NCERT Class XI Chemistry The s-Block Elements Solutions

Question : 26

Total: 32

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) E° for M(aq)2++2e– → M(s) (where M = Ca, Sr or Ba) is nearly constant.

Solution:

(a) This is attributed to the hydration of the cation in water. As a result, size of the cation increases and its mobility decreases. Due to the smallest size, Li+ ion is hydrated to the maximum and has least mobility while Cs+ ion due to least hydration has maximum mobility.
(b) Lithium is a very strong reducing agent. As a result, it directly combines with nitrogen to form its nitride (Li3N).
(c) E° of any M2+/M electrode depends upon three factors : (i) enthalpy of vaporisation, (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 potentials are nearly constant.

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