JEE Main · 2022 · Shift-IIeasyRDX-010

Which one of the following is an example of disproportionation reaction?

Redox Reactions · Class 11 · JEE Main Previous Year Question

Question

Which one of the following is an example of disproportionation reaction?

Options
  1. a

    3MnO42−+4H+→2MnO4−+MnO2+2H2O\mathrm{3MnO_4^{2-} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O}

    ✓
  2. b

    MnO4−+4H++4e−→MnO2+2H2O\mathrm{MnO_4^{-} + 4H^{+} + 4e^{-} \rightarrow MnO_2 + 2H_2O}

  3. c

    10I−+2MnO4−+16H+→2Mn2++8H2O+5I2\mathrm{10I^{-} + 2MnO_4^{-} + 16H^{+} \rightarrow 2Mn^{2+} + 8H_2O + 5I_2}

  4. d

    8MnO4−+3S2O32−+H2O→8MnO2+6SO42−+2OH−\mathrm{8MnO_4^{-} + 3S_2O_3^{2-} + H_2O \rightarrow 8MnO_2 + 6SO_4^{2-} + 2OH^{-}}

Correct Answera

3MnO42−+4H+→2MnO4−+MnO2+2H2O\mathrm{3MnO_4^{2-} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O}

Detailed Solution

Disproportionation = same element simultaneously oxidised and reduced in the same reaction.

Step 1 — Check option (1): 3MnO42−+4H+→2MnO4−+MnO2+2H2O\mathrm{3MnO_4^{2-} + 4H^{+} \rightarrow 2MnO_4^{-} + MnO_2 + 2H_2O}

  • Mn in MnO42−\mathrm{MnO_4^{2-}}: +6+6 (reactant)
  • Mn in MnO4−\mathrm{MnO_4^{-}}: +7+7 → oxidised
  • Mn in MnO2\mathrm{MnO_2}: +4+4 → reduced
  • Same element (Mn) both oxidised and reduced → Disproportionation ✓

Step 2 — Eliminate others:

  • (2): Half-reaction only (reduction only) ✗
  • (3): I−\mathrm{I^-} oxidised, Mn\mathrm{Mn} reduced — two different elements ✗ (not disproportionation)
  • (4): S\mathrm{S} oxidised, Mn\mathrm{Mn} reduced — two different elements ✗

Answer: Option (1)

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