NCERT Solutions
Class 12 Chemistry
Electrochemistry

Q.4
Calculate the standard cell potentials of galvanic cell in which the following reactions take place:
- 2Cr(s) + 3Cd2+ (aq) → 2Cr3+ (aq) + 3Cd
- Fe2+ (aq) + Ag+ (aq) → Fe3+ (aq) + Ag(s)
Calculate the ΔrG (-) and equilibrium constant of the reactions
(i) E0 Cr3+ / Cr = - 0.74 V E0 Cd2+ / Cd = - 0.40 V
The galvanic cell of the given reaction is depicted as:
Cr(s) |Cr3+ (aq) ||Cd2+ (aq) |Cd(s)
Now, the standard cell potential is
Ecell (-) = (E R (-) – EL (-))
= (0.40 – (-0.74))
=+0.34 V
ΔrG (-) = -nFE cell (-)
In the given equation, n =6
F = 96487 C mol−1
Ecell (-) =+0.34 V
Then ΔrG (-) = (−6 × 96487 C mol−1 × 0.34 V)
= −196833.48 CV mol−1
= −196833.48 J mol−1
= −196.83 kJ mol−1
Again,
ΔrG (-) = -RT ln K
=> ΔrG (-) =-2.303 Rt ln K
=> log K = - (ΔrG (-) /2.303 RT)
=-(196.83 x103)/ (2.303 x 8.314 x298)
= 34.496
K = antilog (34.496) = 3.13 × 1034
(ii) E0 Fe3+ /Fe2+ = 0.77 V E0 Ag+ / Ag = 0.80 V
The galvanic cell of the given reaction is depicted as:
Fe2+ (aq) |Fe3+ (aq) ||Ag+ (aq) |Ag(s)
Now, the standard cell potential is
Ecell (-) = (E R (-) – EL (-))
= (0.80-0.77)
= 0.03 V Here, n = 1.
Then, ΔrG (-) = -nFE cell (-)
= (−1 × 96487 C mol−1 × 0.03 V)
= -2894.61 Jmol-1
= -2.89 KJ mol-1
Again, ΔrG (-) =-2.303 Rt ln K
log K = - (ΔrG (-) /2.303 RT)
=-(2.894.61)/ (2.303 x 8.314 x298)
= 0.5073
Therefore, K = antilog (0.5073)
= 3.2 (approximately)