Question

Account for the following :
(I) The E$^\circ_{Mn^{2+}/Mn}$ value for manganese is highly negative, whereas E$^\circ_{Mn^{3+}/Mn^{2+}}$ is highly positive.

Hint:

Stable half-filled subshell → unusual E$^\circ$ values.

Answer 1

Manganese has stable half-filled d$^5$ configuration in Mn$^{2+}$ (3d$^5$).
Removing an electron to form Mn$^{3+}$ breaks this stable configuration → requires more energy.
Thus, Mn$^{2+}$ to Mn is difficult (highly negative E$^\circ$) but Mn$^{3+}$ to Mn$^{2+}$ is favourable (high E$^\circ$).