Question

In which reaction is hydrogen acting as an oxidising agent?
A.With iodine to give hydrogen iodide
B.With lithium to give lithium hydride
C.With nitrogen to give ammonia
D.With sulphur to give hydrogen sulphide

Answer 1

An oxidizing agent can be basically explained as a reactant in a redox reaction, that causes other substances to lose electrons, and then accept these electrons for itself. A reducing agent performs the exact opposite operation to this. A reducing agent itself loses its electrons and makes them available for getting accepted by other reactants.

Complete step by step answer:
Before we move towards the solution of this question, let us first understand some basic concepts.
Now, hydrogen is a very special element. It has only one electron in its 1s orbital. This orbital has the capacity for holding only two electrons, so it is half filled in this case. Now, while forming compounds with other elements, hydrogen either loses this electron to get more stable, or it gains an electron to completely fill its valence shell. Because of this behaviour exhibited by hydrogen, we can that it acts as both an oxidising agent and a reducing agent.
The nature of the reagent can be determined by the change in its oxidation state. If the oxidation number of a substance reduces, then it has undergone reduction and hence is an oxidising agent. From all the options mentioned above, the oxidation state of hydrogen reduces in only one case.
While reacting with lithium, the oxidation state of hydrogen changes from (0) to (-1). Hence, it acts as an oxidising agent in this case. The reaction for this can be given as:
$2Li + 3{H_2} \to 2LiH{}_3$

Hence, Option B is the correct option.

Note:
One simple way for understanding the nature of a reagent, remember this: oxidising agents themselves undergo reduction, while reducing agents themselves undergo oxidation.