Answer: 8.
- A 4
- B 8
- C 5
- D 2
Correct answer: B. 8
Explanation: MnO₄⁻ → Mn²⁺: Mn changes from +7 to +2 (gains 5e⁻). Balance O by adding 4H₂O on the right; balance H by adding 8H⁺ on the left; add 5e⁻ on the left: MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
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.