Question

Comment on each of the following observations :
(a) The mobilities of the alkali metal ions in aqueous solution are `Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)`.
(b) Lithium is the only alkali metal which forms nitride directly .
(c) `E^(@)` for `M^(2+) (aq) + 2e^(-) to M(s) ` (where M = Ca , Sr or Ba) is nearly constant .

Answer 1

(a) Smaller the size of the ion , more highly it is hydrated and hence greater is the mass of the hydrated ion and hence lower is its ionic mobility . Since the extent of hydration decreases in the order :
`Li^(+) gt Na^(+) gt K^(+) gt Rb^(+) gt Cs^(+)`
therefore , ionic mobility increases in the reverse order :
`Li^(+) lt Na^(+) lt K^(+) lt RB^(+) lt Cs^(+)`
(b) Due to samll size, `Li^(+)` has high polarising power and hence it strongly attracts the larger negatively charged nitride ion `(N_(3)^(-))` towards itself . As a result , lattice energy of `Li_(3)N` is high and due to this high lattice energy `Li_(3)N` is easily formed.
The other alkali metals do not form their corresponding nitrides because their lattice energies are not only low but also decrease further as their sizes increase from Na to Cs.
`6Li(s)+N_(2)(g) overset (Delta) to 2 Li_(3)N (s)`
(c) `E^(@)` of any `M^(2+)//M` electrode depends upon three factors :(i) enthalpy of vapourization,(ii) ionization enthalpy , (iii) enthalpy of hydration . Since the combined effect of these factors is approximately the same for Ca , Sr and Ba , therefore , their electrode potential are nearly constant .