Which of the following are electron-transfer reactions? For those that are, indicate which reactant is the reducing agent and which reactant is the oxidizing agent. (a) \(\mathrm{PF}_{3}+3 \mathrm{H}_{2} \mathrm{O} \rightarrow \mathrm{H}_{3} \mathrm{PO}_{3}+3 \mathrm{HF}\) (b) \(\mathrm{H}_{2}+\mathrm{Cl}_{2} \rightarrow 2 \mathrm{HCl}\) (c) \(2 \mathrm{Cr}_{2} \mathrm{O}_{3}+3 \mathrm{Si} \rightarrow 4 \mathrm{Cr}+3 \mathrm{SiO}_{2}\) (d) \(\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H}_{2} \mathrm{O}\)

For each given reaction, we should first determine the oxidation state of the elements in each compound both in the reactants and products. This will help identify any changes in oxidation state and classify the reactions as electron-transfer reactions or not. (a) \(\mathrm{PF_{3}}+3 \mathrm{H_{2}O} \rightarrow \mathrm{H_{3}PO_{3}}+3 \mathrm{HF}\) Oxidation states: P in \(\mathrm{PF_{3}}\): +3 F in \(\mathrm{PF_{3}}\): -1 P in \(\mathrm{H_{3}PO_{3}}\): +3 F in \(\mathrm{HF}\): -1 No change in the oxidation state of any element, so it's not an electron-transfer reaction. (b) \(\mathrm{H_{2}}+\mathrm{Cl_{2}} \rightarrow 2 \mathrm{HCl}\) Oxidation states: H in \(\mathrm{H_{2}}\): 0 Cl in \(\mathrm{Cl_{2}}\): 0 H in \(\mathrm{HCl}\): +1 Cl in \(\mathrm{HCl}\): -1 There are changes in the oxidation states, so it's an electron-transfer reaction. (c) \(2 \mathrm{Cr_{2}O_{3}}+3 \mathrm{Si} \rightarrow 4 \mathrm{Cr}+3 \mathrm{SiO_{2}}\) Oxidation states: Cr in \(\mathrm{Cr_{2}O_{3}}\): +3 O in \(\mathrm{Cr_{2}O_{3}}\): -2 Si in \(\mathrm{Si}\): 0 Cr in \(\mathrm{Cr}\): 0 O in \(\mathrm{SiO_{2}}\): -2 Si in \(\mathrm{SiO_{2}}\): +4 There are changes in the oxidation states, so it's an electron-transfer reaction. (d) \(\mathrm{HCl}+\mathrm{NaOH} \rightarrow \mathrm{NaCl}+\mathrm{H_{2}O}\) Oxidation states: H in \(\mathrm{HCl}\): +1 Cl in \(\mathrm{HCl}\): -1 Na in \(\mathrm{NaOH}\): +1 O in \(\mathrm{NaOH}\): -2 H in \(\mathrm{H_{2}O}\): +1 O in \(\mathrm{H_{2}O}\): -2 Na in \(\mathrm{NaCl}\): +1 Cl in \(\mathrm{NaCl}\): -1 No change in the oxidation state of any element, so it's not an electron-transfer reaction.

For the electron-transfer reactions identified in Step 1, we should now indicate which reactant is the reducing agent and which reactant is the oxidizing agent. (b) \(\mathrm{H_{2}}+\mathrm{Cl_{2}} \rightarrow 2 \mathrm{HCl}\) Oxidation: H in \(\mathrm{H_{2}}\) goes from 0 to +1, losing electrons - reducing agent Reduction: Cl in \(\mathrm{Cl_{2}}\) goes from 0 to -1, gaining electrons - oxidizing agent (c) \(2 \mathrm{Cr_{2}O_{3}}+3 \mathrm{Si} \rightarrow 4 \mathrm{Cr}+3 \mathrm{SiO_{2}}\) Oxidation: Si in \(\mathrm{Si}\) goes from 0 to +4, losing electrons - reducing agent Reduction: Cr in \(\mathrm{Cr_{2}O_{3}}\) goes from +3 to 0, gaining electrons - oxidizing agent So, the final solutions are: (a) Not an electron-transfer reaction (b) Electron-transfer reaction; \(\mathrm{H_{2}}\) is the reducing agent and \(\mathrm{Cl_{2}}\) is the oxidizing agent. (c) Electron-transfer reaction; \(\mathrm{Si}\) is the reducing agent and \(\mathrm{Cr_{2}O_{3}}\) is the oxidizing agent. (d) Not an electron-transfer reaction