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प्रश्न
A solution of \[\ce{KMnO4}\] on reduction yields either a colourless solution or a brown precipitate or a green solution depending on pH of the solution. What different stages of the reduction do these represent and how are they carried out?
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उत्तर
\[\ce{KMnO4}\] acts as an oxidising agent in alkaline, neutral and acidic mediums, i.e., oxidising behaviour of \[\ce{KMnO4}\] depends on pH of the solution.
In alkaline medium (pH > 7),
\[\ce{MnO^{-}4 + e^{-} -> MnO^{2-}4}\]
i.e., permanganate is changed to manganate which gives green solution.
\[\ce{2KMnO4 + 2KOH -> \underset{Green}{2K2MnO4} + H2O + [O]}\]
Reducing agent + \[\ce{[O] -> Product}\].
In neutral solution (pH = 7), permanganate is changed to manganese dioxide (brown ppt).
\[\ce{MnO^{-}4 + 2H2O ->[+3e^{-}] \underset{Brown ppt}{MnO2} + 4OH^{-}}\]
\[\ce{2KMnO4 + H2O -> 2KOH + 2MnO2 + 3[O]}\]
In acidic medium (pH < 7), permanganate is changed to manganous ion (Colourless).
\[\ce{MnO^{-}4 + 8H^{+}5e^{-} -> Mm^{2+} + 4H2O}\]
\[\ce{2KMnO4 + 3H2SO4 -> \underset{\underset{solution}{Colourless}}{2MnSo4} + K2SO4 + 3H2O + 5[O]}\]
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