Engineering Chemistry S Datta

ELECTROCHEMICAL CELLS 239
Highlights:
Uses of standard potential (reduction) values:
1. Prediction of an oxidation-reduction reaction.
2. Prediction regarding liberation of hydrogen by a metal from a hydracid.
3. Prediction of feasibility of a chemical reaction.
Example. Predict whether the reaction will occur spontaneously or not.
Zn +2 + Cu Zn + Cu +2
∈° + 2 = – 0.762 ∈° + 2
Zn /Zn
Cu /Cu
= 0.344
The two half cell reactions are:
Zn +2 + 2e Zn [∈° = – 0.762]
Cu – 2e Cu +2 [∈° = 0.344]
Adding Zn +2 + Cu Zn + Cu +2 [E° Cu
= – 1.106]
Since, E° cell
= – ve.
So, the reaction will not occur spontaneously.
Example. Find whether Cu + disproportionate in aqueous solution into Cu +2 and Cu°
i.e., cell reaction is
2Cu + Cu +2 + Cu°, Given, ∈° + 2 + = + 0.15 V
∈° +
Cu /Cu °
= 0.52 V
The two half cell reactions are:
Cu + + e Cu° [∈° = 0.52 V]
Cu + – e Cu +2 [∈° = 0.15 V]
Adding Cu + + Cu + Cu +2 + Cu° [E° cell
= 0.37 V]
Since E° cell
is positive, Cu + will disproportionate spontaneously in aqueous solution into
Cu +2 and Cu°.
Example. Ag will not displace hydrogen from HCl, given ∈° Ag + /Ag = 0.798.
Sol. Cell reaction: 2Ag + 2H + ⎯⎯→ 2Ag + + H 2
The two half cell reactions are:
2Ag – 2e 2 Ag + [∈° = – 0.798]
2H + + 2e H 2
[∈°H + /H = 0.00 V]
Adding 2Ag + 2H + 2Ag + + H 2
[E° cell
= – 0.798]
Since, the E° cell
value is negative, Ag cannot displace H 2
from HCl.
Concentration Cell
When a metal is dipped into two solutions of one of its salts of different concentrations,
a very interesting situation arises:
Cu
/Cu