The following reaction can be best described as:
3MnO42− + 4H+ → 2MnO4− + MnO2 + 2H2O
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Disproportionation
A disproportionation reaction is a redox reaction in which the same element, in one oxidation state, is oxidised to a higher state and reduced to a lower state at the same time.
The oxidation state x of manganese in each species follows from the charge balance (O is −2):
\( \ce{MnO4^2-} \): x + 4(−2) = −2, so Mn is +6.
\( \ce{MnO4^-} \): x + 4(−2) = −1, so Mn is +7.
\( \ce{MnO2} \): x + 2(−2) = 0, so Mn is +4.
\( \displaystyle \ce{3MnO4^2- + 4H^+ -> 2MnO4^- + MnO2 + 2H2O} \)
Of the three Mn(+6) atoms, two go up to +7 (oxidation) and one goes down to +4 (reduction). The electrons balance: the two oxidised Mn atoms lose 1 electron each (2 e− in all), and the one reduced Mn atom gains 2 electrons. So the green manganate ion disproportionates into purple permanganate and MnO2. This is why manganate is stable only in alkaline solution.
Why the other options are incorrect
Option (1) Coupling — Incorrect. A coupling reaction joins two molecules or groups into one, as when benzenediazonium chloride couples with phenol to give an azo dye. Here no two species join; one ion splits into two products with different oxidation states of Mn.
Option (3) Displacement — Incorrect. In a displacement reaction one element replaces another in a compound, as in \( \ce{Zn + CuSO4 -> ZnSO4 + Cu} \). Here only manganese changes its oxidation state, and no other element takes its place.
Option (4) Oxidative fusion — Incorrect. Oxidative fusion is the step that makes manganate: MnO2 is fused with KOH in the presence of air or KNO3. It is the step before the given reaction:
\( \displaystyle \ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O} \)
NCERT (2026-27 edition):
Potassium permanganate KMnO4 – “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.
\( \displaystyle \ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O} \)
\( \displaystyle \ce{3MnO4^2- + 4H^+ -> 2MnO4^- + MnO2 + 2H2O} \)
Commercially it is prepared by the alkaline oxidative fusion of MnO2 followed by the electrolytic oxidation of manganate (VI).”