Q88 / 180 · AIIMS 2006 · Chemistry
Assertion : $E^{\circ}$ for $\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}$ is more positive than $\mathrm{Cr}^{3+} / \mathrm{Cr}^{2+}$.
Reason : The third ionization energy of $\mathrm{Mn}$ is larger than that of $\mathrm{Cr}$.
- AIf both assertion and reason are true and reason is the correct explanation of the assertion
- Correct answerBIf both assertion and reason are true but reason is not the correct explanation of the assertion
- CIf assertion is true, but reason is false
- DBoth assertion and reason are false statements.
Solution
It is known that the value of $E^{\circ}$ becomes more negative down the series. As the chromium is present below in electrochemical series than $\mathrm{Mn}$, so $\mathrm{Mn}$ has more positive $E^{\circ}$ value than $\mathrm{Cr}$.
$$
\begin{array}{ll}
\mathrm{Mn} \rightarrow 3 d^5 4 s^2, & \mathrm{Mn}^{2+} \rightarrow 3 d^5 4 s^0 \\
\mathrm{Cr} \rightarrow 3 d^5 4 s^1, & \mathrm{Cr}^{2+} \rightarrow 3 d^4 4 s^0
\end{array}
$$
For elimination of one electron from $\mathrm{Mn}^{2+}$, more energy is required due to its stable configuration. That is why third ionization energy of $\mathrm{Mn}$ is larger than $\mathrm{Cr}$.
Hence, the correct answer is option: 'B'.