Question

Which of the following is an oxidation-reduction reaction?
ZnS(s) + 2O2(g) Right arrow. ZnSO4(s)
CaO(s) + H2O(l) Right arrow. Ca(OH)2(s)
6Li2O(s) + P4O10(g) Right arrow. 4Li3PO4(s)
SO2(g) + H2O(l) Right arrow. H2SO3(aq)

Answer 1

Only ZnS(s) + 2O2(g) → ZnSO4(s) is a redox reaction.

Reason: assign oxidation numbers.
- ZnS: Zn = +2, S = −2; O2: O = 0.
- ZnSO4: Zn = +2, S = +6, O = −2.

Sulfur is oxidized (−2 → +6) and oxygen is reduced (0 → −2). The other three reactions show no change in oxidation states.