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Question

Give reasons.
$E^{o}$ value for $M n^{3 +} / M n^{2 +}$ couple is much more positive than that for $F e^{3 +} / F e^{2 +}$ .

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Solution

$M n^{3 +} + e^{-} \to M n^{2 +}$
Outer electronic configuration of $M n^{3 +}$ is $3 d^{4}$ .
Outer electronic configuration of $M n^{2 +}$ is $3 d^{5}$ . $M n^{3 +} + e^{-} \to M n^{2 +}$ corresponds to $3 d^{4} \to 3 d^{5}$ . $3 d^{5}$ is half filled and stable. Thus, in this half cell, less stable electronic configuration is converted to more stable electronic configuration. So this half reaction has more tendency to occur.
$F e^{3 +} + e^{-} \to F e^{2 +}$
Outer electronic configuration of $F e^{3 +}$ is $3 d^{5}$ .
Outer electronic configuration of $F e^{2 +}$ is $3 d^{6}$ . $F e^{3 +} + e^{-} \to F e^{2 +}$ corresponds to $3 d^{5} \to 3 d^{6}$ . Thus, in this half cell, more stable electronic configuration is converted to less stable electronic configuration.
So this half reaction has less tendency to occur.
Hence, $E^{o}$ value for $M n^{3 +} / M n^{2 +}$ couple is much more positive than that for $F e^{3 +} / F e^{2 +}$ .

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