Acidified potassium permanganate oxidises oxalate ions at 333 K. Which of the following are formed from the permanganate ion and from the oxalate ion, respectively?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \( \ce{Mn^2+} \) and \( \ce{CO2} \)
Permanganate is a strong oxidising agent, and what it is reduced to depends on the hydrogen ion concentration. In acid solution manganese goes from +7 all the way to +2:
\( \displaystyle \ce{MnO4^- + 8H+ + 5e^- -> Mn^2+ + 4H2O} \)
The oxalate ion is oxidised: carbon goes from +3 to +4, and each ion gives two molecules of carbon dioxide:
\( \displaystyle \ce{C2O4^2- -> 2CO2 + 2e^-} \)
Adding the two half-reactions (5 oxalate ions for every 2 permanganate ions) gives the full reaction:
\( \displaystyle \ce{5C2O4^2- + 2MnO4^- + 16H+} \xrightarrow{\text{333 K}} \ce{2Mn^2+ + 8H2O + 10CO2} \)
Why the other options are incorrect
Option (1) \( \ce{MnO2} \) and \( \ce{CO2} \) — Incorrect. Permanganate gives MnO2 in neutral or faintly alkaline solution, as in the oxidation of iodide to iodate, not in acid solution.
Option (3) \( \ce{Mn^2+} \) and \( \ce{CO} \) — Incorrect. Carbon in CO is +2, lower than +3 in oxalate. Forming CO would be a reduction of oxalate, but here oxalate is the substance being oxidised.
Option (4) \( \ce{MnO4^2-} \) and \( \ce{CO2} \) — Incorrect. Manganate (+6) comes from a one-electron reduction of permanganate; it is not the product in acid, where manganate itself disproportionates to permanganate.
NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “1. In acid solutions:” … “(c) Oxalate ion or oxalic acid is oxidised at 333 K: 5C2O42– + 2MnO4– + 16H+ → 2Mn2+ + 8H2O + 10CO2”