Question

Which of the following reactions are disproportionation reactions?
a) $2C{u}^{+}$ $\to$ $C{u}^{2+}$ + $C{u}^0$

b) $3Mn{O}_4^{2-}$ + $4H^{+}$ $\to$ $2Mn{O}_4^{-}$ +$Mn{O}_2$ + $2H_{2}O$

c) $2KMn{O}_4$ $\stackrel{△}{\to }$ $K_{2}Mn{O}_4$ + $Mn{O}_2$ + $O_2$

d) $2Mn{O}_4^{-}$ + $3M{n}^{2+}$ + $2H_{2}O$ $⟶$ $5Mn{O}_2$ + $4H^{+}$

• A. (a) and (b) only
• B. (a), (b) and (c)
• C. (a), (c) and (d)
• D. (a) and (d) only

Answer 1

The correct option is A (a) and (b) only

Disproportionation reactions are those in which the same reactant species is oxidised and reduced, this happens in a) and b) since the oxidation number of:
Copper changes from +1 to +2 and 0 in a)
Manganese changes from +6 to +7 and +4 in b)
In c), Mn goes from + 7 to +6 and +4, this is reduction only.
In d), Mn has two oxidation states - +7 and +2 which become +4. This is the reverse of disproportionation.