Engineering Chemistry S Datta

250 ENGINEERING CHEMISTRY
of blotting paper. To prevent drying of the paste, the top of the cell is sealed with pitch. A small
hole is at the seal. The e.m.f. of the cell is 1.45 V.
This type of cell is widely used in torches, transistors, signalling equipments, telephone
etc.
Alkaline Battery
It is a modification of dry cell where NH 4
Cl has been replaced by KOH and acts as an
electrolyte. In this type of battery, zinc is used in powder form and makes a gel with KOH. And
the graphite-rod is kept surrounded by a paste of MnO 2
. The cell reaction is
Anode: Zn(s) + 2OH – (aq) ⎯⎯→ Zn(OH) 2
(s) + 2e –
Cathode: 2MnO 2
(s) + H 2
O(l) + 2e – ⎯⎯→ Mn 2
O 3
(s) + 2OH – (aq)
Net reaction:
Zn(s) + 2MnO 2
(s) + H 2
O(l) ⎯⎯→ Zn(OH) 2
(s) + Mn 2
O 3
(s)
Advantage. The life of alkaline battery is longer than the dry cell because zinc does not
corrode.
Use. It is mainly used in camera, calculators, watches etc.
Nickel-Cadmium Battery
These battery is constructed of cadmium anode and a paste of NiO(OH)(s) cathode. The
cell reaction is
Anode: Cd(s) + 2OH – (aq) ⎯⎯→ Cd(OH) 2
(s) + 2e –
Cathode: 2NiO . OH(s) + 2H 2
O(l) + 2e – ⎯⎯→ 2Ni(OH) 2
(s) + 2OH – (aq)
Net reaction: 2NiO.OH(s) + Cd(s) + 2H 2
O(l) ⎯⎯→ Cd(OH) 2
(s) + 2Ni(OH) 2
(s)
Here the reaction products adhere to the electrode surfaces and the reaction can be
easily reversed. This type of battery is portable and reachargeable. The main advantage of this
battery is that, it undergoes no deterioration as no gases are produced during charging and
discharging.
Use: It is used in calculators, electronic flash units etc.
Mercury Battery
This type of battery is constructed of zinc-amalgam (a solution of Zn in mercury) as
anode and a paste of KOH, Zn(OH) 2
and HgO as cathode. The anode and the cathode are
divided by a paper. The paper permits the migration of ions. The cell reaction is:
Anode: Zn-amalgam(s) + 2OH – (aq) ⎯⎯→ ZnO(s) + H 2
O(l) + 2e –
Cathode: HgO(s) + H 2
O(l) + 2e – ⎯⎯→ Hg(l) + 2OH – (aq)
Net reaction: Zn-amalgam(s) + HgO(s) ⎯⎯→ ZnO(s) + Hg(l)
The entire cell is covered in a stainless steel case. The size is small. It is expensive.
Use: It finds its use in pacemakers, hearing aids, digital watches etc.
Fuel Cells
In these cells combustion of the fuel is employed as chemical reaction to generate
electricity.
Fuel + O 2
⎯⎯→ Combustion products + electricity