Answer: 6.
- A 2
- B 3
- C 5
- D 6
Correct answer: D. 6
Explanation: Each Cr goes from +6 to +3 (gains 3e⁻); two Cr atoms gain 6e⁻ total. Each Fe²⁺ loses 1e⁻. To balance: 6 Fe²⁺ are oxidised per Cr₂O₇²⁻ reduced.
Electron transfer in a redox reaction: zinc loses electrons (oxidised, acts as reducing agent) and copper(II) ions gain those same electrons (reduced, acts as oxidising agent).
Concept context
Master electron transfer in chemistry: assign oxidation states, balance half-reactions, identify oxidising and reducing agents, and connect redox to everyday reactions.