JEE Mains Chemistry · The d- and f-Block Elements

Potassium Permanganate and Manganese Compounds

MnO₂ fused with KOH and an oxidant gives green manganate, which disproportionates or is oxidised to purple permanganate; permanganate is then a five-electron oxidant in acid, ending at Mn²⁺, and a three-electron oxidant in neutral or faintly alkaline solution, ending at MnO₂.

All The d- and f-Block Elements notes JEE Mains Chemistry strategy

Why this matters

Twenty-eight PYQs, eleven of them numeric, and three from 2026. Fourteen follow MnO₂ to K₂MnO₄ and KMnO₄: the disproportionation of manganate, the effect of heat, and the two tetrahedral ions; fourteen use permanganate as an oxidant, where the product depends on whether the solution is acidic or neutral. Most of the numeric answers are an oxidation-state change or a magnetic moment of the manganese product.

Concept 1 of 2: Making KMnO₄: manganate, permanganate and disproportionation

Manganese climbs in two steps. Fusing

\mathrm{MnO_2}

(+4) with alkali and an oxidant gives green manganate, Mn(VI). Manganate is stable only in alkali. In neutral or acid solution it disproportionates: two Mn(VI) go up to Mn(VII) and one comes down to Mn(IV). Industry avoids wasting a third of the manganese by oxidising manganate electrolytically instead.

Definition

Fusion: $\mathrm{2MnO_2 + 4KOH + O_2 \rightarrow 2K_2MnO_4 + 2H_2O}$ (air or $\mathrm{KNO_3}$ as the oxidant).
Disproportionation (neutral or acid): $\mathrm{3MnO_4^{2-} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O}$ . Products +7 and +4, a difference of 3.
Commercial route: manganate is oxidised electrolytically in alkaline solution to permanganate.
Laboratory route: $\mathrm{Mn^{2+}}$ is oxidised by peroxodisulphate: $\mathrm{2Mn^{2+} + 5S_2O_8^{2-} + 8H_2O \rightarrow 2MnO_4^{-} + 10SO_4^{2-} + 16H^{+}}$ .
Heating: $\mathrm{2KMnO_4 \xrightarrow{513\ K} K_2MnO_4 + MnO_2 + O_2}$ .
The two ions: both tetrahedral, with π bonds between oxygen p and manganese d orbitals. Manganate is green, Mn +6, d¹, paramagnetic (1.73 BM). Permanganate is purple, Mn +7, d⁰, diamagnetic; its colour is charge transfer.

Disproportionation of manganate

\mathrm{3\overset{+6}{Mn}O_4^{2-} + 4H^{+} \rightarrow 2\overset{+7}{Mn}O_4^{-} + \overset{+4}{Mn}O_2 + 2H_2O}

Worked example

0.9 mol of manganate is acidified. How many moles of permanganate and of

\mathrm{MnO_2}

form, and what are the spin-only moments of manganese in each product?

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2025 · 24 Jan 2025 · Q37Moderate

Example 1 · The d- and f-Block Elements · Potassium Permanganate and Manganese Compounds

Preparation of potassium permanganate from

MnO_{2}

involves two step process in which the

1^{\text{st~}}

step is a reaction with KOH and

KNO_{3}

to produce

Manganate +6, permanganate +7

The names are easy to swap. Manganate,

\mathrm{MnO_4^{2-}}

, is green, +6 and paramagnetic. Permanganate,

\mathrm{MnO_4^{-}}

, is purple, +7 and diamagnetic.

Peroxodisulphate goes all the way to permanganate

Oxidising a Mn(II) salt with peroxodisulphate gives

\mathrm{MnO_4^{-}}

, not manganate. A statement that it stops at manganate is false.

Drill 13 more on making kmno₄: manganate, permanganate and disproportionation

Concept 2 of 2: Permanganate as an oxidant: acid against neutral

Permanganate always gains electrons, but how many depends on the medium. In acid there is enough H⁺ to strip all four oxygens off as water, so Mn falls all the way to +2: five electrons. In neutral or faintly alkaline solution it stops at insoluble

\mathrm{MnO_2}

, +4: three electrons. The product of the reducing agent can change with the medium too.

Definition

Acid: $\mathrm{MnO_4^{-} + 8H^{+} + 5e^{-} \rightarrow Mn^{2+} + 4H_2O}$ , $E^\circ = +1.52$ V. n-factor 5.
Iodide: $\mathrm{10I^{-} + 2MnO_4^{-} + 16H^{+} \rightarrow 2Mn^{2+} + 5I_2 + 8H_2O}$ .
Iron(II): $\mathrm{5Fe^{2+} + MnO_4^{-} + 8H^{+} \rightarrow Mn^{2+} + 5Fe^{3+} + 4H_2O}$ .
Oxalate: $\mathrm{5C_2O_4^{2-} + 2MnO_4^{-} + 16H^{+} \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O}$ .
Also $\mathrm{S^{2-} \rightarrow S}$ , $\mathrm{SO_3^{2-} \rightarrow SO_4^{2-}}$ , $\mathrm{NO_2^{-} \rightarrow NO_3^{-}}$ .
Neutral or faintly alkaline: $\mathrm{MnO_4^{-} + 2H_2O + 3e^{-} \rightarrow MnO_2 + 4OH^{-}}$ . n-factor 3.
Iodide goes to iodate: $\mathrm{2MnO_4^{-} + H_2O + I^{-} \rightarrow 2MnO_2 + 2OH^{-} + IO_3^{-}}$ .
Thiosulphate goes to sulphate: $\mathrm{8MnO_4^{-} + 3S_2O_3^{2-} + H_2O \rightarrow 8MnO_2 + 6SO_4^{2-} + 2OH^{-}}$ .
Titrations are done in dilute $\mathrm{H_2SO_4}$ , never HCl: permanganate oxidises chloride to chlorine.
With Mohr's salt: $\mathrm{2KMnO_4 + 10FeSO_4 + 8H_2SO_4 \rightarrow K_2SO_4 + 2MnSO_4 + 5Fe_2(SO_4)_3 + 8H_2O}$ .

Two media, two products

\text{acid: } \mathrm{Mn^{+7} \rightarrow Mn^{2+}}\ (5e^{-}) \qquad \text{neutral/faintly alkaline: } \mathrm{Mn^{+7} \rightarrow MnO_2}\ (3e^{-})

Worked example

What volume of 0.02 M

\mathrm{KMnO_4}

oxidises 25 mL of 0.1 M

\mathrm{FeSO_4}

in dilute sulphuric acid?

Practice this conceptself-check · 5 quick reps

The same idea in a real exam question:

JEE Mains · 2023 · 30 January 2023 · Q138Moderate

Example 2 · The d- and f-Block Elements · Potassium Permanganate and Manganese Compounds

KMnO_{4}

oxidises

I^{-}

in acidic and neutral/faintly alkaline solution, respectively, to

Permanganate oxidises; it never reduces

Acidified permanganate OXIDISES oxalate, nitrite and iodide. A statement that it 'reduces oxalate, nitrite and iodide' is false, however familiar the list looks.

Count the water of crystallisation when the question does

If the titration uses ferrous ammonium sulphate HEXAHYDRATE, the 10 formula units bring 60 water molecules of their own. With the 8 formed in the reaction that is 68 per 2

\mathrm{KMnO_4}

Drill 13 more on permanganate as an oxidant: acid against neutral

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Making KMnO₄: manganate, permanganate and disproportionation
Disproportionation of manganate
$\mathrm{3\overset{+6}{Mn}O_4^{2-} + 4H^{+} \rightarrow 2\overset{+7}{Mn}O_4^{-} + \overset{+4}{Mn}O_2 + 2H_2O}$
Permanganate as an oxidant: acid against neutral
Two media, two products
$\text{acid: } \mathrm{Mn^{+7} \rightarrow Mn^{2+}}\ (5e^{-}) \qquad \text{neutral/faintly alkaline: } \mathrm{Mn^{+7} \rightarrow MnO_2}\ (3e^{-})$

Watch out for (4)

Manganate +6, permanganate +7
→ Making KMnO₄: manganate, permanganate and disproportionation
Peroxodisulphate goes all the way to permanganate
→ Making KMnO₄: manganate, permanganate and disproportionation
Permanganate oxidises; it never reduces
→ Permanganate as an oxidant: acid against neutral
Count the water of crystallisation when the question does
→ Permanganate as an oxidant: acid against neutral

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Related notes

Magnetic Moment and Colour — moments of the manganese products →