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 metal to form a nitride directly.

(c) \( E^0\) for \( M^{2+}\)(aq) + \( 2e^{–}\) → M(s) (where M = Ca, Sr or Ba) is nearly constant.

Asked by Abhisek | 1 year ago |  52

1 Answer

Solution :-

(a) The ionic and atomic sizes of the metals tend to increase while going down the alkali group.

The increasing order of the ionic sizes of the alkali metal ions is as shown below:

Li+ < Na+ < K+ < Rb+ < Cs+

Smaller the size of an ion, greater is its ability to get hydrated. Li+ ion gets heavily hydrated since it is the smallest in size whereas Cs+ has the largest size and is the least hydrated ion. The alkali metal ions when arranged in the decreasing order of their hydrations is as shown below:

Li+ > Na+ > K+ > Rb+ > Cs+

Higher the mass of a hydrated ion, the lesser is its ionic mobility. Thus, hydrated Li+ is the least mobile ion whereas hydrated Cs+ is the most mobile ion.

The ionic mobility of the alkali metal ions are in the following order:

Li+ < Na+ < K+ < Rb+ < Cs+

(b) The only metal that can form a nitride directly is Lithium because Li+ has a smaller size and is easily compatible with the N3– ion. Thus, the lattice energy released is very high which is enough to overcome the amount of energy needed to form N3- ion.

(c) Electrode potential (E°) of any \( \dfrac{ M^{2+}}{M}\) electrode is decided by three factors:

(i) Enthalpy of hydration

(ii) Enthalpy of vaporisation

(iii) Ionisation enthalpy

The cumulative effect of these factors on Ba, Sr, and Ca is almost the same.

As a result, their electrode potentials are also same.

Answered by Abhisek | 1 year ago

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