JEE Main 2022 · 26 July · morning · Single correct · +4 / −1
Which of the given reactions is not an example of disproportionation reaction?
- (a) 2 H₂ O₂ → 2 H₂ O+O₂
- (b) 2 NO₂+H₂ O → HNO₃+HNO₂
- (c) MnO₄⁻+4 H⁺+3 e⁻ → MnO₂+2 H₂ O
- (d) 3 MnO₄²⁻+4 H⁺ → 2 MnO₄⁻+MnO₂+2 H₂ O
Answer and solution
Answer (c) MnO₄⁻+4 H⁺+3 e⁻ → MnO₂+2 H₂ O
Solution
Chemistry ma’am · 3 steps
Disproportionation needs one element in one state going both up and down; a half-reaction only goes one way.
- What to look for. In disproportionation, atoms of one element in one oxidation state are both oxidised and reduced in the same reaction. So among the products, that element must show up in a higher and a lower oxidation state.
Reaction Element Reactant state Product states Disproportionation? (a) H₂O₂ decomposes O -1 -2 in H₂O, 0 in O₂ yes (b) NO₂ in water N +4 +5 in HNO₃, +3 in HNO₂ yes (c) MnO₄⁻ takes 3e⁻ Mn +7 +4 in MnO₂ only no, reduction only (d) MnO₄²⁻ in acid Mn +6 +7 in MnO₄⁻, +4 in MnO₂ yes - Why (c) fails. Option (c) is a reduction half-reaction. Mn goes from +7 to +4 by accepting electrons, and no Mn atom is oxidised, so it cannot be disproportionation.
- So. The reaction that is not a disproportionation is (c).
The idea to keep. For disproportionation, find one element in one oxidation state and check that the products show it both higher and lower. Electrons written as a reactant mean a half-reaction, which only goes one way.
Answer from the NTA final answer key. Solution by PrepWiser.