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MCQEasyJEE 2026Oxidation Number & Redox Reactions

JEE Chemistry 2026 Question with Solution

The oxidation state of chromium in the final product formed in the reaction between KI and acidified K2Cr2O7K_2Cr_2O_7 solution is:

  • A

    +3+3

  • B

    +6+6

  • C

    +2+2

  • D

    +4+4

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: KI reacts with acidified K2Cr2O7K_2Cr_2O_7 solution.

Find: The oxidation state of chromium in the final product.

In acidic medium, dichromate ion Cr2O72−Cr_2O_7^{2-} acts as a strong oxidizing agent and gets reduced, while I−I^- gets oxidized to I2I_2.

The balanced ionic reaction is:

Cr2O72−+6I−+14H+→2Cr3++3I2+7H2OCr_2O_7^{2-} + 6I^- + 14H^+ \to 2Cr^{3+} + 3I_2 + 7H_2O

Oxidation State Identification

From the reaction, the final chromium-containing product is Cr3+Cr^{3+}.

Therefore, the oxidation state of chromium in the final product is +3+3. The correct option is A.

Common mistakes

  • Assuming chromium remains in the +6+6 oxidation state because it starts in Cr2O72−Cr_2O_7^{2-}. This is wrong because dichromate is reduced in acidic medium. Always identify the product species before assigning oxidation state.

  • Confusing acidic and basic medium products of chromium. This is wrong because the chromium product depends on the medium. In acidic medium, the reduced product is Cr3+Cr^{3+}, not chromate or chromium hydroxide.

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