Engineering Chemistry S Datta

248 ENGINEERING CHEMISTRY
Current density
Decomposition
voltage
Voltage
Fig. 10.13 Decomposition Potential.
At this stage, if the applied e.m.f. is gradually increased, electrolysis continues
uninterrupted. So, a minimum voltage must always be applied to start the electrolysis process.
Highlights:
• The decomposition potential is different for different electrolytes.
• The decomposition potential of CuSO 4
is 1.5 V with Cu-electrodes and that of
ZnSO 4
is 2.55 V with Zn-electrodes. When an e.m.f. of less than 2-5 V is applied
between Cu-electrodes only Cu will get deposited at the cathode while Zn will
remain in solution if a mixture of CuSO 4
and ZnSO 4
is used as electrolyte.
Overvoltage
Theoretical voltage required for the decomposition of an acid solution should be equal to
the e.m.f. of the reversible cell constructed with H 2
(g) at 1 atm. But it is seen that the value of
applied e.m.f. is always higher than this theoretical value. Electrolysis of dil. H 2
SO 4
with Ptelectrodes
requires 1.7 V in place of 1.23 V (theoretical value). This difference between the
theoretical voltage and actual applied voltage necessary for electrolysis is known as
overvoltage.
Battery
Cells are devices where electric current is generated at the cost of some physicochemical
processes going inside the cell. In order to apply exact thermodynamic principles to cell reaction
it is customary to distinguish between reversible and irresversible cells. Because the thermodynamic
principle is only applicable to reversible cells. It is well known that any chemical
reaction conducted reversibly can yield some external work and the useful work available
from the process, provided the process has been conducted reversibly at constant temperature
and under a given pressure is equal to (the decreasing Gibbs free energy – ∆G). Since electrical
work is equal to nFE, where F = faraday and E = e.m.f. of the cell
so, we have
– ∆G = nFE or ∆G = – nFE
A battery is an electrochemical cell which is used as the source of direct current of a
constant voltage. Battery works on the above thermodynamic principle.
Battery
Primary
[The cell reaction
is not reversible]
Secondary
[The cell reaction
is not reversible]
Flow
[The reactants, products and
electrolytes pass through the battery
and during the passage electric current
is generated at the expense of chemical
reactions as in electrochemical cells]