Question

Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with oxalic acid? Write the ionic equation for the reaction.

Answer 1

Potassium permanganate is prepared by the 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.

\[\ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O}\]

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

In an acidic medium (dilute sulphuric acid), permagnate ion reacts with oxalate ion \[\ce{(C2O^{2-}_4)}\] or oxalic acid at 333 K to evolve carbon dioxide gas.

Reduction half-reaction: \[\ce{[MnO^-_4 + 8H+ + 5e- -> Mn^{2+} + 4H2O] \times 2}\]

Oxidation half-reaction: \[\ce{[C2O^-_4 -> 2CO2 + 2e-] \times 5}\]

Net ionic equation: \[\ce{2MnO^-_4 + 16H+ + 5C2O^{2-}_4 -> 2Mn^{2+} + 8H2O + 10CO2}\]