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In acidic solution, dichromate ion (Cr₂O₇²⁻) acts as a strong oxidising agent. The half-reaction is:
ACr₂O₇²⁻ + 7H⁺ + 3e⁻ → 2Cr³⁺ + 7H₂O
BCrO₄²⁻ + H₂O → Cr₂O₇²⁻ + 2OH⁻
CCr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V)
DCr₂O₇²⁻ + 14H⁺ + 12e⁻ → 2Cr + 7H₂O
Answer & Solution
Correct answer: C. Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O (E° = +1.33 V)
**Cr₂O₇²⁻ + 14H⁺ + 6e⁻ → 2Cr³⁺ + 7H₂O, E° = +1.33 V**. Each Cr goes from +6 to +3 (3-electron change × 2 Cr = 6 electrons). Strong oxidiser — oxidises I⁻ to I₂, S²⁻ to S, Sn²⁺ to Sn⁴⁺, Fe²⁺ to Fe³⁺.
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