Specify which of the following are oxidation–reduction reactions, and identify the oxidizing agent, the reducing agent, the substance being oxidized, and the substance being reduced. a. $\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(a q) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)$ b. $\mathrm{HCl}(g)+\mathrm{NH}_{3}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)$ c. $\mathrm{SiCl}_{4}(i)+2 \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 4 \mathrm{HCl}(a q)+\mathrm{SiO}_{2}(s)$ d. $\mathrm{SiCl}_{4}(l)+2 \mathrm{Mg}(s) \rightarrow 2 \mathrm{MgCl}_{2}(s)+\mathrm{Si}(s)$ e. $\mathrm{Al}(\mathrm{OH})_{4}-(a q) \rightarrow \mathrm{AlO}_{2}^{-}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)$

In order to determine if an oxidation-reduction reaction has occurred, we must first find the oxidation states of each element in the reactants and products. Remember that the oxidation state is the positive or negative number assigned to an atom to indicate its degree of oxidation or reduction.

If the oxidation state of an element changes during the reaction, this implies a transfer of electrons and thus an oxidation-reduction reaction. a. \(\mathrm{Cu}(s)+2 \mathrm{Ag}^{+}(a q) \rightarrow 2 \mathrm{Ag}(s)+\mathrm{Cu}^{2+}(a q)\) Oxidation states: Cu: 0 -> +2, Ag: +1 -> 0 Cu is oxidized, and Ag is reduced. The reaction is an oxidation-reduction reaction. The oxidizing agent is Ag+, the reducing agent is Cu, the substance being oxidized is Cu, and the substance being reduced is Ag+. b. \(\mathrm{HCl}(g)+\mathrm{NH}_{3}(g) \rightarrow \mathrm{NH}_{4} \mathrm{Cl}(s)\) Oxidation states: H: +1 -> +1, Cl: -1 -> -1, N: -3 -> -3, H in NH3: +1 -> +1 No change in oxidation states. This reaction is not an oxidation-reduction reaction. c. \(\mathrm{SiCl}_{4}(i)+2 \mathrm{H}_{2} \mathrm{O}(l) \rightarrow 4 \mathrm{HCl}(a q)+\mathrm{SiO}_{2}(s)\) Oxidation states: Si: +4 -> +4, Cl: -1 -> -1, H: +1 -> +1, O: -2 -> -2 No change in oxidation states. This reaction is not an oxidation-reduction reaction. d. \(\mathrm{SiCl}_{4}(l)+2 \mathrm{Mg}(s) \rightarrow 2 \mathrm{MgCl}_{2}(s)+\mathrm{Si}(s)\) Oxidation states: Si: +4 -> 0, Cl: -1 -> -1, Mg: 0 -> +2 Si is reduced, and Mg is oxidized. The reaction is an oxidation-reduction reaction. The oxidizing agent is SiCl4, the reducing agent is Mg, the substance being oxidized is Mg, and the substance being reduced is SiCl4. e. \(\mathrm{Al}(\mathrm{OH})_{4}-(a q) \rightarrow \mathrm{AlO}_{2}^{-}(a q)+2 \mathrm{H}_{2} \mathrm{O}(l)\) Oxidation states: Al: +3 -> +3, O in OH: -2 -> -2, H: +1 -> +1, O in AlO2: -2 -> -2 No change in oxidation states. This reaction is not an oxidation-reduction reaction.