2024 H2 Chem P1 Q15 — Redox Solution

The answer is A (magnesium chloride).
All the elements start at oxidation number 0. The metal is oxidised, so it is the reducing agent; the non-metal is reduced, so it is the oxidising agent. Compare the size of each change:
• A: \(\text{MgCl}_2\): Mg 0 to +2 (change of 2); Cl 0 to \(-1\) (change of 1). The reducing agent changes more, so A is correct.
• B is wrong: MgS: Mg 0 to +2 (change of 2); S 0 to \(-2\) (change of 2). The changes are equal.
• C is wrong: NaBr: Na 0 to +1 (change of 1); Br 0 to \(-1\) (change of 1). The changes are equal.
• D is wrong: \(\text{Na}_2\text{O}\): Na 0 to +1 (change of 1); O 0 to \(-2\) (change of 2). The oxidising agent changes more.