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  2. Comment on each of the following observations :(a)The mobilities of the alkali metal ions in aqueous solution are Li+ < Na+ < K+ < Rb+ < Cs+

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Comment on each of the following observations :(a)The mobilities of the alkali metal ions in aqueous solution are Li+ < Na+ < K+ < Rb+ < Cs+

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.

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R Rakesh

(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^{+}<Na^{+}<K^{+}<Rb^{+}<Cs^{+}

(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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