A solution of KMnO4 on reduction yields either a colourless solut… - Vidyadip

Question

A solution of 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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Answer

Oxidising behaviour of KMnO₄ depends on pH of the solution.
In acidic medium $\text{(pH<7)}$
$\text{MnO}_4^-+8\text{H}^++5\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{Mn}^{2+}+4\text{H}_2\text{O}\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Colourless)}$
In alkaline medium $\text{pH<7}$
$\text{MnO}_4^-+\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{MnO}^{2-}_4\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Green)}$
In neutral medium $\text{(pH=7)}$
$\text{MnO}_4^-+2\text{H}_2\text{O}+3\text{e}^-\xrightarrow{\ \ \ \ \ \ }\text{MnO}_{2}+4\text{OH}^-\\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ ^\text{(Brown precipitate)}$

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