Question

Question 10.26 Comment on each of the following observations:

(a) The mobilities of the alkali metal ions in aqueous solution are Li+ < Na+ < K+ < Rb+ < Cs+

(b) Lithium is the only alkali metal to form a nitride directly.

(c) (where M = Ca, Sr or Ba) is nearly constant.

Class XI The s-Block Elements Page 306

Answer 1

(a) we know, smaller the size , higher is the hydration energy.because A small ion can move faster.

The mobility of the alkali metal ions in aqueous solution are :
Li+ < Na+ < K+ < Rb+ <Cs+
due to smaller the size of the ion, more highly it is hydrated and greater the hydration of the ion, lower is its ionic mobility. Since the extent of hydration decreases in order.
Li+ >Na+ >K+ > Rb+> Cs+
Therefore , ionic mobility increases in the reverse order.
e.g., Li+ < Na+ < K+ < Rb+ < Cs+


(b) because of its smaller in size and diagonal relationship with Mg form Nitride directly.
6Li + N2 ---->2Li3N


(c) E°for M2+ +2e--->M
where M = Ca,Sr,Ba is almost same because,E°of any M2+/M electrode depends upon there factors :(i) enthalpy of vaporization (ii) Ionisation 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.