Which of the following reactions are disproportionation reactions?
(A) Cu⁺ → Cu²⁺+Cu
(B) 3 MnO₄²⁻+4 H⁺ → 2
MnO₄⁻+MnO₂+2 H₂ O
(C) 2 KMnO₄ → K₂ MnO₄+MnO₂+O₂
(D) 2 MnO₄⁻+3 Mn²⁺+2 H₂ O → 5 MnO₂+4 H⁺
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Disproportionation needs one element in one species to go both up and down in oxidation state.

1. The test. In a disproportionation, the same element in the same reactant is both oxidised and reduced. That element starts at one oxidation state and ends at one higher state and one lower state.

   Reaction	Oxidation states of the key element	What happens	Disproportionation?
   (A)	Cu: +1 → +2 and 0	one Cu goes up, one goes down	Yes
   (B)	Mn: +6 → +7 (in MnO₄⁻) and +4 (in MnO₂)	two Mn go up, one goes down	Yes
   (C)	Mn: +7 → +6 and +4; O: -2 → 0	Mn is only reduced; O is the one oxidised	No
   (D)	Mn: +7 and +2 → +4	two states meet in the middle	No (comproportionation)

2. C and D fail. In (C) both Mn products are lower than +7, so Mn is never oxidised. The oxidation happens on a different element, oxygen. In (D) two different Mn species combine into one middle state, which is the reverse of disproportionation.
3. So. Only (A) and (B) are disproportionation reactions, which is option (a).

The idea to keep. For disproportionation, track one element: it must split from a single state into a higher and a lower one. Two states merging into one is comproportionation, the opposite.

Answer from the NTA final answer key. Solution by PrepWiser.