Question

Classify each of the following half-reactions as oxidation or reduction. (a) \(\mathrm{TiO}_{2}(s) \longrightarrow \mathrm{Ti}^{3+}(a q)\) (b) \(\mathrm{Zn}^{2+}(a q) \longrightarrow \mathrm{Zn}(s)\) (c) \(\mathrm{NH}_{4}^{+}(a q) \longrightarrow \mathrm{N}_{2}(g)\) (d) \(\mathrm{CH}_{3} \mathrm{OH}(a q) \longrightarrow \mathrm{CH}_{2} \mathrm{O}(a q)\)

Short answer

Question: Classify each of the following half-reactions as oxidation or reduction: (a) \(\mathrm{TiO}_{2}(s) \longrightarrow \mathrm{Ti}^{3+}(a q)\) (b) \(\mathrm{Zn}^{2+}(a q) \longrightarrow \mathrm{Zn}(s)\) (c) \(\mathrm{NH}_{4}^{+}(a q) \longrightarrow \mathrm{N}_{2}(g)\) (d) \(\mathrm{CH}_{3} \mathrm{OH}(a q) \longrightarrow \mathrm{CH}_{2}\mathrm{O}(a q)\) Answer: (a) Reduction (b) Reduction (c) Oxidation (d) Oxidation

Step-by-step solution

Half-reaction (a)

For the half-reaction \(\mathrm{TiO}_{2}(s) \longrightarrow \mathrm{Ti}^{3+}(a q)\), first determine the oxidation states of both titanium species. In \(\mathrm{TiO}_{2}(s)\), titanium has an oxidation state of +4, whereas, in \(\mathrm{Ti}^{3+}(a q)\), the oxidation state is +3. Since the oxidation state is decreasing from +4 to +3, this indicates that titanium is gaining an electron. Thus, this half-reaction is a reduction.

Half-reaction (b)

In the half-reaction \(\mathrm{Zn}^{2+}(a q) \longrightarrow \mathrm{Zn}(s)\), we have zinc going from an oxidation state of +2 in \(\mathrm{Zn}^{2+}(a q)\) to an oxidation state of 0 in \(\mathrm{Zn}(s)\). The decrease in oxidation state indicates that electrons are being gained, so this half-reaction is a reduction.

Half-reaction (c)

For the half-reaction \(\mathrm{NH}_{4}^{+}(a q) \longrightarrow \mathrm{N}_{2}(g)\), let's find the oxidation state of nitrogen in each species. In \(\mathrm{NH}_{4}^{+}(a q)\), nitrogen has an oxidation state of -3. In \(\mathrm{N}_{2}(g)\), nitrogen has an oxidation state of 0. The oxidation state of nitrogen increases from -3 to 0, which means electrons are being lost. Consequently, this half-reaction is an oxidation.

Half-reaction (d)

In the final half-reaction \(\mathrm{CH}_{3} \mathrm{OH}(a q) \longrightarrow \mathrm{CH}_{2}\mathrm{O}(a q)\), we need to assess the oxidation state of carbon. In \(\mathrm{CH}_{3} \mathrm{OH}(a q)\), carbon has an oxidation state of -2. In \(\mathrm{CH}_{2}\mathrm{O}(a q)\), carbon has an oxidation state of 0. The oxidation state increases from -2 to 0, indicating that electrons are being lost. Therefore, this half-reaction is an oxidation. In conclusion, we have classified each half-reaction as follows: (a) Reduction (b) Reduction (c) Oxidation (d) Oxidation