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Questions
Explain why does colour of KMnO4 disappear when oxalic acid is added to its solution in acidic medium.
Why does pink colour of aqueous KMnO4 solution disappear when warmed with oxalic acid solution in acidic medium?
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Solution
The pink colour disappears because the permanganate ion \[\ce{(MnO^-_4)}\], which is purple/pink, is reduced to the manganese(II) ion (Mn2+), which is colourless.
\[\ce{5C2O^{2-}4 + \underset{(Coloured)}{2MnO^-4} + 16H+ -> \underset{(Coloured)}{2Mn^2+} + 8H2O + 10CO2}\]
The reaction is naturally very slow at room temperature. Warming the solution provides the activation energy needed to start the reaction. Once the reaction begins, the Mn2+ ions produced act as an autocatalyst, further accelerating the decolourisation.
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