Question: 75. The \mathrm{E}^{\circ} value for the \mathrm{Mn}^{3+} / \mathrm{Mn}^{2+} couple is more positive than that of \mathrm{Cr}^{3+} / \mathrm{Cr}^{2+} or \mathrm{Fe}^{3+} / \mathrm{Fe}^{2+} due to change of
(1) \mathrm{d}^{5} to \mathrm{d}^{4} configuration
(2) d^{5} to d^{2} configuration
(3) d^{4} to d^{5} configuration
(4) d^{3} to d^{5} configuration
Answer: Option (3)
Explanation:
The standard electrode potential depends on the relative stability of oxidation states,
which is influenced by electronic configuration.
For manganese, the electronic configuration changes as follows:
\mathrm{Mn}^{3+} : 3d^{4} \mathrm{Mn}^{2+} : 3d^{5}The d^{5} configuration is exceptionally stable due to half-filled subshell stability,
which provides maximum exchange energy and symmetrical distribution of electrons.
During the reduction of \mathrm{Mn}^{3+} to \mathrm{Mn}^{2+},
the configuration changes from d^{4} to the more stable d^{5}.
This large increase in stability makes the reduction highly favorable.
As a result, the \mathrm{E}^{\circ} value for the \mathrm{Mn}^{3+} / \mathrm{Mn}^{2+} couple is more positive compared to similar couples of chromium and iron.
Therefore, the higher standard electrode potential is due to the change from d^{4} to d^{5} configuration.
Hence, the correct answer is Option (3).