Select the disproportionation reaction which is, expected to be spontaneous in aq. solution.

Explanation

A disproportionation reaction occurs when a single species is simultaneously reduced and oxidized, producing two different products. Let's analyze both options:

MnO₄²⁻ → MnO₄⁻ + MnO₂ (Option a)

In this reaction, the manganese in MnO₄²⁻ is both oxidized (from +6 to +7 in MnO₄⁻) and reduced (from +6 to +4 in MnO₂). This reaction is possible and often spontaneous in acidic or neutral aqueous solutions. Manganese in the +6 oxidation state is relatively unstable and tends to disproportionate into more stable oxidation states (+7 and +4).

Cu⁺ → Cu²⁺ + Cu (Option b)

This is another example of disproportionation where Cu⁺ is oxidized to Cu²⁺ and reduced to metallic Cu. The reaction is spontaneous in aqueous solutions because Cu⁺ is less stable than Cu²⁺, and Cu²⁺ is more likely to be reduced to metallic Cu.

Both reactions are examples of spontaneous disproportionation in aqueous solutions, so the correct answer is "Both (a) and (b)".