JEE Main 2023 · 25 January · evening · Single correct · +4 / −1
Potassium dichromate acts as a strong oxidizing agent in acidic solution. During this process, the oxidation state changes from :
- (a) + 2 to + 1
- (b) + 6 to + 2
- (c) + 6 to + 3
- (d) + 3 to + 1
Answer and solution
Answer (c) + 6 to + 3
Solution
Chemistry ma’am · 3 steps
Find chromium's oxidation state in dichromate first, then remember it ends up as green Cr³⁺.
- Start in dichromate. In Cr₂O₇²⁻, let each Cr be x. Seven oxygens give -14, so 2x - 14 = -2 and x = +6.
Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O
- Where it ends. In acid, dichromate takes up 6 electrons, 3 for each Cr atom. Each Cr drops from +6 to +3, and the orange solution turns green from Cr³⁺.
- So. The oxidation state of chromium changes from +6 to +3, which is option (c).
The idea to keep. Acidified K₂Cr₂O₇ always goes Cr +6 → +3 (orange to green), taking 6 electrons per dichromate ion. Acidified KMnO₄ goes Mn +7 → +2 with 5 electrons.
Answer from the NTA final answer key. Solution by PrepWiser.