Give reason for the folloiwng :
(a) `E_("value")^(@)` for `Mn^(3+)//Mn^(2+)` couple is much more positive than that of `Fe^(3+)//Fe^(2+)` couple.
(b) Iron has higher enthalpy of atomization then that of copper.
(c) `Sc^(3+)` is colourless in aqueous solution whereas `Ti^(3+)` is coloured.

Explanation

(a) Reduction potential of `Mn^(3+)//Mn^(2+)` is more positive because `Mn^(2+)` is `3d^(5)` which is half filled and hence stable , but reduction potential of `Fe^(3+)//Fe^(2+)` is less positive because `Fe^(2+)` is `3d^(6)` and `Fe^(3+)` is `3d^(5)` which is more stable due to half filled configuration . Hence `Fe^(3+)//Fe^(2+)` has lower reduction potential value .
(b) Enthalpy of atomization depends upon strength of metallic bond which further depends on no. of unpaired electrons . Greater the no. of unpaired electrons stronger will be the metallic bond . In case of Fe(26) , 4 unpaired electrons are there while in copper , there is only 1 unpaired electron . Hence , iron has higher enthalpy of atomization than that of copper .
(c) `Sc^(3+)` is `3d^(@)` , no unpaired electron in valence shell . Hence , it is colourless in aqueous solution where as `Ti^(3+)` is `3d^(1)` , 1 unpaired electron . Due to d-d transition (from `t^(2)` g to eg ) , `Ti^(3+)` is coloured .