2025 O Level Chem P1 Q22 — Redox Reactions Solution

Answer: D. Redox = change in oxidation state.

1) \(\text{Ba(NO}_3)_2 + \text{Na}_2\text{SO}_4 \rightarrow \text{BaSO}_4 + 2\text{NaNO}_3\) — pure precipitation, no oxidation state changes.

2) Cu + dilute HCl — copper is below hydrogen in the reactivity series, so this doesn't react. Not redox (not even a reaction).

3) Ethanol + oxygen — combustion: \(\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\). Carbon's oxidation state changes (–2 in ethanol to +4 in CO₂); oxygen reduced (0 to –2). Redox.

4) Molten PbO + C — carbon reduces lead(II) oxide: \(\text{PbO} + \text{C} \rightarrow \text{Pb} + \text{CO}\). Pb²⁺ reduced to Pb⁰; C⁰ oxidised to C²⁺. Redox.

So 3 and 4 → D.