Identify the reducing agent in the following reactions:
$(a)$ $4 NH_{3} + 5 O_{2} \longrightarrow 4 NO + 6 H_{2}O$
$(b)$ $H_{2}O + F_{2} \longrightarrow HF + HOF$
$(c)$ $Fe_{2}O_{3} + 3 CO \longrightarrow 2 Fe + 3 CO_{2}$
$(d)$ $2 H_{2} + O_{2} \longrightarrow 2 H_{2}O$

(N/A) In $4 NH_{3} + 5 O_{2} \longrightarrow 4 NO + 6 H_{2}O$,the oxidation state of nitrogen in $NH_{3}$ increases from $-3$ to $+2$. Thus,$NH_{3}$ is the reducing agent.
$(b)$ In $H_{2}O + F_{2} \longrightarrow HF + HOF$,the oxidation state of oxygen in $H_{2}O$ increases from $-2$ to $0$ (in $HOF$). Thus,$H_{2}O$ is the reducing agent.
$(c)$ In $Fe_{2}O_{3} + 3 CO \longrightarrow 2 Fe + 3 CO_{2}$,the oxidation state of carbon in $CO$ increases from $+2$ to $+4$. Thus,$CO$ is the reducing agent.
$(d)$ In $2 H_{2} + O_{2} \longrightarrow 2 H_{2}O$,the oxidation state of hydrogen in $H_{2}$ increases from $0$ to $+1$. Thus,$H_{2}$ is the reducing agent.
$A$ reducing agent is a substance that donates electrons or increases its own oxidation state during a chemical reaction.