Reduction of the metal centre in aqueous permanganate ion involves :

Engineering

Chemistry

pH Scale and Relative Strength of Acids

Balancing of Redox Reaction

Dichromate and Permanganate Ion Reactions

Question

Reduction of the metal centre in aqueous permanganate ion involves :

3 electrons in neutral medium

3 electrons in alkaline medium

5 electrons in acidic medium

5 electrons in neutral medium

$\underset{}{\to}$ In alkaline solution, KMnO₄ is first reduced to magnate and then to insoluble MnO₂

_{$\to_{}^{} {\overset{+ 7}{2 KMnO}}_{4} + H_{2} O \overset{Neutral}{\to} {\overset{+ 4}{2 MnO}}_{4} + 2 KOH + 3 [O]$}

$\to_{}^{} {MnO}_{4}^{-} + 8 H^{+} + 5 e^{-} \overset{acidic}{\to} {Mn}^{+ 2} + 4 H_{2} O$

The reduction of permanganate ion (MnO₄^-) depends on the medium (acidic, neutral, or alkaline) because the stability of different oxidation states of manganese varies with pH. The number of electrons gained by manganese during reduction changes accordingly.

In acidic medium, MnO₄^- is reduced to Mn²⁺ (manganous ion). The change in oxidation state of Mn is from +7 to +2, which involves a gain of 5 electrons:

{MnO}_{4}^{-} + 8 H^{+} + 5 e^{-} \to {Mn}^{2 +} + 4 H_{2} O

In neutral or weakly alkaline medium, MnO₄^- is reduced to MnO₂ (manganese dioxide). The change in oxidation state is from +7 to +4, involving a gain of 3 electrons:

{MnO}_{4}^{-} + 2 H_{2} O + 3 e^{-} \to MnO 2 + 4 {OH}^{-}

In strongly alkaline medium, MnO₄^- can be reduced to manganate ion (MnO₄^2-), where the oxidation state changes from +7 to +6, involving a gain of 1 electron. However, this is less common in typical reduction contexts compared to the other two.

Therefore, reviewing the options:

"3 electrons in neutral medium" is correct.
"3 electrons in alkaline medium" is also generally correct for neutral to weakly alkaline conditions.
"5 electrons in acidic medium" is correct.
"5 electrons in neutral medium" is incorrect; it's 3 electrons in neutral medium.

The question asks for the reduction involving the metal centre, and based on standard chemistry, the correct statements are the ones describing 3 electrons in neutral/alkaline medium and 5 electrons in acidic medium.

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