aqueous formaldehyde and the catalyst are introduced into a reaction vessel fitted with a
stirrer and heated to 70 -75 0 C by means of steam.
The resol resin is mixed and heated along with hexamethylene tetramine, (CH2)6N4, a cross
linking agent, to give bakelite resin. OH
CH 2OH CH 2OH
Phenol + Formaldehyde (excess) Alkali catalyst n
OH OH OH OH
CH2 CH2 CH2
CH2 CH2 CH2 CH2
CH2 CH2 CH2
OH OH OH OH
Structure of Bakelite resin
CH 2OH (Resol)
Heat
With
Hexamethylenetetramine
Properties: They are very good OH insulators. They are rigid, hard, scratch resistant and water
resistant. They are attacked by alkalies due to the presence of -OH group in their structure.
Applications: They are employed as sealants for sealing electric bulbs to their metal
holders. They are also used for bonding sheets of paper, cardboard and wood. Bakelite is
OH
62

used in the fabrication of a wide range of products, particularly electrical fittings such as
plugs and switches and telephone parts, cabinets for radio and TV. They are used as ion
exchangers in water treatment.
(b) Polyurethanes
Polyurethane is formed by the reaction of a diisocyanate with a glycol.
(n +1) HO-(CH2)4-OH + n OCN-(CH2)6-NCO Tertiary amine
Butane diol 1,6 – hexamethylene Diisocyanate N 2 atmosphere
H O-(CH2)4-O –C- N-(CH2)6-N-C OH
O H H O
Polyurethane
Properties: They are excellent thermal insulators; they have high tensile strength and high
abrasion resistance. They have very good adhesion to metals, glass, ceramics, etc. They are
firm, light weight and soft.
Applications: Polyurethanes are used as rigid and flexible foams, elastomers, fibers, and
surface coatings.
Their flexible foams are used cushions for furniture and automobiles, packaging, fabrics,in
immobilizing the enzymes, etc. Rigid foams are used in insulation of refrigerators, deep
freezers, air conditioners. They are sued in tips of aircraft wings, radar covers, roof
insulation for cars, buses, railway coaches. Elastomer polyurethanes are used in rollers for
printing press, in foot wear and as wire coating enamel.
ELASTOMERS:
SYNTHETIC RUBBERS
Elastomers are high polymers which can be stretched to several times their length when
subjected to an external force but readily regain their original shape when the force is
withdrawn.
Natural rubber has many deficiencies. Synthetic rubbers have many advantages over
natural rubber. Some of these are given below.
Deficiencies of natural rubber and advantages of synthetic rubber
Natural rubber Synthetic rubbers
(a) Attacked by sunlight and air Not attacked by air and sunlight
(e.g. neoprene rubber. nitrile rubber)
(b) Less resistance to heat and cold Greater resistance to heat and
cold (e.g. nitrile rubber)
(c) Holds less air and water at Hold more air and water even
high pressure at high pressures (e.g. butyl rubber
used in inner tubes of cars)
(d) Softens and swells on storing Do not swell and can hold organic
organic Solvents solvents better (e.g.polysulphide)
(e) Rubber property lost at extreme Rubber property retained over wide
temperature range of temperature
n
63

(a) Butyl rubbers
Butyl rubbers are copolymers of isobutylene mixed with 1.5 to 4.5 % of isoprene. The
reaction mixture is cooled to -95°C and stirred thoroughly with anhydrous Aluminium
chloride in methyl chloride. The product separates out in the reactor.
CH3 CH3 CH3 CH3
| | | |
n CH3 - C = CH2 + n CH2 = C-CH=CH2 AlCl 3 in CH 3Cl CH2 - C = CH- CH2 - C-CH2
Isobutylene Isoprene | n
CH3
Butyl rubber
Properties: It has good resistance to heat, abrasion, and to chemicals. It exhibits low gas
permeability.
Applications: Used in making of inner tubes for tyres, for insulation of high voltage wires
and cables.
(b) Neoprene: This is also called as polychloroprene. It is produced by the emulsion
polymerization of 2-chloro-1,3-butadiene (known as chloroprene) in presence of
ammonium persulphate as initiator.
n CH2 = C-CH=CH2 -(CH2- C=CH-CH2)-n
| |
Cl Cl
2-chloro-1,3-butadiene Neoprene rubber
Properties: It can be vulcanized (process of cross linking using sulphur at double bond
position). Vulcanized products of neoprene have excellent tensile strength, higher oil
resistance. The presence of chlorine substituted double bonds make the polymer rather
unreactive and leads to good resistance to most chemicals, oxygen and ozone.
Applications: It is used for providing oil resistant insulation coatings to wires and cables
and for producing shoe soles, tyres, gloves, etc.
ADHESIVES; EPOXY RESINS
An adhesive is defined as a polymeric substance that binds together two or more
similar or dissimilar materials so that the resulting material can be used as a single
piece.
Adhesives may be broadly classified as
Natural e.g. gum and glue
Synthetic. Phenol-formaldehyde, urea-formaldehyde, resorcinol-formaldehyde,
silicones and epoxides.
Epoxy resins
Epoxy resins are polymeric materials containing the epoxy group R-CH -CH-
O
Epoxy resins are synthesized by condensation polymerization of bisphenol-A and
epichlorohydrin
64

CH3 O
|
n HO C OH + n CH2 - CH - CH2Cl
|
CH3
-HCl
O CH3 OH
| |
n CH 2-CH-CH 2 o C oCH 2-CH - CH 2
|
CH3
CH 3 O
| + nHCl
O C O-CH 2-CH-CH 2
|
CH 3
Epoxy resin
Properties: Epoxy resins contain an epoxy group at both the ends of the polymer chain.
Epoxy resins are linear, low molecular weight polymers soluble in organic liquids and
highly resistant to acids, alkalies and abrassion. They have good adhesion capacity and can
be used for binding various surfaces – rubber, wood, leather, ceramics, metals and
polymers. They have very good electrical insulating property.
Applications: Used as structural adhesives, in laminating the materials and used to impart
shrinkage resistance in fabrics.
CONDUCTING POLYMERS
Definition:
The organic polymers having conjugated double and single bond alternatively when doped
with oxidizing or reducing agents conducts electricity are called as conducting polymers.
(Or) Organic polymers with highly delocalized Π electron system having electrical
conductance of the order of a conductor are called as conducting polymers.
Doping delocalizes the electrons responsible for conduction. Ex: Polyaniline
Mechanism of conduction in conducting polymers:
A neutral polymer having conjugation in its carbon back bone has a band model identical to
that of an insulator. The polymer may be transformed into a conductor by doping it with an
electron donor (reductive dopant, Ex: Sodium Napthalide) or an electron acceptor
(oxidative dopant, Ex: I2, FeCl3, AlCl3). This resembles doping of Si with As or Ga. In
doped conjugated polymers, there are two types of Π electron energy bands – the Π
bonding molecular orbital constitute the valence band, while Π* anti bonding molecular
orbital form the conduction band.
The doping creates a positive charge and a free radical on the carbon chain, this leads to
delocalization of electrons throughout the chain, further doping removes free radical and
reduces energy gap, with heavy doping a band electrons will be formed between valence
band and conduction band, which leads to conduction in the polymer chain.
65

Mechanism of conduction in Polyacetylene:
Polyacetylene is a semiconductor on its own, but when doped, its conductivity can be
increased. It can be doped by oxidative dopant such as I2 (p-doping) of by reductive dopant
such as sodium naphthalide (n-doping). Oxidative doping is more common.
When I2 is added as dopant, it attracts an electron from the polyacetylene chain. The
electron is removed from the top of the valence band of polyacetylene creating a vacancy or
hole. The polyacetylene, now positively charged is called a radical cation or polaron. The
lone electron remaining on the chain (free radical), can more easily, hence sets up
delocalization. If polyacetylene is further doped, the positive charge on the chain is doubled
by the removal of free radical, results in the formation of soliton. With heavy doping,
several charged soliton forms a soliton band. This band merges the valence band and
conduction band, thus exhibiting conductivity.
H2
H
N
+
Polyacetylene chain
I2
+ Polaron
N
I2 (Further doping)
+
+ Soliton
I2 (Further doping)
+
Delocalization
Heavy doping with I2
Band of solitons will be formed between valence band and conduction band.
Neutral Chain Polaron soliton Soliton band
H
N
H
N
n
Conduction
band
Valence band
Fig. 8.12 Band diagram of polyacetylene as the amount of oxidative dopant is increased
Structure of Poly aniline:
H
66

FUELS
1. Define fuels and give the classification of fuels with example
Definition
Fuel is a carbonaceous combustible substance, which on combustion liberates a large
amount of energy in the form of heat, that can be effectively used for domestic and
industrial applications.
Classification
On the basis of occurrence, fuels are classified as primary and secondary fuels
Primary fuels occur in nature and are used without processing.
Secondary fuels are obtained by chemical processing of primary fuels.
On the basis of physical state, fuels are classified as solid, liquid and gaseous fuels
E.g. Coal Crude oil Natural gas Charcoal Petrol Coal gas
Wood (Petroleum) Coke Diesel Water gas
Definition
Calorific Value
Primary
Fuels
Secondary
Solid Liquid Gaseous Solid Liquid Gaseous
The quality of a fuel is determined by the amount of energy released per unit mass or
volume referred to as calorific value.
Definition
Calorific value of a fuel is the amount of heat liberated when a unit mass or a unit
volume of the fuel is burnt completely in air or oxygen.
Fuels generally contain hydrogen in addition to carbon. During combustion, the
hydrogen is converted to steam.
In the determination of calorific value of the fuel if the products of combustion are
cooled to ambient temperature (room temperature), the latent heat of steam is also
included. This is referred to as gross calorific value (GCV) or higher calorific value.
67

In practice, the products of combustion are allowed to escape and the amount of
heat realized is lesser than the GCV (since the latent heat of vaporization is not
released). This is net calorific value (NCV) or lower calorific value.
GCV = NCV + latent heat of steam
Gross Calorific value is the amount of heat liberated when a unit mass or a unit
volume of the fuel is burnt completely in air or oxygen and the products of
combustion are cooled to ambient temperature.
Net Calorific value is the amount of heat liberated when a unit mass or a unit
volume of the fuel is burnt completely in air or oxygen and the products of
combustion are allowed to escape.
2. Explain the determination of Calorific Value of a Solid Fuel Using Bomb
Calorimeter with neat diagram.
Sample
Stirrer
Oxygen
Wires for ignition
Thermometer
Construction
The bomb calorimeter (shown in the fig.) consists of an outer cylindrical steel vessel
(bomb) with an airtight screw and an inlet for oxygen.
The bomb has a platinum crucible with a loop of wire. The ends of the wire project
out and can be connected to a source of electric current.
The bomb is immersed in a rectangular vessel (calorimeter) containing water, which
is continuously stirred.
A Beckmann thermometer is introduced into the calorimeter.
Working
A known mass of the fuel is made into a pellet and taken in the crucible.
Oxygen is passed through the bomb.
A known mass of water is taken in the calorimeter and is closed with the lid.
The initial temperature of water is noted.
A
B
Lid
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The ends of the wire are connected to an electric source so as to ignite the fuel.
The heat released is absorbed by water. The temperature of water rises.
The final temperature is noted.
Calculation
Let
m = mass of fuel
W = mass of water
w = water equivalent of calorimeter
t1 = initial temperature of water
t2 = final temperature of water
s = specific heat of water
GCV ( solid fuel) = (W+w) (t2-t1) s
m
If the fuel contains x% hydrogen, NCV of the fuel is calculated as follows
2 atoms of hydrogen produce one molecule of water
2g of hydrogen produce 18 g of water
x g of hydrogen produce 9 g of water
x % hydrogen 9 x g of water = 0.09 x g of water
100
NCV = GCV - latent heat of steam formed
= GCV - 0.09 x latent heat of steam
Latent heat of steam = 2454 kJ kg -1
1 calorie = 4.187 kJ kg -1
The calorific value of a liquid fuel can be determined using bomb calorimeter.
Formulae for Solving Numerical Problems:
GCV (solid fuel) = (W+w) (t2-t1) s
m
NCV (solid fuel) = GCV - latent heat
= G.C.V. - (0.09 % of H) latent heat
GCV( gaseous fuel) = W s (t2- t1)
V
NCV ( gaseous fuel) = GCV – latent heat
= G.C.V. – amount of water collected latent heat
V
= G.C.V. – v latent heat
V
(1 cm 3 of water 1 g of water)
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Numerical Problems
Problem 1: Calculate the gross calorific value and net calorific value of a sample of coal 0.
5g of which when burnt in a bomb calorimeter, raised the temperature of 1000g of water
from 293K to 301.6K. The water equivalent of calorimeter is 350 g. The specific heat of
water is 4.187 kJ kg -1 , latent heat of steam is 2457.2kJkg -1 . The coal sample contains 93%
carbon, 5% hydrogen and 2% ash.
m = mass of the fuel = 0.5 g
W = mass of water taken = 1000 g
w = water equivalent of calorimeter = 350 g
t1 = initial temperature of water = 293 K
t2 = final temperature of water = 296.4 K
s = specific heat of water = 4.187 kJ kg -1 K -1
GCV (solid fuel) = (W+w) (t2-t1) s
m
= (1000 +350) g (296.4 -293)K 4.187 kJ kg -1 K -1
0.5g
= 1350 g 3.4 K 4.187 kJ kg -1 K -1
1. 5g
= 3 8437 kJ kg -1
NCV (solid fuel) = GCV - latent heat
= G.C.V. - (0.09 % of H) latent heat
= 38437 kJ kg -1 - (0.09 5) 1105.7 kJ kg -1
= 38437 kJ kg -1 – 1106 kJ kg -1
= 37331 kJ kg -1
Problem 2: Calculate the gross calorific value and net calorific value of a gaseous fuel,
0.012 g of which when burnt raised the temperature of 3.5kg of water by 8.2K. Specific
heat of water is 4.2 kJ kg -1 K -1. Latent heat of steam is 2.45 kJ kg -1 . The volume of water
collected is 6.5 cm 3 . Latent heat of steam is 2457.2kJ kg -1
V = volume of the gas burnt = 0.015 g
W = mass of water = 3.5 kg
t2- t1 = rise in temperature = 15.6 K
s = specific heat of water = 4.2kJ kg -1 K -1
v = volume of water collected = 6.5 cm 3
GCV( gaseous fuel) = W s (t2- t1)
V
= 3.5 kg 4.2 kJkg -1 K -1 15.6 K
0.012m 3
= 11073 kJm -3
NCV( gaseous fuel) = GCV – latent heat
= G.C.V. - amount of water collected latent heat
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V
= 11073 kJm -3 – 6.5 10 -3 kg 2457.2kJkg -1
0.012
(1 cm 3 of water 1 g of water)
= 11073 kJm -3 – 6.5 10 -3 kg 2457.2kJkg -1
0.015
= 11073 kJm -3 – 1065 kJm -3
= 10008 kJm -3
Cracking of Petroleum
Decomposition of high molecular weight hydrocarbons into low molecular weight
hydrocarbons is known as cracking.
Ex : C10 H22 C8 H18 + C2 H4
Decane Octane Ethylene
There are two types of cracking.
1. Thermal cracking : This process is conducted at higher temperature (600 0 C) and
pressure (5-10 Kg / cm 2 ) , where high molecular weight hydrocarbons are converted
into low molecular weight hydrocarbons.
2. Catalytic cracking : This process is conducted at a temperature 500 0 C and a
pressure 1-5 in presence of catalysts such as alumina , aluminosilicates or zeolites,
where high molecular weight hydrocarbons are converter into low molecular weight
hydrocarbons at a faster rate.
There are two types of catalytic cracking:
1. Fixed bed catalytic cracking : In this process the catalyst particles are spared onto
trays in a reactor and preheated heavy oil is sent into the reactor , the high molecular
weight hydrocarbons are converted into low molecular weight hydrocarbons . Here
catalyst can not be regenerated.
2. Fluidized Bed Catalytic Cracking
Q. 3. explain Fluidized Bed Catalytic Cracking
Heavy oil is cracked using zeolite (Y type) catalyst with a rare earth oxide.
Heavy oil is heated to about 300°C in a preheater and passed through a riser column
(shown in fig.) into a reactor.
The reactor contains finely powdered catalyst maintained at about 500°C.
The heavy oil undergoes cracking.
The cracked product is fractionated to give petrol.
Exhaust gases Cracked vapours into
fractionating column
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600°C 500°C
Regenerator Reactor
P=1-5 kg cm -2
Crude
Oil Spent catalyst
Oil + Catalyst Spent catalyst
+ hot air Hot air
Preheater Pump
Regeneration of Catalyst
After some time, the catalyst gets deactivated in the reactor due to the deposition of
carbon and oil on its surface.
Steam is passed through the riser column into the reactor.
The deactivated catalyst is forced into a regenerator along with hot air.
The regenerator temperature will be maintained at about 600°C.
Air oxidizes C to CO2 and steam removes the oil.
The regenerated catalyst is sent again to reactor with fresh oil.
Reformation of Petrol
Reformation is a process of bringing about structural changes in the hydrocarbons
with the primary objective of improving the octane number of petrol.
The changes in structure could be isomerization, cyclization, aromatization or
polymerization.
There are two types of reformation
1. Thermal reformation : Thermal reformation is carried out by heating the gasoline to
500 o C – 600 o C at a presence of 85 atmospheres in a reactor. The conditions are
controlled by quenching the hot vapours with cold spray of oil to avoid the formation of
gases.
2. Catalytic reformation: catalytic reformation is carried out by passing the petrol
through Pt (0.75%) supported on alumina at about 500oC and 50kg cm-2 pressure.
Q. 4 Write any three petroleum reformation reactions.
Isomerization straight chain hydrocarbons are converted to branched
hydrocarbons
CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH3 CH3 - CH - CH2 - CH2 - CH2 - CH3
n- heptane
CH3
methyl hexane
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Cyclization straight chain hydrocarbons are converted to cyclic compounds
CH3 - CH2 - CH2 - CH2 - CH2 - CH2 - CH3 - CH3
n- heptane methyl cyclohexane
Aromatization cyclic compounds are dehydrogenated.
- CH3 - CH3
methyl cyclohexane toluene
Q. Explain the mechanism of knocking.
Knocking in IC Engines
The power output and efficiency of an IC engine depends on the Compression ratio
which is the ratio of the volume of the cylinder at the end of the suction stroke to the
volume of the cylinder at the end of the compression stroke.
Compression ratio =
Volume of (Fuel + air )in the cylinder at end of suction stroke
Volume of (Fuel + air ) in cylinder at end of compression stroke
Under ideal conditions, in an IC engine the petrol-air mixture drawn into the
cylinder of the engine undergoes compression and then ignited.
The hydrocarbons in petrol undergo complete combustion and the flame propagates
smoothly.
Sometimes, due to deposits of carbon on the walls of the cylinder the hydrocarbons
in petrol form peroxy compounds.
The accumulated peroxides decompose suddenly and burst into flames producing
shock waves.
The shock wave hits the walls of the engine and the piston with a rattling sound.
This is knocking.
The reactions that take place in an IC engine are given below (taking ethane as an
example for the hydrocarbon present in petrol):
Under ideal conditions
C2H6 + 7/2 O2 2 CO2 + 3H2O (Normal combustion)
Under knocking conditions (Explosive combustion)
C2H6 + O2 CH3 –O-O- CH3
(Dimethyl peroxide)
CH3 –O-O- CH3 CH3CHO + H2O
CH3CHO + 3/2 O2 HCHO + CO2 + H2O
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HCHO + O2 H2O + CO2
Note that the overall reaction is the same under both the conditions. One molecule
of ethane reacts with 7/2 molecules of oxygen forming carbon dioxide and water
with the release of energy.
Under ideal conditions, the energy is released at a uniform rate.
Under knocking conditions, the energy is released slowly at first followed by a lag
(formation of peroxides) and finally the energy is released at a very fast rate
(decomposition of peroxides).
Ill effects of knocking
4. Decreases life of engine
5. Causes Piston wrap
6. Consumption of fuel is more
Q 6. Define octane and cetane number
Octane Number
The resistance to knocking offered by petrol is expressed in terms of an arbitrary
scale called octane number
Isooctane has least tendency to knock and n-heptane has more tendency to knock.
The octane value of isooctane is arbitrarily taken as 100 and that of n – heptane as
zero.
Octane number is the percentage by volume of isooctane present in a standard
mixture of isooctane and n – heptane, which has the same knocking characteristic as
the petrol under test.
Different standard mixtures ( 90:10; 80:20, 75:25 etc) of isooctane and n–heptane
are prepared and the compression ratio of each of these is determined under
standard conditions.
The compression ratio of the fuel under test is determined under the same
conditions.
Suppose the compression ratio of the fuel is same as that of 80 :20 mixture, the
octane number of the fuel is 80.
Cetane Number:
The resistance to knocking offered by diesels is expressed in terms of an arbitrary
scale called cetane number
Cetane (hexadecane) has least tendency to knock and - methyl naphthalene has
more tendency to knock. The cetane value of Cetane is arbitrarily taken as 100 and
that of - methyl naphthalene as zero.
It is the percentage by volume of cetane present in a mixture of cetane and -
methyl naphthalene which has the same knocking characteristic as the diesel under
test.
Q. 7. Write a note on antiknocking agents
Prevention of Knocking
Addition of tetraethyl lead (TEL) to Petrol:
Tetraethyl Lead decomposes the peroxides formed and prevents knocking. In the
process, lead gets deposited on the inner walls of the engines and at spark plugs. Hence
74

dichloroethane and dibromoethane are added along with tetraethyl lead. These convert
the lead into lead halides, which are volatile and escape with exhaust gases.
The release of lead compounds pollutes the atmosphere.
Catalytic converters (rhodium catalyst) are used in IC engines to convert CO in the
exhaust to CO2. Tetraethyl Lead used as anti knocking agent poisons the catalyst
and hence leaded petrol is not advisable in such IC engines.
Nowadays usage of leaded petrol is phased out completely due to pollution caused
by the lead present in it.
Addition of MTBE:
Methyl tertiary butyl ether (MTBE) is added to petrol (unleaded petrol) to boost its
octane number. The oxygen present in ether group of MTBE brings about complete
combustion of petrol preventing peroxide formation and hence knocking is prevented.
MTBE can be used as antiknocking agent in IC engines with catalytic converter.
Q.8. Write a note on power alcohol
Power Alcohol:
This is alcohol-blended petrol.
Gasohol is a blend of 10 – 85% of absolute ethanol and 90 – 15% of petrol by
volume and is used as a fuel in the United States. Absolute alcohol is used in the
preparation of Power alcohol to prevent phase separation.
Alcohol contains higher percentage of oxygen than MTBE and hence brings about
complete oxidation of petrol more effectively.
Therefore power alcohol has better antiknocking characteristics than unleaded
petrol.
Advantages of power alcohol
power output is high
does not release CO, causes less pollution.
alcohol is obtained from molasses, a agricultural product and hence renewable.
biodegradable.
Q.9. Write a note on Bergius process and Fischer Tropsch process.
Bergius process
In the Bergius process, lignite is hydrogenated to give liquid hydrocarbons for use as
synthetic petrol. Hydrogen is obtained by the reaction of water gas on coal or by partial
oxidation of natural gas. Lignite is ground to a fine dust. The dust is mixed with heavy oil
and made into a paste. Iron oxide or nickel catalyst is added. The mixture is pumped into a
reactor maintained at about 500 – 550 0 C and a pressure of about 250 atmospheres .
Hydrogen gas is passed through the reactor. Lignite gets hydrogenated and a mixture of
hydrocarbons is obtained. This mixture is passed through a fractionating column to get
petrol.
Lignite
dust +
Heavy
Oil
Paste
Crude Oil Vapours
500 -550°C
250 atm
Cooler
Gasoline
Catalyst Water
Out
Kerosene
75

FISCHER TROPSCH PROCESS
Water in
Heavy
oil
Compressor Reactor H2 gas Fractionating Cracking
Column
Gasoline
A schematic diagram of Bergius process
Crude Oil Vapours Cooler
CO+ H2 Water Gasoline
Out
Catalyst
H2 Kerosene
200-300°C
Water in
Heavy Oil
Pre-Heater
Compressor Catalytic Fractionating Cracking
Converter
reactor
Column
Gasoline
A schematic diagram of Fischer – Tropsch process
In the Fischer – Tropsch process, water gas ( a mixture of CO and H2) is mixed with half of
its volume of hydrogen and passed over a catalyst such as cobalt mixed with oxides of
magnesium and thorium at about 230 – 300 0 C under a pressure of about 200 atmospheres.
The product consists of a mixture of hydrocarbons and is fractionated to yield petrol and
other liquid fuels.
(2n+1) H2 + 2n CO CnH2n+2 + nH2O
2n H2 + n CO CnH2n + nH2O
2n H2 + n CO CnH2n+1 OH + (n-1)H2O
------------------------------------
76