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Question
Describe the preparation of potassium permanganate. How does the acidified permanganate solution react with iron (II) ions? Write the ionic equation for the reaction.
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Solution
Potassium permanganate is prepared by 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.
\[\ce{2MnO2 + 4KOH + O2 -> 2K2MnO4 + 2H2O}\]
Potassium manganate is extracted with water and thereafter disproportionates in neutral or acidic solution to yield potassium permanganate.
\[\ce{3MnO^{2-}_4 + 4H^+ -> 2MnO^-_4 + MnO2 + 2H2O}\]
In an acidic medium of dilute sulphuric acid, KMnO4 acts as a strong oxidising agent, and it reacts as:
\[\ce{MnO^-_4 + 8H^+ + 5e- -> Mn^2+ + 4H2O}\]
Ferrous (Fe2+) ion solution to ferric (Fe3+) ion solution
\[\ce{MnO^-_4 + 8H+ + 5e- -> Mn^{2+} + 4H2O}\]
\[\ce{5Fe^{2+} -> 5Fe^{3+} + 5e-}\]
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\[\ce{MnO^-_4 + 5Fe^{2+} + 8H+ -> Mn^{2+} + 5Fe^{3+} + 4H2O}\]
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