2023 O Level Chem P1 Q24 — Redox Solution

Answer: D. Reduction = gain of electrons / decrease in oxidation state.

(1) \(\text{Cu} \rightarrow \text{Cu}^{2+}\): copper loses 2 electrons (0 → +2). This is oxidation, not reduction. ✗
(2) \(\text{I}_2 \rightarrow 2\text{I}^-\): iodine gains electrons (0 → −1). Reduction. ✓
(3) \(2\text{NO}_3^- \rightarrow \text{N}_2\text{O}_5\): N is +5 on both sides (\(\text{NO}_3^-\) and \(\text{N}_2\text{O}_5\)), so N is not reduced. To balance charge, oxygen must change state — some O is oxidised from −2 to 0 (forming \(\text{O}_2\)). This is an oxidation of oxygen, not a reduction. ✗
(4) \(\text{SO}_4^{2-} \rightarrow \text{SO}_2\): S goes from +6 to +4, gaining electrons. Reduction. ✓

So 2 and 4 — option D.