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MCQEasyJEE 2025Oxidation Number & Redox Reactions

JEE Chemistry 2025 Question with Solution

The species which does not undergo disproportionation reaction is:

  • A

    ClO2−\text{ClO}_2^-

  • B

    ClO4−\text{ClO}_4^-

  • C

    ClO3−\text{ClO}_3^-

  • D

    ClO2\text{ClO}_2

Answer

Correct answer:B

Step-by-step solution

Standard Method

Given: The species are ClO2−\text{ClO}_2^-, ClO4−\text{ClO}_4^-, ClO3−\text{ClO}_3^- and ClO2\text{ClO}_2.

Find: Which species does not undergo disproportionation reaction.

A disproportionation reaction is a redox reaction in which the same species undergoes both oxidation and reduction to give two different products.

Species in intermediate oxidation states can usually undergo disproportionation because they can move to both higher and lower oxidation states.

For the given chlorine species:

  • ClO2−\text{ClO}_2^- can disproportionate to form ClO3−\text{ClO}_3^- and Cl−\text{Cl}^-.
  • ClO3−\text{ClO}_3^- can disproportionate to form ClO4−\text{ClO}_4^- and Cl−\text{Cl}^-.
  • ClO2\text{ClO}_2 can disproportionate to form higher and lower oxidation state chlorine species.
  • In ClO4−\text{ClO}_4^-, chlorine is already in its highest oxidation state, +7+7, so it cannot be oxidized further.

Therefore, ClO4−\text{ClO}_4^- does not undergo disproportionation.

The correct option is B.

Oxidation State Check

Given: The options are chlorine oxy-species.

Find: The species that cannot undergo disproportionation.

Check the oxidation state of chlorine in each species:

In ClO2−:x+2(−2)=−1⇒x=+3In ClO3−:x+3(−2)=−1⇒x=+5In ClO4−:x+4(−2)=−1⇒x=+7In ClO2:x+2(−2)=0⇒x=+4\begin{aligned} \text{In } \text{ClO}_2^- &: x + 2(-2) = -1 \Rightarrow x = +3 \\ \text{In } \text{ClO}_3^- &: x + 3(-2) = -1 \Rightarrow x = +5 \\ \text{In } \text{ClO}_4^- &: x + 4(-2) = -1 \Rightarrow x = +7 \\ \text{In } \text{ClO}_2 &: x + 2(-2) = 0 \Rightarrow x = +4 \end{aligned}

A species undergoes disproportionation only if its oxidation state can increase as well as decrease.

Here, +3+3, +4+4 and +5+5 are intermediate oxidation states, so these species can undergo disproportionation. But +7+7 is the maximum oxidation state of chlorine, so ClO4−\text{ClO}_4^- cannot be oxidized further.

Hence, the species which does not undergo disproportionation reaction is ClO4−\text{ClO}_4^-.

The correct option is B.

Common mistakes

  • Assuming every oxyanion undergoes disproportionation is incorrect. Disproportionation requires an intermediate oxidation state. First check whether the central atom can be both oxidized and reduced.

  • Ignoring the maximum oxidation state of chlorine leads to a wrong choice. In ClO4−\text{ClO}_4^-, chlorine is already at +7+7, so oxidation is not possible. Verify the oxidation state before deciding.

  • Confusing ClO4−\text{ClO}_4^- with ClO3−\text{ClO}_3^- is a common conceptual error. ClO3−\text{ClO}_3^- has chlorine in an intermediate state and can disproportionate, whereas ClO4−\text{ClO}_4^- cannot.

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