Question

Why is the $E^{\ominus }$ value for the $M{n}^{3+}/M{n}^{2+}$ couple much more positive than that for $C{r}^{3+}/C{r}^{2+}$ or $F{e}^{3+}/F{e}^{2+}$? Explain.

Answer 1

$\text{Higher}{E}^0\text{value for}M{n}^{3+}/M{n}^{2+}$

Because $M{n}^{3+}$ has the outer electronic configuration of $3d^4$ and $M{n}^{2+}$ has the outer electronic configuration of $3d^5$. Thus, the conversion of $M{n}^{3+}$ to $M{n}^{2+}$ will be a favourable reaction since $3d^5$ is a very stable configuration as it is half filled configuration.
Hence, $E^0$ value for $M{n}^{3+}/M{n}^{2+}$ couple is positive.

$\text{Lower}{E}^0\text{value for}C{r}^{3+}/C{r}^{2+}\text{or}F{e}^{3+}/F{e}^{2+}$

$C{r}^{3+}toC{r}^{2+}$ undergoes a change in outer electronic configuration from $3d^3$ to $3d^4$. $F{e}^{3+}toF{e}^{2+}$ undergoes a change in outer electronic configuration from $3d^5$ to $3d^6$.

Both these configurations of the resultant are not stable and hence have a lower $E^0$ value.