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Standard Electrode Potentials

Account for t...

Question

Account for the following:
(i) In the series Sc to Zn, the enthalpy of atomisation of zinc is the lowest.
(ii) $E^{o}$ value for the $M n^{3 +} / M n^{2 +}$ couple is much more positive than that for $C r^{3 +} / C r^{2 +}$ .

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Solution

(i) Enthalapy of atomisation of zin C is lowest i.e., $126 k J m o l^{- 1}$
Due to the absence of these unpaired electrons, the inter atomic electron bonding is the weakest in Zn and as a result it has least enthalapy of atomisation
(ii) $M n^{+ 3} + e^{-} \to M n^{+ 2}$
$[A r] 3 d^{4} 4 s^{o} [A r] 3 d^{5} 4 s^{o} E_{1}^{o}$
$C r^{+ 3} + e^{-} \to C r^{+ 2}$
$[A r] 3 d^{3} 45^{o} [A r] 3 d^{4} 4 s^{o} E_{2}^{o}$
Due to half filled stability in $M n^{+ 2}$ $E_{1}^{o}$ is more $+ V e$ than $E_{2}^{o}$

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