An aqueous solution containing Na+, Sn2+, Cl− and SO42− ions, all at unit concentration, is electrolysed between a silver anode and a platinum cathode. What changes occur at the electrodes when current is passed through the cell?
Given: E°(Ag+|Ag) = 0.799 V; E°(Sn2+|Sn) = −0.14 V; E°(Sn4+|Sn2+) = 0.13 V; E°(Cl2|Cl−) = 1.36 V; E°(S2O82−|SO42−) = 2.00 V; E°(H2O|H2, OH−) = −0.83 V
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → Sn2+ is reduced and Sn2+ is oxidised.
When several species compete at an electrode, the cathode takes the reduction with the higher E°, and the anode takes the oxidation of the couple with the lower E°. The silver anode is a reactive electrode, so its own oxidation is one of the candidates.
Cathode (reductions): E°(Sn2+|Sn) = −0.14 V is higher than E°(H2O|H2, OH−) = −0.83 V (and far higher than Na+|Na), so Sn2+ is reduced:
\( \displaystyle \ce{Sn^2+ + 2e^- -> Sn} \)
Anode (oxidations): the couples are Sn4+|Sn2+ (0.13 V), Ag+|Ag (0.799 V), Cl2|Cl− (1.36 V) and S2O82−|SO42− (2.00 V). The lowest is 0.13 V, so Sn2+ is oxidised before the silver anode dissolves:
\( \displaystyle \ce{Sn^2+ -> Sn^4+ + 2e^-} \)
NCERT (2026-27 edition):
2.5.1 Products of Electrolysis – “… The products of electrolysis depend on the different oxidising and reducing species present in the electrolytic cell and their standard electrode potentials. … At the cathode there is competition between the following reduction reactions: … The reaction with higher value of Eo is preferred … At the anode the following oxidation reactions are possible: … The reaction at anode with lower value of Eo is preferred …”