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MCQEasyJEE 2026Oxidation Number & Redox Reactions

JEE Chemistry 2026 Question with Solution

MnO₄²⁻, in acidic medium, disproportionates to:

  • A

    Mn₂O₇ and MnO

  • B

    Mn₂O₇ and MnO₂

  • C

    MnO₄⁻ and MnO

  • D

    MnO₄⁻ and MnO₂

Answer

Correct answer:D

Step-by-step solution

Standard Method

Given: The species is MnO₄²⁻ in acidic medium.

Find: The products formed in its disproportionation reaction.

A disproportionation reaction is one in which the same species undergoes oxidation as well as reduction.

First find the oxidation state of Mn in MnO₄²⁻:

x+4(−2)=−2x + 4(-2) = -2 x−8=−2x - 8 = -2 x=+6x = +6

So manganese is in the +6 oxidation state.

In disproportionation, Mn(+6) forms one product in a higher oxidation state and another in a lower oxidation state. The stable higher oxidation state is +7 in MnO₄⁻, and the lower oxidation state taken here is +4 in MnO₂.

So the unbalanced reaction is:

MnO42−→MnO4−+MnO2\text{MnO}_4^{2-} \rightarrow \text{MnO}_4^- + \text{MnO}_2

Balancing the electron change gives:

3MnO42−→2MnO4−+MnO23\text{MnO}_4^{2-} \rightarrow 2\text{MnO}_4^- + \text{MnO}_2

Now balance it in acidic medium:

3MnO42−+4H+→2MnO4−+MnO2+2H2O3\text{MnO}_4^{2-} + 4\text{H}^+ \rightarrow 2\text{MnO}_4^- + \text{MnO}_2 + 2\text{H}_2\text{O}

Thus, the products are MnO₄⁻ and MnO₂.

Therefore, the correct option is D.

Oxidation State Based Explanation

Given: Manganate ion MnO₄²⁻ is in acidic medium.

Find: Which pair of products is formed after disproportionation.

The oxidation state of manganese in MnO₄²⁻ is +6. In disproportionation, the same Mn(+6) species is both oxidized and reduced.

  • Oxidation side: Mn(+6) \rightarrow Mn(+7) giving MnO₄⁻
  • Reduction side: Mn(+6) \rightarrow Mn(+4) giving MnO₂

The balanced reaction in acidic medium is:

3MnO42−+4H+→2MnO4−+MnO2+2H2O3\text{MnO}_4^{2-} + 4\text{H}^+ \rightarrow 2\text{MnO}_4^- + \text{MnO}_2 + 2\text{H}_2\text{O}

Hence the disproportionation products are MnO₄⁻ and MnO₂, so the correct option is D.

Common mistakes

  • Assuming the reduction product must be Mn²⁺ in every acidic medium is incorrect. For this disproportionation, the typical reduced product is MnO₂. Check the specific redox pathway instead of using a memorized oxidation state blindly.

  • Confusing manganate MnO₄²⁻ with permanganate MnO₄⁻ leads to a wrong oxidation state. Always calculate the oxidation state of manganese first before predicting disproportionation products.

  • Choosing MnO as the reduced product is incorrect because it corresponds to manganese in the +2 state for this oxide formulation and does not match the standard disproportionation outcome described here. Use the balanced reaction to verify the products.

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