Answer: H 2 O 2 (H 2 O 2 oxidises Cr 3+ to CrO 4 2- ).
- A Cr<sup>3+</sup>, the species that is itself oxidised to chromate in this reaction
- B H<sub>2</sub>O<sub>2</sub> (H<sub>2</sub>O<sub>2</sub> oxidises Cr<sup>3+</sup> to CrO<sub>4</sub><sup>2-</sup>)
- C H<sub>2</sub>O, the product formed and therefore not the oxidising agent
- D OH-, the basic medium ion that does not change oxidation state
Correct answer: B. H<sub>2</sub>O<sub>2</sub> (H<sub>2</sub>O<sub>2</sub> oxidises Cr<sup>3+</sup> to CrO<sub>4</sub><sup>2-</sup>)
Explanation: Cr goes from +3 to +6 (loses 3e⁻ per Cr; oxidised); H₂O₂ accepts electrons (O goes from -1 to -2): H₂O₂ is the oxidising agent. In basic medium the reaction is: 2Cr³⁺ + 3H₂O₂ + 10OH⁻ → 2CrO₄²⁻ + 8H₂O.
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.