JEE Main 2024 — Redox Reactions Question with Solution

When a salt reacts with $${\mathrm{MnO}}_2$$ and concentrated $${\mathrm{H}}_2{\mathrm{SO}}_4$$, the type of gas evolved depends on the anion present in the salt. In this case, a greenish yellow gas is evolved, which indicates the formation of chlorine gas ($${\mathrm{Cl}}_2$$). $${\mathrm{Cl}}_2$$ gas is greenish-yellow in color and is typically produced from halide salts (namely chlorides, bromides, or iodides) when they are oxidized. Among the given options, we need to identify a salt that contains a halide which can produce $${\mathrm{Cl}}_2$$ upon reaction with $${\mathrm{MnO}}_2$$ and concentrated $${\mathrm{H}}_2{\mathrm{SO}}_4$$.

Option A ($${\mathrm{NH}}_4\mathrm{Cl}$$) contains chloride ions, and when heated with $${\mathrm{MnO}}_2$$ and concentrated $${\mathrm{H}}_2{\mathrm{SO}}_4$$, it can undergo a redox reaction where the chloride ions are oxidized to $${\mathrm{Cl}}_2$$ gas. The corresponding reaction can be represented as follows:

$${\mathrm{MnO}}_2+4\mathrm{HCl}⟶{\mathrm{MnCl}}_2+{\mathrm{Cl}}_2+2{\mathrm{H}}_2\mathrm{O}$$

Option B ($${\mathrm{CaI}}_2$$) contains iodide ions, which would lead to the liberation of iodine or $${\mathrm{I}}_2$$, not a greenish yellow gas.

Option C ($${\mathrm{KNO}}_3$$) contains nitrate ions and does not produce a halogen gas upon reaction with $${\mathrm{MnO}}_2$$ and $${\mathrm{H}}_2{\mathrm{SO}}_4$$.

Option D ($$\mathrm{NaBr}$$) contains bromide ions which would lead to the formation of bromine ($${\mathrm{Br}}_2$$), a reddish-brown gas, not greenish yellow.

Therefore, the correct answer is Option A ($${\mathrm{NH}}_4\mathrm{Cl}$$), since it is the salt that can produce a greenish yellow gas ($${\mathrm{Cl}}_2$$) when reacted with $${\mathrm{MnO}}_2$$ and concentrated $${\mathrm{H}}_2{\mathrm{SO}}_4$$.