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MCQEasyJEE 2023Oxidation Number & Redox Reactions

JEE Chemistry 2023 Question with Solution

During the reaction of permanganate with thiosulphate, the change in oxidation of manganese occurs by a value of 33. Identify which of the below medium will favor the reaction:

  • A

    Aqueous neutral

  • B

    Aqueous acidic

  • C

    Both aqueous acidic and neutral

  • D

    Both aqueous acidic and faintly alkaline

Answer

Correct answer:A

Step-by-step solution

Standard Method

Given: During the reaction of permanganate with thiosulphate, the change in oxidation of manganese is 33.

Find: Which medium favors this reaction.

In a neutral medium, permanganate is reduced to MnO2\mathrm{MnO_2}, where the oxidation state of manganese changes from +7+7 to +4+4.

Δoxidation state=7−4=3\Delta \text{oxidation state} = 7 - 4 = 3

In an acidic medium, permanganate is reduced to Mn2+\mathrm{Mn^{2+}}, where the oxidation state changes from +7+7 to +2+2.

Δoxidation state=7−2=5\Delta \text{oxidation state} = 7 - 2 = 5

Since the question specifies that the change in oxidation state of manganese is 33, the reaction must occur in aqueous neutral medium.

Therefore, the correct option is A.

Common mistakes

  • Assuming acidic medium is always preferred for permanganate reactions is incorrect because the reduction product of permanganate depends on the medium. Here, use the oxidation-state change given in the question to identify the correct medium.

  • Confusing the reduction of permanganate to Mn2+\mathrm{Mn^{2+}} with reduction to MnO2\mathrm{MnO_2} is wrong. In neutral medium, manganese goes from +7+7 to +4+4, not to +2+2.

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