Potassium Permanganate and Potassium Dichromate — Part 1: Potassium permanganate: visual lesson, ISC Class 12 Chemistry

Lesson

Purple power: potassium permanganate

Part 1 — how KMnO₄ is made, why it is purple, and how it oxidises in different media.

In this lesson you will

Describe the structure and shape of the permanganate ion
Write the equations for the preparation of KMnO₄ from pyrolusite
Write balanced ionic equations for permanganate oxidations in acidic, basic and neutral media
Explain why KMnO₄ acts as a self-indicator and solve titration problems

Concept

Properties of KMnO₄

Dark purple, almost black, crystals; moderately soluble in water giving an intense purple solution; manganese is in the +7 state (3d⁰). On heating to about 513 K it decomposes, releasing oxygen.

\ce{2KMnO4 ->[513 K] K2MnO4 + MnO2 + O2}

Table of properties of potassium permanganate
Property	KMnO₄
Appearance	Dark purple crystals
Oxidation state of Mn	+7 (3d⁰)
Magnetism	Diamagnetic
On heating (513 K)	K₂MnO₄ + MnO₂ + O₂
Uses	Oxidant, disinfectant, titrant

Diagram

Structure and shape of the permanganate ion

$\ce{MnO4-}$ is tetrahedral: Mn at the centre, four O at the corners, with three Mn=O bonds and one Mn–O⁻ bond drawn (all four are equivalent by resonance). The green manganate ion $\ce{MnO4^2-}$ is also tetrahedral but has Mn +6 (3d¹, paramagnetic). The purple colour of permanganate comes from charge transfer (O → Mn), not a d–d transition.

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