Question

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

Answer 1

Hint:For solubility, compare the trends in the lattice energies and the hydration energies. For thermal stability, consider various factors such as diagonal relationship, electronegativity and cationic size.

Complete answer:
(i) Solubility:
Alkali metal nitrates, carbonates and sulphates have water solubility. When you move down the group of alkali metals, you will observe that the solubility of alkali metal nitrates, carbonates and sulphates increases. This is due to a more rapid decrease in the lattice energies when compared to the hydration energies.
Nitrates of alkaline earth metal have water solubility. When you go from top to bottom in the group of alkaline earth metals, the solubility of nitrates decreases. This is due to more rapid decrease in the hydration energies as compared to the lattice energies.
The solubility of carbonates and sulphates of alkaline earth metals is lower than that of alkali metal carbonates and sulphates. On moving down the group of alkaline earth metals, the solubility of carbonates and sulphates decreases as the hydration energy decreases.

(ii) Thermal stability:
When you heat nitrates of alkali metals and alkaline earth metals, you observe the decomposition. When you heat the nitrates of alkali metal such as sodium, potassium, rubidium and cesium, they will decompose and will give metal nitrites and oxygen.
 \[2{\text{ MN}}{{\text{O}}_3}{\text{ }}\xrightarrow{\Delta }2{\text{ MN}}{{\text{O}}_2}{\text{ + }}{{\text{O}}_2}\]
But when you heat the nitrates of alkaline earth metals such as beryllium, magnesium, calcium, strontium and barium, they will decompose and form metal oxide, nitrogen dioxide and oxygen.
\[2{\text{ M}}{\left( {{\text{N}}{{\text{O}}_3}} \right)_2}{\text{ }}\xrightarrow{\Delta }2{\text{ M}}{{\text{O}}_2}{\text{ + 4N}}{{\text{O}}_2}{\text{ + }}{{\text{O}}_2}\]
Lithium nitrate behaves like magnesium nitrate and decomposes to form metal oxide, nitrogen dioxide and oxygen.
 \[4{\text{ Li}}{\left( {{\text{N}}{{\text{O}}_3}} \right)_2}{\text{ }}\xrightarrow{\Delta }2{\text{ Li}}{{\text{O}}_2}{\text{O + 4N}}{{\text{O}}_2}{\text{ + }}{{\text{O}}_2}\]
This is due to the fact that lithium cation, having a small size is not able to stabilize nitrate ions.
The carbonates of alkaline earth metal carbonates are thermally less stable and they will readily decompose to form carbon dioxide.
\[{\text{ MC}}{{\text{O}}_3}{\text{ }}\xrightarrow{\Delta }{\text{ MO + C}}{{\text{O}}_2}\]
With moving from top to bottom, in the group of alkaline earth metals, you will observe that there is an increase in the stability of alkaline earth metal carbonates.
Lithium shows diagonal relationship with magnesium. Hence, the thermal decomposition of lithium carbonate is similar to that of magnesium carbonate.
\[{\text{L}}{{\text{i}}_2}{\text{C}}{{\text{O}}_3}{\text{ }}\xrightarrow{\Delta }{\text{ L}}{{\text{i}}_2}{\text{O + C}}{{\text{O}}_2}\]
Other alkali metal carbonates are thermally stable.
When you heat the sulphates of alkaline earth metals, they will decompose to form sulphur trioxide
When you move down the group, the alkaline earth metal sulphates show an increase in the electropositive character and the thermal stability.
The alkali metal sulphates (with exception of lithium sulphate) are thermally stable.

Note:
You can say that a salt is thermally stable, if on heating, it does not undergo decomposition. Thermally unstable salts will decompose on heating, and usually, a gas is evolved and a solid residue is left behind.