Which of the following reactions are disproportionation reactions? (1) Cu+ Cu2+ + Cu (2) 3MnO42- + 4H+ 2MnO4- + MnO2 +…

Step 1 — Check reaction (1): $\mathrm{2Cu^{+} \rightarrow Cu^{2+} + Cu}$

• Cu: $+1 \rightarrow +2$ (oxidised) and $+1 \rightarrow 0$ (reduced)
• Same element simultaneously oxidised and reduced → Disproportionation ✓

Step 2 — Check reaction (2): $\mathrm{3MnO_4^{2-} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O}$

• Mn in $\mathrm{MnO_4^{2-}}$: $+6$
• Mn in $\mathrm{MnO_4^{-}}$: $+7$ (oxidised)
• Mn in $\mathrm{MnO_2}$: $+4$ (reduced)
• Same element (Mn) simultaneously oxidised and reduced → Disproportionation ✓

Step 3 — Check reaction (3): $\mathrm{2KMnO_4 \rightarrow K_2MnO_4 + MnO_2 + O_2}$

• Mn in $\mathrm{KMnO_4}$: $+7$
• Mn in $\mathrm{K_2MnO_4}$: $+6$ (reduced)
• Mn in $\mathrm{MnO_2}$: $+4$ (reduced)
• O in $\mathrm{KMnO_4}$: $-2$; O in $\mathrm{O_2}$: $0$ (oxidised)
• Different elements change: Mn is only reduced, O is oxidised → NOT disproportionation ✗ (this is a decomposition redox)

Step 4 — Check reaction (4): $\mathrm{2MnO_4^{-} + 3Mn^{2+} + 2H_2O \rightarrow 5MnO_2 + 4H^{+}}$

• Mn in $\mathrm{MnO_4^-}$: $+7$ (reduced to $+4$)
• Mn in $\mathrm{Mn^{2+}}$: $+2$ (oxidised to $+4$)
• This is comproportionation (opposite of disproportionation) → NOT disproportionation ✗

Answer: Option (1) — (1) and (2)