JEE Main 2024 — D And F Block Elements Question with Solution

To determine the correct answer to this question, let's analyze the catalysis mechanism involving Iron (III) in the reaction between iodide ($${\mathrm{I}}^{-}$$) and persulfate ($${\mathrm{S}}_2{\mathrm{O}}_8^{2-}$$) ions.

Iron (III), or $${\mathrm{Fe}}^{3+}$$, can act as a catalyst by undergoing redox reactions, cycling between $${\mathrm{Fe}}^{2+}$$ and $${\mathrm{Fe}}^{3+}$$. Here’s how it typically works:

1. $${\mathrm{Fe}}^{3+}$$ can oxidize iodide ions to iodine. The reaction will proceed as follows:

$$2F{\mathrm{e}}^{3+}+2{\mathrm{I}}^{-}\to 2F{\mathrm{e}}^{2+}+{\mathrm{I}}_2$$

2. $${\mathrm{Fe}}^{2+}$$, now formed, can be oxidized back to $${\mathrm{Fe}}^{3+}$$ by reducing persulfate ions. The reaction will be:

$$2F{\mathrm{e}}^{2+}+{\mathrm{S}}_2{\mathrm{O}}_8^{2-}\to 2F{\mathrm{e}}^{3+}+2S{\mathrm{O}}_4^{2-}$$

Based on this mechanism, $${\mathrm{Fe}}^{3+}$$ is responsible for oxidizing the iodide ions, and $${\mathrm{Fe}}^{2+}$$ is responsible for reducing the persulfate ions. Thus, we can conclude:

A. $${\mathrm{Fe}}^{3+}$$ oxidises the iodide ion: This statement is correct.

B. $${\mathrm{Fe}}^{3+}$$ oxidises the persulphate ion: This statement is incorrect because $${\mathrm{Fe}}^{3+}$$ does not oxidize persulfate ions.

C. $${\mathrm{Fe}}^{2+}$$ reduces the iodide ion: This statement is incorrect because $${\mathrm{Fe}}^{2+}$$ does not reduce iodide ions.

D. $${\mathrm{Fe}}^{2+}$$ reduces the persulphate ion: This statement is correct.

Therefore, the most appropriate answer is:

Option C: A and D only