(a) Complete the following chemical equations :
(i) `MnO_(4)^(-) (aq) + S_(2)O_(3)^(2-) (aq) + H_(2)O(l) to `
(ii) ` Cr_(2)O_(7)^(2-)(aq)+Fe^(2+) (aq) + H^(+) (aq) to `
(b) Explain the following observatons :
(i) `La^(3+) (Z = 57) and Lu^(3+) (Z = 71)` do not show any colour in solutions.
(ii) Among the divalent cations in the first series of transition elements, manganese exhibits the maximum paramagnetism.
(iii) `Cu^(+)` ion is not known in aqueous solutions.

Explanation

(a) (i) `8 MnO_(4)^(-)(aq)+ 3 S_(2)O_(3)^(2-) (aq) + H_(2)O(l) to 8 MnO_(2) (s) + 6 SO _(4)^(2-) (aq) + 2OH^(-) (aq)`
(ii) `Cr_(2)O_(7)^(2-)(aq)+6 Fe^(2+) (aq) + 14 H^(+) (aq) to 2 Cr^(3+) (aq) + 6 Fe^(3+) (aq) + 7 H_(2) O (l) `
(b) (i) It is due to obsence of unpaired electrons, `(e^(-)s)` they cannot undergo f - f transion.
(ii) ` Mn^(2+)` has maximum number of unpaired electrons
(iii) Since` Cu^(+)` in aqueous solution undergoes disproportionation i.e.
` 2 Cu^(+) (aq ) to Cu^(2+) (aq ) + Cu(s)`
` Cu^(2+)`(aqu) has more negative hydration energy which can over come ` 2nd` I. E energy.