During the electrolysis of concentrated sulphuric acid, which of the following reactions is preferred at the anode?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( \ce{2SO4^2-(aq) -> S2O8^2-(aq) + 2e^-} \)
Oxidation takes place at the anode. In sulphuric acid two oxidations compete there: water can give oxygen, or sulphate ions can give peroxodisulphate ions:
\( \displaystyle \ce{2H2O(l) -> O2(g) + 4H+(aq) + 4e^-} \qquad E^\circ_{(cell)} = +1.23\ \mathrm{V} \qquad (2.38) \)
\( \displaystyle \ce{2SO4^2-(aq) -> S2O8^2-(aq) + 2e^-} \qquad E^\circ_{(cell)} = 1.96\ \mathrm{V} \qquad (2.39) \)
Which one wins depends on the concentration of the acid, since the electrode potentials change with concentration (Nernst equation). In dilute acid, oxygen is given off (2.38); at higher concentrations of H2SO4, reaction (2.39) is preferred.
Why the other options are incorrect
Option (1) \( \ce{2H2O(l) -> O2(g) + 4H+(aq) + 4e^-} \) — Incorrect. This is the anode reaction preferred for dilute sulphuric acid, not for the concentrated acid.
Option (3) \( \ce{2H+(aq) + 2e^- -> H2(g)} \) — Incorrect. This is a reduction (electrons are taken up), so it takes place at the cathode, not at the anode.
Option (4) \( \ce{2H2O(l) -> H2O2(aq) + 2H+(aq) + 2e^-} \) — Incorrect.
This is an oxidation of water, but it gives hydrogen peroxide.
It is not one of the two processes possible at the anode in the electrolysis of sulphuric acid, (2.38) and (2.39); in the concentrated acid the sulphate ions are oxidised to peroxodisulphate.
NCERT (2026-27 edition):
2.5.1 Products of Electrolysis – “The standard electrode potentials are replaced by electrode potentials given by Nernst equation … to take into account the concentration effects. During the electrolysis of sulphuric acid, the following processes are possible at the anode:”
\( \displaystyle \ce{2H2O(l) -> O2(g) + 4H+(aq) + 4e^-} \qquad E^\circ_{(cell)} = +1.23\ \mathrm{V} \qquad (2.38) \)
\( \displaystyle \ce{2SO4^2-(aq) -> S2O8^2-(aq) + 2e^-} \qquad E^\circ_{(cell)} = 1.96\ \mathrm{V} \qquad (2.39) \)
“For dilute sulphuric acid, reaction (2.38) is preferred but at higher concentrations of H2SO4, reaction (2.39) is preferred.”