# 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) E 0 for M2+ (aq) + 2e– → M(s) (where M = Ca, Sr or Ba) is nearly constant.

(a) As we know that down the group ionic size increases. Lithium-ion is the smallest in size and we know smaller the size of ion, the more hydrated it is. So lithium ion is most hydrated and $Cs^{+}$ion is bigger in size so it is least hydrated.

Greater the size of hydrated ion less is mobility. So that the order of mobility of ions $Li^{+}

(b) Lithium unlike other alkali metals direct react with nitrogen and forms lithium nitride ($Li_{3}N$). This is because lithium-ion is the smallest in size and therefore it has very high size compatibility with $N^{3-}$. Hence the lattice energy released is very high.

(c)   Electrode potential ($E^{0}$) for $M^{2+}$ is depend on mainly three-factor- (i)ionisation enthalpy (ii)hydration enthalpy (iii)enthalpy of vaporisation.  But we consider the overall effect of these three factors that should be the same for all $Ca,Sr,Ba$. Hence their electrode potential is constant.

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