Explain the following :
(a) Out of `Sc^(3+), Co^(2+) " and "Cr^(3+) "ions, only "Sc^(3+)` is colourless in aqueous solutions. (Atomic no.: Co =27, Sc = 21 and Cr =24)
(b) The `E^(@)Cu^(2+)//Cu` for copper metal is positive (+0.34), unlike the remaining members of the first transition series.
(c) `La(OH)_(3) " is more basic than Lu "(OH)_(3).`

Explanation

(a) `Co^(2) : [Ar]3d^(7) " " Sc^(3+) : [Ar]3d_^(0) " " Cr^(3+) :[Ar]3d^(3)`
`Co^(2+) " and "Cr^(3+)` have unpaired electrons. Thus, they are coloured in aqueous solution. `Sc^(3+)` has no unpaired electron. Thus, it is colourless.
(b) Metal copper has high enthalpy of atomisation and enthalpy of ionisation.
Therefore the high energy required to convert `Cu(s) " to "Cu^(2+)(aq)` is not balanced by its hydration enthalpy.
(c) Due to lanthanoid contraction the size of lanthanoid ion decreases regularly with increases in atomic size. Thus, covalent character between lanthanoid ion and `OH^(-) " increases from "La^(3+) "to "Lu^(3+)`. Thus, the basic character of hydroxides decreases from `La(OH)_(3) " to" Lu(OH)_(3).`