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Question
Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with oxalic acid? Write the ionic equation for the reaction.
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Solution
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: \[\ce{2MnO^-_4 + 16H+ + 5C2O^{2-}_4 -> 2Mn^{2+} + 8H2O + 10CO2}\]
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