JEE Main · 2022 · 26 Jul 2022 · Shift 1 · Redox Reactions and Electrochemistry

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Which of the given reactions is not an example of disproportionation reaction?
  1. (A)2H₂O₂ → 2H₂O + O₂
  2. (B)2NO₂ + H₂O → HNO₃ + HNO₂
  3. (C)MnO₄⁻ + 4H⁺ + 3e⁻ → MnO₂ + 2H₂O✓ Correct
  4. (D)3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O

Answer: (C)

Solution

A disproportionation reaction is one in which the same element undergoes both oxidation and reduction simultaneously.
  • (A) 2H₂O₂ → 2H₂O + O₂: Oxygen goes from -1 (in H₂O₂) to -2 (in H₂O) and 0 (in O₂). Disproportionation.
  • (B) 2NO₂ + H₂O → HNO₃ + HNO₂: Nitrogen goes from +4 (in NO₂) to +5 (in HNO₃) and +3 (in HNO₂). Disproportionation.
  • (C) MnO₄⁻ + 4H⁺ + 3e⁻ → MnO₂ + 2H₂O: Mn goes from +7 to +4. This is only reduction, not disproportionation.
  • (D) 3MnO₄²⁻ + 4H⁺ → 2MnO₄⁻ + MnO₂ + 2H₂O: Mn goes from +6 to +7 and +4. Disproportionation.

Answer: (C).

JEE Main · 2022 · 26 Jul 2022 · Shift 1 · Single correct option

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