AIIMS 2006 Chemistry Standard Electrode Potentials Assertion Reason Question

To analyze the assertion and reason provided in the question, we need to delve into the concepts of standard electrode potentials and ionization energies.

Assertion Analysis

Assertion: $E^\circ$ for $\text{Mn}^{3+}/\text{Mn}^{2+}$ is more positive than $\text{Cr}^{3+}/\text{Cr}^{2+}$.

This statement is true. The standard electrode potential ($E^\circ$) reflects the tendency of a species to be reduced. In the case of manganese ($\text{Mn}^{3+}/\text{Mn}^{2+}$), it has a more positive $E^\circ$ value compared to chromium ($\text{Cr}^{3+}/\text{Cr}^{2+}$). This can be attributed to the electronic configuration and stability of the ions involved. Manganese, being in the d-block, has a half-filled d-subshell in its +3 oxidation state, leading to greater stability and hence a higher tendency to gain an electron compared to chromium.

Reason Analysis

Reason: The third ionization energy of Mn is larger than that of Cr.

This statement is also true. The third ionization energy ($IE_3$) refers to the amount of energy required to remove the third electron from an atom. For manganese, the removal of the third electron leads to a more stable configuration, as it transitions from $[Ar] 3d^5$ to $[Ar] 3d^4$. In contrast, chromium’s removal of its third electron does not lead to a similarly stable arrangement as it goes from $[Ar] 3d^5 4s^1$ to $[Ar] 3d^4$.

Why the Correct Answer is B

While both the assertion and reason are true, the reason provided does not directly explain why the assertion holds. The higher positive value of $E^\circ$ for $\text{Mn}^{3+}/\text{Mn}^{2+}$ compared to $\text{Cr}^{3+}/\text{Cr}^{2+}$ is mainly influenced by the stability of the resulting ions and their respective electronic configurations rather than solely by the third ionization energy differences. Therefore, the correct choice is B: both the assertion and reason are true, but the reason is not the correct explanation of the assertion.

Why Other Options are Incorrect

• Option A: Incorrect because, while both statements are true, the reason does not adequately explain the assertion.
• Option C: Incorrect because both the assertion and the reason are true.
• Option D: Incorrect because both statements are, in fact, true.

Summary

Understanding the relationship between standard electrode potentials and ionization energies is crucial in electrochemistry. The assertion is supported by the electronic stability of the ions involved, while the reason, although true, does not provide a direct link to the assertion’s validity. This nuanced understanding is key when tackling similar problems in NEET/JEE contexts.