Which of the following are redox reactions? (a) \(2 \mathrm{Na}+2 \mathrm{H}_{2} \mathrm{O} \rightarrow 2 \mathrm{NaOH}+\mathrm{H}_{2}\) (b) \(\mathrm{MgBr}_{2}+2 \mathrm{NaF} \rightarrow \mathrm{MgF}_{2}+2 \mathrm{NaBr}\) (c) \(2 \mathrm{CO}+\mathrm{O}_{2} \rightarrow 2 \mathrm{CO}_{2}\) (d) \(\mathrm{SO}_{2}+\mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{2} \mathrm{SO}_{3}\)

(a) \(2\ \text{Na} + 2\ \text{H}_{2}\text{O}\ \rightarrow\ 2\ \text{NaOH} + \text{H}_{2}\) Oxidation states: \(\text{Na}\): before reaction is 0, after reaction is +1. \(\text{H}\): in \(\text{H}_{2}\text{O}\) is +1, in \(\text{NaOH}\) is +1, in \(\text{H}_{2}\) is 0. \(\text{O}\): in \(\text{H}_{2}\text{O}\) is -2, in \(\text{NaOH}\) is -2. (b) \(\text{MgBr}_{2} + 2\ \text{NaF}\ \rightarrow\ \text{MgF}_{2} + 2\ \text{NaBr}\) Oxidation states: \(\text{Mg}\): before reaction is +2, after reaction is +2. \(\text{Br}\): before reaction is -1, after reaction is -1. \(\text{Na}\): before reaction is +1, after reaction is +1. \(\text{F}\): before reaction is -1, after reaction is -1. (c) \(2\ \text{CO} + \text{O}_{2}\ \rightarrow\ 2\ \text{CO}_{2}\) Oxidation states: \(\text{C}\): in \(\text{CO}\) is +2, in \(\text{CO}_{2}\) is +4. \(\text{O}\): in \(\text{CO}\) is -2, in \(\text{O}_{2}\) is 0, in \(\text{CO}_{2}\) is -2. (d) \(1\ \text{SO}_{2} + \text{H}_{2}\text{O}\ \rightarrow\ \text{H}_{2}\text{SO}_{3}\) Oxidation states: \(\text{S}\): in \(\text{SO}_{2}\) is +4, in \(\text{H}_{2}\text{SO}_{3}\) is +4. \(\text{O}\): in \(\text{SO}_{2}\) is -2, in \(\text{H}_{2}\text{O}\) is -2, in \(\text{H}_{2}\text{SO}_{3}\) is -2. \(\text{H}\): in \(\text{H}_{2}\text{O}\) is +1, in \(\text{H}_{2}\text{SO}_{3}\) is +1.

We can now analyze the reactions based on their oxidation states: (a) There is a change in oxidation state for Na (from 0 to +1) and H (from +1 to 0). Since there's a transfer of electrons, this reaction is a redox reaction. (b) There is no change in oxidation states for any of the elements involved. Thus, this reaction is not a redox reaction. (c) There is a change in oxidation state for C (from +2 to +4) and O (from 0 to -2). Since there's a transfer of electrons, this reaction is a redox reaction. (d) There is no change in oxidation states for any of the elements involved. Thus, this reaction is not a redox reaction. In conclusion, reactions (a) and (c) are redox reactions, while reactions (b) and (d) are not.