Assertion : $E^{\circ}$ for $\mathrm{Mn}^{3+} / \mathrm{Mn}^{2+}$ is more positive than $\mathrm{Cr}^{3+} /…
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 $\mathrm{Cr}$.
If both assertion and reason are true and reason is the correct explanation of the assertion
If both assertion and reason are true but reason is not the correct explanation of the assertion
If assertion is true, but reason is false
Both 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 $\mathrm{Cr}$.