Question
The $E^{\circ}$ value for the ${Mn}^{3 +}/{Mn}^{2 +}$ couple is more positive than that of ${Cr}^{3 +}/{Cr}^{2 +}$ or ${Fe}^{3 +}/{Fe}^{2 +}$ due to change of
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Solution
Sol. [Key Concept: Stability of Mn²⁺ and reduction potential]
$E_{{Mn}^{3 +}/{Mn}^{2 +}}^{\circ} > E_{{Cr}^{3 +}/{Cr}^{2 +}}^{\circ}$ or $E_{{Fe}^{3 +}/{Fe}^{2 +}}^{\circ}$
Electronic configuration of ${Mn}^{3 +} = \lbrack Ar\rbrack 3d^{4}$
Electronic configuration of ${Mn}^{2 +} = \lbrack Ar\rbrack 3d^{5}$
Electronic configuration of ${Cr}^{3 +} = \lbrack Ar\rbrack 3d^{3}$
Electronic configuration of ${Cr}^{2 +} = \lbrack Ar\rbrack 3d^{4}$
As ${Mn}^{3 +}$ from $d^{4}$ configuration goes to more stable $d^{5}$ configuration (Half filled), due to more exchange energy in $d^{5}$ configuration
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