(a) Give reasons for the following:
(i) `Mn^(3+)` is a good oxidising agent.
(ii) `E^(@)_(M^(2+)//M)` values are not regular for first row transition metals (3d series).
(iii) Although 'F' is more electronegative than 'O', the highest Mn fluoride is `MnF_(4)`, whereas the highest oxide is `Mn_(2)O_(7)`.
(b) Complete the following equations :
(i) `2CrO_(4)^(2-)2H^(+)to`
(ii) `KMnO_(4)overset("heat")to`

Explanation

(a) (i) Electronic configuration of `Mn^(3+)` is `d^(4)` in the outer shell. It acts as oxidising agent or it gets reduced to `Mn^(2+)`. In doing so its configuration changes to do which is a half-filled stable configuration.
(ii) `E^(@)_(M^(2+)//M)` values are not regular for first row transition metals (3d series). The values of `E^(@)` for Mn, Ni and Zn are more negative than expected from the trend. The stability of the half-filled d sub-shell in `Mn^(2+)` and the completely filled `d^(10)` configuration of `Zn^(2+)` are related to their `E^(@)` values. `E^(@)` for Ni is accounted for by the highest negative `Delta_(hyd)H^(@)`.
(iii) The ability of O to stabilise these high oxidation states exceeds that of fluorine. Thus, the highest Mn fluoride is `MnF_(4)` whereas the highest oxide is `Mn_(2)O_(7)`. The ability of oxygen to form multiple bonds to metals explains this. In the covalent oxide `Mn_(2)O_(7)`, each Mn is tetrahedrally surrounded by O.s including a Mn - O - Mn bridge.
(b) (i) `2CrO_(4)^(2-)+2H^(+)+Cr_(2)O_(7)^(2-)+H_(2)O`
(ii) `2KMnO_(4)overset(Heat)toK_(2)MnO_(4)+MnO_(2)+O_(2)`