Which of the following reactions are disproportionation reactions?
A. \(2Cu^+ \longrightarrow Cu^{2+} + Cu\)
B. \(3MnO_4^{2-} + 4H^+ \longrightarrow 2MnO_4^- + MnO_2 + 2H_2O\)
C. \(2KMnO_4 \longrightarrow K_2MnO_4 + MnO_2 + O_2\)
D. \(2MnO_4^- + 3Mn^{2+} + 2H_2O \longrightarrow 5MnO_2 + 4H^+\)
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → A, B only
In a disproportionation reaction an element in one oxidation state is oxidised and reduced at the same time, giving one product with a higher and one with a lower oxidation state.
(A) 2Cu+ → Cu2+ + Cu — True. Cu goes from +1 to +2 (oxidation) and from +1 to 0 (reduction); \( \ce{Cu^{+}} \) disproportionates in aqueous solution.
(B) 3MnO42− + 4H+ → 2MnO4− + MnO2 + 2H2O — True. In \( \ce{MnO4^{2-}} \), x + 4(−2) = −2 gives Mn +6; it goes to +7 in \( \ce{MnO4^{-}} \) (x − 8 = −1) and to +4 in \( \ce{MnO2} \). The green manganate ion disproportionates in neutral or acidic solution.
(C) 2KMnO4 → K2MnO4 + MnO2 + O2 — False. Mn goes from +7 to +6 (\( \ce{K2MnO4} \): 2 + x − 8 = 0) and to +4 (\( \ce{MnO2} \)): both are reductions. The element oxidised is oxygen (−2 → 0 in \( \ce{O2} \)), so this is the thermal decomposition of \( \ce{KMnO4} \), not a disproportionation.
(D) 2MnO4− + 3Mn2+ + 2H2O → 5MnO2 + 4H+ — False. Mn(+7) and Mn(+2) both end up as Mn(+4); two oxidation states combine into one, the reverse of disproportionation (comproportionation).
NCERT (2026-27 edition):
Potassium Permanganate – “Potassium permanganate is prepared by fusion of MnO2 with an alkali metal hydroxide and an oxidising agent like KNO3. This produces the dark green K2MnO4 which disproportionates in a neutral or acidic solution to give permanganate.”
\( \ce{3MnO4^2- + 4H+ -> 2MnO4^- + MnO2 + 2H2O} \)