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Disproportionation of Manganate (MnO4^2-) Ion

Chemistry · Redox Equilibrium · NEET

The manganate ion MnO4^2- has manganese in the +6 oxidation state. In acidic solution it is not stable: the same +6 manganese is at the same time oxidised to +7 (permanganate MnO4^-, purple) and reduced to +4 (MnO2, brown solid). This one-element-two-fates reaction is called disproportionation. Memory hook: "6 splits into 7 and 4" — the middle oxidation state (+6) is squeezed out into the higher and lower states around it.
Disproportionation of Manganate (Mn +6)MnO4^2- (green)Mn = +6MnO4^- (purple)Mn = +7 (oxidised)MnO2 (brown)Mn = +4 (reduced)+1 up-2 down3 MnO4^2- + 4 H^+ -> 2 MnO4^- + MnO2 + 2 H2O
The +6 manganese in green manganate splits both ways at once: part rises to +7 (purple permanganate) and part falls to +4 (brown MnO2). Acid (H^+) drives this because manganate is stable only in strong base.

Your doubts, answered

Is manganate the same as permanganate? They look almost identical.

No, and NEET uses this to trick you. Manganate is MnO4^2- (charge 2 minus), Mn is +6, and it is green. Permanganate is MnO4^- (charge 1 minus), Mn is +7, and it is purple. One extra electron and one extra negative charge separate them. In the disproportionation, green manganate turns into purple permanganate plus brown MnO2.

Why does MnO4^2- (+6) disproportionate but MnO4^- (+7) does not?

Disproportionation needs an element in an intermediate oxidation state, so it has room to go both up and down. Mn in MnO4^2- is +6, which sits between +4 and +7, so it can rise to +7 and fall to +4. Mn in MnO4^- is +7, the highest possible oxidation state for manganese (group number). It cannot be oxidised any further, so permanganate cannot disproportionate.

What is the balanced equation and how do I check it?

The reaction is 3 MnO4^2- + 4 H^+ -> 2 MnO4^- + MnO2 + 2 H2O. Check electrons: 2 Mn atoms go from +6 to +7 (lose 1 electron each = 2 electrons lost), and 1 Mn atom goes from +6 to +4 (gains 2 electrons). Electrons lost (2) equal electrons gained (2), so it balances. Three +6 atoms produce two +7 atoms and one +4 atom.

What colour change tells me manganate has disproportionated?

You start with a green solution (manganate, MnO4^2-). On adding acid it turns purple (permanganate, MnO4^-) and a brown or black solid (MnO2) settles out. Green to purple plus a brown precipitate is the visible signature of this disproportionation.

Why does acid (H^+) drive this reaction?

Manganate is only stable in strongly alkaline (basic) solution. Adding H^+ removes the alkali and shifts the balance, so the intermediate +6 state collapses into the more stable +7 and +4 states. That is why the equation has H^+ on the left. In strong base, MnO4^2- stays green and stable.

⚠️ The NEET trap
✗ Students see 2 KMnO4 -> K2MnO4 + MnO2 + O2 and call it disproportionation of Mn.
✓ That thermal decomposition is NOT a disproportionation, because oxygen also changes (from -2 to 0 in O2). In a true disproportionation only ONE element in ONE oxidation state must split into higher and lower states. The real manganate disproportionation is 3 MnO4^2- + 4 H^+ -> 2 MnO4^- + MnO2 + 2 H2O, where only Mn changes.
🧠 Disproportionation = one element, one starting state, two ending states. If a second element also changes oxidation number, it is not disproportionation.

Real NEET questions

NEET 2019

Which of the following reactions are disproportionation reactions? (a) 2Cu+ -> Cu2+ + Cu (b) 3MnO4^2- + 4H+ -> 2MnO4- + MnO2 + 2H2O (c) 2KMnO4 -> K2MnO4 + MnO2 + O2 (d) 2MnO4- + 3Mn2+ + 2H2O -> 5MnO2 + 4H+

A · (a) and (b) only ✓
B · (a), (b) and (c)
C · (a), (c) and (d)
D · (a) and (d) only
Solution: In disproportionation the same element in one oxidation state is both oxidised and reduced. (a) Cu+ (+1) -> Cu2+ (+2) and Cu (0): yes. (b) Mn in MnO4^2- (+6) -> MnO4- (+7) and MnO2 (+4): yes, this is manganate disproportionation. (c) is thermal decomposition (oxygen also changes -2 to 0), not disproportionation. (d) is comproportionation (two states +7 and +2 converge to one +4), the reverse. So only (a) and (b), option A.

Solved Redox Equilibrium NEET PYQs

Try the real previous-year questions from this chapter — each with the answer and a full solution.

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Frequently asked

What is the oxidation state of Mn in the manganate ion?

In MnO4^2-, each oxygen is -2 (total -8) and the ion charge is -2, so Mn = -2 - (-8) = +6. Manganese is in the +6 state, which is the intermediate state that allows disproportionation.

What are the two products of manganate disproportionation?

Purple permanganate ion MnO4^- (Mn in +7, oxidised) and brown manganese dioxide MnO2 (Mn in +4, reduced). The balanced equation is 3 MnO4^2- + 4 H^+ -> 2 MnO4^- + MnO2 + 2 H2O.

In which medium is manganate stable?

Manganate MnO4^2- is stable only in strongly alkaline (basic) solution. In neutral or acidic solution it disproportionates into permanganate and manganese dioxide.

Is manganate disproportionation the same as comproportionation?

No, it is the opposite. Disproportionation: one intermediate state splits into a higher and a lower state (+6 -> +7 and +4). Comproportionation: a higher and a lower state combine into one intermediate state (for example +7 and +2 give +4).

Why can permanganate not disproportionate?

Mn in permanganate MnO4^- is +7, the maximum oxidation state of manganese. It cannot be oxidised higher, and disproportionation requires the element to go both up and down, so permanganate cannot disproportionate.

Related concepts:
Why Cu+ Disproportionates in WaterWhat Is a Comproportionation ReactionPredicting Disproportionation from a Latimer DiagramKMnO4 Behaviour in Acidic, Neutral and Basic MediumWhy H2O2 Acts as Both Oxidiser and Reducer
Practise Chemistry for NEET:
Chemistry MCQs (free, chapter-wise)Redox Equilibrium PYQs