Question

In which of the following reactions, hydrogen is acting as an oxidizing agent?

• A. With $Li$ to form $LiH$
• B. With $I_{2}$ to give $HI$
• C. With $S$ to give $H_{2}S$
• D. None of the above

Answer 1

Answer: (A)

Oxidising agent is one which undergoes reduction. Decreasing in oxidation number shows reduction. Find oxidation number in all given reactions to decide in which of them hydrogen is oxidising agent.
(a) $Li +\overset{0}{ H }_{2} \longrightarrow \overset{+1}{2 Li }\, \overset{-1}{ H}$
Oxidation number of hydrogen is decreasing from 0 to $-1$.
So $H _{2}$ is acting as oxidising agent in this reaction.
(b) $\overset{0}{H} _{2}+ I _{2} \longrightarrow \overset{+}{2HI}$
$\because$ Oxidation number of hydrogen is increasing. $H _{2}$ is not acting as oxidising agent in this reaction.
(c) $\overset{0}{ H _{2}}+ S \longrightarrow \overset{+1}{H} _{2} S$
$\because$ Oxidation number of hydrogen is increasing
$\therefore H_{2}$ is not acting as oxidising agent in this reaction.