(a) Account for the following:
(i) `Cu^(+)` is unstable in an aqueous solution.
(ii) Transition metals form complex compounds.
(b) Complete the following equation:
`Cr_(2)O_(7)^(2-)+8H^(+)+3NO_(2)^(-)to`

Explanation

(a) (i) `Cu^(+)` is unstable in an aqueous solution. It changes to `Cu^(2+)`. This is due to much more negative `Delta_(hyd)H^(@)` of `Cu^(2+)` than `Cu^(+)`, which more than compensates the second ionisation enthalpy of Cu.
(ii) Transition metals form complex compounds due to comparatively smaller sizes of the metal ions, high ionic charges and the availability of d-orbitals for bond formation.
(b) `Cr_(2)O_(7)^(2-)+14H^(+)+6e^(-)to2Cr^(3+)+7H_(2)O`
`(3NO_(2)^(-)+3H_(2)O to3NO_(3)^(-)+6H^(+)+6e^(-))/(Cr^(2)O_(7)^(2-)+8H^(+)+3NO_(2)^(-)to2Cr^(3+)+3NO_(3)^(-)+4H_(2)O)`