Compare the solubility and thermal stability of the following compounds of the alkali metals with those of the alkaline earth metals. (a) Nitrates, (b) Carbonates, (c) Sulphates.

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Solution

(i) Solubility
Nitrates, carbonates and sulphates of alkali metals are water soluble. On moving down the group, the solubility increases. This is because their lattice energies decreases more rapidly than their hydration energies.
Alkaline earth metal nitrates are water soluble. On moving down the group, their solubility decreases. This is because their hydration energies decreases more rapidly than their lattice energies.
Alkaline earth metal carbonates and sulphates have lower solubility than alkali metal carbonates and sulphates. The solubility of alkaline metal carbonates and sulphates decreases with decrease in hydration energy as we move down the group.
(ii) Thermal stability
Alkali and alkaline earth metal nitrates decompose on heating. On heating alkali metal (Na, K, Rb and Cs) decompose to form metal nitrites and oxygen.
$2 M N O_{3} h e a t - - \to 2 M n O_{2} + O_{2}$
Alkaline earth metal nitrates decompose on heating to give metal oxide, nitrogen dioxide and oxygen.
$2 M (N O_{3})_{2} h e a t - - \to 2 M O + 4 N O_{2} + O_{2} (M = B e, M g, C a, S r, B a)$
Lithium nitrate (like magnesium nitrate) decomposes to form metal oxide, nitrgen dioxide and oxygen.
$4 L i N O_{3} H e a t - -- \to 2 L i_{2} O + 4 N O_{2} + O_{2}$
This is because small sized Li+ ion cannot stabilize nitrate ion.
Alkaline earth metal carbonates have less stability towards heat and decompose to carbon dioxide.
$M C O_{3} \to M O + C O_{2}$
On moving down the group, the stability of alkaline earth metal carbonates increases. Because of diagonal relationship, lithium carbonate decomposes similar to magnesium carbonate.
$L i_{2} C O_{3} h e a t - - \to L i_{2} O + C O_{2}$
Other alkali metal carbonates being stable to heat do not decompose.
On heating, alkaline earth metal sulphates decompose to form $S O_{3}$
$M S O_{4} h e a t - - \to M O + S O_{3}$
On moving down the group, the electropositive characteer and the thermal stability of alkaline earth metal sulphates increases.
Except $L i_{2} S O_{4}$ , the alkali metal sulphates are stable and do not decompose easily.
$L i_{2} S O_{4}$ decomposes like $M g S O_{4}$ to give $S O_{3}$
$L i_{2} S O_{4} h e a t - - \to L i_{2} O + S O_{3}$
$M g S O_{4} h e a t - - \to M g O + S O_{3}$

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Present a comparative account of the alkali and alkaline earth metals with respect to the following charactieristics.

(i) Tendency to form ionic / covalent compounds

(ii) Nature fo oxides and their solubility in water

(iii) Formation of oxosalts

(iv) Solubility of oxosalts.

(v) Thermal stability of oxosalts.

△ H

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