Which of the following oxidation reactions are carried out by both K₂ Cr₂ O₇ and KMnO₄ in acidic medium?
A. Γ⁻ → I₂
B. S²⁻ → S
C. Fe²⁺ → Fe³⁺
D. I⁻ → IO₃⁻
E. S₂ O₃ ²⁻ → SO₄²⁻
Choose the correct answer from the options given below :

Solution

Chemistry ma’am · 3 steps

Use the NCERT list of reactions for each reagent, and note the medium printed next to each one.

1. Acidic dichromate. In acid, Cr₂O₇²⁻ goes to Cr³⁺. NCERT lists it oxidising I⁻ to I₂, H₂S (that is, S²⁻) to S, Fe²⁺ to Fe³⁺ and Sn²⁺ to Sn⁴⁺.
2. Permanganate by medium. In acid, MnO₄⁻ goes to Mn²⁺ and also oxidises I⁻ to I₂, S²⁻ to S and Fe²⁺ to Fe³⁺. The changes I⁻ to IO₃⁻ and S₂O₃²⁻ to SO₄²⁻ are listed only for neutral or faintly alkaline KMnO₄, where it goes to MnO₂.

   Change	Acidic K₂Cr₂O₇	Acidic KMnO₄
   A. I⁻ → I₂	yes	yes
   B. S²⁻ → S	yes	yes
   C. Fe²⁺ → Fe³⁺	yes	yes
   D. I⁻ → IO₃⁻	no	no, neutral or faintly alkaline only
   E. S₂O₃²⁻ → SO₄²⁻	no	no, neutral or faintly alkaline only

3. So. Only A, B and C are carried out by both oxidants in acidic medium, so the answer is (a).

The idea to keep. With KMnO₄, always check the medium. I⁻ to IO₃⁻ and S₂O₃²⁻ to SO₄²⁻ happen in neutral or faintly alkaline solution, not in acid.

Answer from the NTA final answer key. Solution by PrepWiser.