(a) Identify the reducing agent in the following reactions:

(i) $4\mathrm{NH}_3 + 5\mathrm{O}_2 \rightarrow 4\mathrm{NO} + 6\mathrm{H}_2\mathrm{O}$

(ii) $\mathrm{H}_2\mathrm{O} + \mathrm{F}_2 \rightarrow \mathrm{HF} + \mathrm{HOF}$

(iii) $\mathrm{Fe}_2\mathrm{O}_3 + 3\mathrm{CO} \rightarrow 2\mathrm{Fe} + 3\mathrm{CO}_2$

(iv) $2\mathrm{H}_2 + \mathrm{O}_2 \rightarrow 2\mathrm{H}_2\mathrm{O}$

(b) Define a redox reaction in terms of gain or loss of oxygen.

(a)

(i) $\text{NH}_3$ (Ammonia): It undergoes oxidation by losing hydrogen and gaining oxygen to form $\text{NO}$, hence it acts as the reducing agent.

(ii) $\text{H}_2\text{O}$ (Water): It reduces fluorine ($\text{F}_2$) to $\text{HF}$ and $\text{HOF}$ while the oxygen in water is oxidized from an oxidation state of $-2$ to $0$, hence it acts as the reducing agent.

(iii) $\text{CO}$ (Carbon monoxide): It gains oxygen to form $\text{CO}_2$ (gets oxidized), hence it acts as the reducing agent.

(iv) $\text{H}_2$ (Hydrogen): It gains oxygen to form $\text{H}_2\text{O}$ (gets oxidized), hence it acts as the reducing agent.

(b) Redox Reaction: A chemical reaction in which one reactant undergoes oxidation (gains oxygen) and another reactant undergoes reduction (loses oxygen) simultaneously is called a redox reaction.