What happens when an acidic solution of potassium permanganate is allowed to stand for some time? Give the equation involved. What is this type of reaction called?
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Solution and Explanation
Answer: On standing, an acidified KMnO4 solution slowly decomposes: the purple colour fades and brown manganese dioxide (MnO2) forms, while oxygen gas is released from the water. The equation is:
4 MnO4- + 4 H+ → 4 MnO2 + 3 O2 + 2 H2O
Here Mn is reduced from +7 to +4 while the oxygen of water is oxidised. Because permanganate acts as the oxidant and water is oxidised, this is an auto-oxidation (self-decomposition) reaction; it is a light-induced/auto-catalytic decomposition of permanganate.
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