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MCQEasyJEE 2023Oxidation Number & Redox Reactions

JEE Chemistry 2023 Question with Solution

Potassium dichromate acts as a strong oxidizing agent in acidic solution. During this process, the oxidation state changes from:

  • A

    +3 to +1+3 \text{ to } +1

  • B

    +6 to +3+6 \text{ to } +3

  • C

    +2 to +1+2 \text{ to } +1

  • D

    +6 to +2+6 \text{ to } +2

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: Potassium dichromate acts as a strong oxidizing agent in acidic solution.

Find: The change in oxidation state of chromium during the process.

In acidic solution, potassium dichromate (K2Cr2O7)\left(\text{K}_2\text{Cr}_2\text{O}_7\right) undergoes reduction. The chromium in dichromate ion (Cr2O72−)\left(\text{Cr}_2\text{O}_7^{2-}\right) is initially in oxidation state +6+6 and is reduced to Cr3+\text{Cr}^{3+}, where chromium has oxidation state +3+3.

Thus, the oxidation state changes from +6+6 to +3+3.

The solution states "The Correct Option is A" but the worked chemistry statement clearly gives the change as +6 to +3+6 \text{ to } +3, which matches option B in the listed options. Therefore, there is a source mismatch.

So, based on the solution working, the defensible answer is B.

Redox reaction diagram showing dichromate in acidic medium reducing from chromium plus six to chromium plus three with balanced ions and water formation.

Oxidation State Identification

Given: Dichromate ion in acidic medium behaves as an oxidizing agent.

Find: The oxidation state change of chromium.

The reduction half-reaction shown is

14H++6e−+Cr2O72−⟶2Cr3++7H2O14\text{H}^+ + 6e^- + \text{Cr}_2\text{O}_7^{2-} \longrightarrow 2\text{Cr}^{3+} + 7\text{H}_2\text{O}

From this, chromium goes from oxidation state +6+6 in Cr2O72−\text{Cr}_2\text{O}_7^{2-} to oxidation state +3+3 in Cr3+\text{Cr}^{3+}.

Therefore, the oxidation state changes from +6+6 to +3+3.

Common mistakes

  • Students often mark the option label written on the solution's without checking whether it matches the actual oxidation-state change. Here, the chemistry working gives +6 to +3+6 \text{ to } +3, so the concept must be trusted over the mislabeled option letter.

  • A common mistake is to think potassium dichromate is oxidized because it is an oxidizing agent. An oxidizing agent itself gets reduced, so chromium goes to a lower oxidation state instead.

  • Some students confuse the oxidation state of chromium in dichromate ion. In Cr2O72−\text{Cr}_2\text{O}_7^{2-}, chromium is +6+6, not +2+2 or +3+3. Always compute oxidation number carefully before choosing the option.

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