Comment on each of the following observation:
a. The mobilities of the alkali metal ions in aqueous solution are `Li^(o+)ltNa^(o+)ltK^(o+)ltRb^(o+)ltCs^(o+)`.
b. Lithium is the only alkali metal to form a nitride directly.
c. `E^(ө)` for `M_(aq)^(2+)+2e^(-)to M_((s))` (where M=Ca,Sr or Ba) is nearly constant.

Explanation

The alkali metal ions are highly hydrated. The smaller the size of the ion, the greater is the degree of hydration. Thus, `Li^(+)` ion gets much more hydrated than `Na^(+)` ion which is more hydrated than `K^(+)` ion and so on. Therefore, the extent of hydration decreases from `Li^(+)` to `Cs^(+)`. As a result of larger hydration of `Li^(+)` ion than `Na^(+)` ion the effective size of `Li^(+)` ion is more than that of `Na^(+)` ion and ionic radii in water decrease as:
`Li^(+) gt Na^(+) gt K^(+) gt Rb^(+) gt Cs^(+)`
Therefore, the mobility of `Li^(+)` ion is slowest and that of `Cs^(+)` ion is maximum. Thus, mobility of ionsin water increases as:
`Li^(+) lt Na^(+) lt K^(+) lt Rb^(+) lt Cs^(+)`
(c) `E^(@)` value of `M^(2+)//M(s)` electrode depends upon three factors :
(i) enthalpy of vaporisation
(ii) ionization enthalpy and
(iii) enthalpy of hydration.The combined effect of these factors is approximately the same for Ca, Sr and Ba so that their electrode potential are nearly same .