WAEC Chemistry 2018 Paper

Concept:In a redox reaction, the oxidizing agent is the substance that is reduced by gaining electrons.Explanation:In $\mathrm{Br}_2$, the oxidation state of bromine is $0$.In $\mathrm{KBr}$, the oxidation state of bromine is $-1$.So bromine changes from $0$ to $-1$, meaning it gains electrons.The half-reaction is: $\mathrm{Br}_2 + 2e^- \to 2\mathrm{Br}^-$.Because bromine accepts electrons, it is reduced.By accepting electrons from $\mathrm{KI}$, it causes iodide ions to be oxidized to $\mathrm{I}_2$.Therefore, bromine acts as an oxidizing agent.Answer:A. an oxidizing agent