Why is the $E^{⊖}$ 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.

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Solution

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

Because $M n^{3 +}$ has the outer electronic configuration of $3 d^{4}$ and $M n^{2 +}$ has the outer electronic configuration of $3 d^{5}$ . Thus, the conversion of $M n^{3 +}$ to $M n^{2 +}$ will be a favourable reaction since $3 d^{5}$ 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.

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

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

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

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