Question -
Answer -
(a) On moving down the alkaligroup, the ionic and atomic sizes of the metals increase. The given alkalimetal ions can be arranged in the increasing order of their ionic sizes as:
Li+ < Na+ < K+ < Rb+ < Cs+
Smaller the size of an ion, the more highlyis it hydrated. Since Li+ is the smallest, it gets heavilyhydrated in an aqueous solution. On the other hand, Cs+ is the largest and so itis the least hydrated. The given alkali metal ions can be arranged in thedecreasing order of their hydrations as:
Li+ > Na+ > K+ > Rb+ > Cs+
Greater the mass of a hydrated ion, thelower is its ionic mobility. Therefore, hydrated Li+ is the least mobileand hydrated Cs+ is the most mobile. Thus, the givenalkali metal ions can be arranged in the increasing order of their mobilitiesas:
Li+ < Na+ < K+ < Rb+ < Cs+
(b) Unlike the otherelements of group 1, Li reacts directly with nitrogen to form lithium nitride.This is because Li+ is very small in size and so its sizeis the most compatible with the N3– ion. Hence, thelattice energy released is very high. This energy also overcomes the highamount of energy required for the formation of the N3– ion.
(c) Electrode potential(E°) of any M2+/M electrode depends upon three factors:
(i)Ionisation enthalpy
(ii)Enthalpy of hydration
(iii)Enthalpy of vaporisation
Thecombined effect of these factors is approximately the same for Ca, Sr, and Ba.Hence, their electrode potentials are nearly constant.