Question

Discuss in detail the oxidising character of potassium permanganate in neutral solutions. Give at least two reactions in the given case.

Answer 1

The following reaction causes potassium permanganate to operate as a moderate oxidizing agent in neutral solution.

\[\ce{2KMnO4 + H2O -> 2KOH + 2MnO2 + 3O}\]

The reaction actually involves the reduction of MnO₄ ion into MnO₂.

\[\ce{MnO^-_4 + 2H2O + 3e- -> MnO2 + 4OH-}\]

Potassium permanganate functions as an oxidizing agent in a neutral medium because MnO₄ ions have a propensity to be reduced. The equation above makes it evident that three moles of electrons are required for one mole of MnO₄ to be reduced. Therefore, in a neutral medium, the equivalent mass of KMnO₄ is one-third of its molecular mass.

Some important oxidising properties of potassium permanganate in neutral medium are as follows.

(i) It dioxide. oxidises manganous sulphate to manganese.

\[\ce{2KMnO4 + H2O -> 2KOH + 2MnO2 + 3O}\]

\[\ce{[MnSO4 + H2O + O -> MnO2 + H2SO4] \times 3}\]

\[\ce{2KOH + H2SO4 -> K2SO4 + 2H2O}\]

\[\ce{\overline{2\text{KMnO}_4 + 3\text{MnSO}_4 + 2\text{H}_2\text{O} \leftarrow 5\text{MnO}_2 + \text{K}_2\text{SO}_4 + 2\text{H}_2\text{SO}_4}}\]

or, 

\[\ce{2MnO^-_4 + 3H2S -> 2KOH + 2MnO2 + 2H2O + 3S}\]

(ii) It oxidises hydrogen sulphide to sulphur.

\[\ce{2KMnO4 + H2O -> 2KOH + 2MnO2 + 3O}\]

\[\ce{[H2S + O -> H2O + S] \times 3}\]

\[\ce{\overline{2\text{KMnO}_4 + 3\text{H}_2 \text{S} \leftarrow 2\text{KOH } + 2\text{MnO}_2 + 2\text{H}_2\text{O} + 3\text{S}}}\]

or,

\[\ce{2MnO^-_4 + 3H2S -> 2OH-  + 2MnO2 + 2H2O + 3S}\]