Which of the following reactions are disproportionation reactions? (a) $2\mathrm{Cu}{}^{+}\to \mathrm{Cu}{}^{2+}+\mathrm{Cu}$ (b) $3\mathrm{MnO}{}_4{}^{2-}+4\mathrm{H}{}^{+}\to 2\mathrm{MnO}{}_4{}^{-}+\mathrm{MnO}{}_2+2\mathrm{H}{}_2\mathrm{O}$ (c) $2\mathrm{KMnO}{}_4\to \mathrm{K}{}_2\mathrm{MnO}{}_4+\mathrm{MnO}{}_2+\mathrm{O}{}_2$ (d) $2\mathrm{MnO}{}_4{}^{-}+3\mathrm{Mn}{}^{2+}+2\mathrm{H}{}_2\mathrm{O}\to 5\mathrm{MnO}{}_2+4\mathrm{H}{}^{+}$ Select the correct option from the following.

Solution & Explanation

\textbf{Answer:} (a) (a) and (b) only \textbf{Solution:} In a disproportionation reaction, the same element in a single oxidation state is simultaneously oxidised and reduced. (a) $\mathrm{Cu}{}^{+}$ ($+1$) goes to $\mathrm{Cu}{}^{2+}$ ($+2$) and $\mathrm{Cu}$ ($0$): Cu disproportionates. (b) Mn in $\mathrm{MnO}{}_4{}^{2-}$ ($+6$) goes to $\mathrm{MnO}{}_4{}^{-}$ ($+7$) and $\mathrm{MnO}{}_2$ ($+4$): Mn disproportionates. (c) $2\mathrm{KMnO}{}_4\to \mathrm{K}{}_2\mathrm{MnO}{}_4+\mathrm{MnO}{}_2+\mathrm{O}{}_2$: here both Mn ($+7$ to $+6$ and $+4$) and O ($-2$ to $0$ in $\mathrm{O}{}_2$) change oxidation state, so this is a thermal decomposition, not a disproportionation of a single species. (d) $2\mathrm{MnO}{}_4{}^{-}$ ($+7$) and $3\mathrm{Mn}{}^{2+}$ ($+2$) go to $5\mathrm{MnO}{}_2$ ($+4$): two different oxidation states of Mn converge to one ($+4$), which is comproportionation, the reverse of disproportionation. Hence only (a) and (b) are disproportionation reactions, giving option (a).