What is meant by 'disproportionation'? Give two examples of disproportionation reaction in aqueous solution.
Disproportionation is a reaction in which a species in an intermediate oxidation state undergoes simultaneous oxidation and reduction, forming two different products—one with a higher oxidation state and another with a lower oxidation state.
Two examples of disproportionation reactions in aqueous solution are:
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Copper(I) disproportionation: 2Cu⁺(aq) → Cu²⁺(aq) + Cu(s) Here, Cu⁺ (oxidation state +1) is simultaneously oxidized to Cu²⁺ (+2) and reduced to Cu (0).
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Manganate disproportionation: 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O Here, MnO₄²⁻ (Mn in +6 state) disproportionates in neutral or acidic solution to give MnO₄⁻ (Mn in +7) and MnO₂ (Mn in +4).
Explanation
The textbook context provides two clear examples of disproportionation. For copper(I), the text states that 'many copper (I) compounds are unstable in aqueous solution and undergo disproportionation' with the reaction 2Cu⁺ → Cu²⁺ + Cu. For manganate, the text explains that K₂MnO₄ 'disproportionates in a neutral or acidic solution to give permanganate' with the reaction 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O. The definition is inferred from these examples where a single species undergoes both oxidation and reduction simultaneously.