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.

Explanation

(i) Solubility : The nitrates, carbonates and sulphates of all alkali metals are soluble in water and their solubility increases on moving down the group because of a rapid decrease in lattice energies as compared to hydration energies.
The nitrates of all alkaline earth metals are also soluble in water but their solubility decreases on moving down the group because their hydration energies decrease more rapidly than the lattice energies.
The alkaline earth metal carbonates are not much soluble in water and the solubility decreases on moving down the group. `BeCO_3` is slightly soluble in water, while `BaCO_3` is almost insoluble. The decrease in solubility on moving down the group is due to a decrease in hydration energy.
The alkaline earth metal sulphates are less soluble in water than the corresponding alkali metal sulphates. Their solubility decreases on moving down the group. The lattice energies of alkaline earth metal sulphates are higher than those of alkali metal sulphates. This is why they are less soluble than alkali metal sulphates. On moving down the group, the hydration energy decreases due to an increase in the size of cations but lattice energy does not change much. Therefore, the solubility of sulphates decreases in going from `BeSO_4" to " BaSO_4`.
(ii) Thermal stability : (a) Nitrates of both alkali metals and alkaline earth metals decompose on heating. The nitrates of alkali metals (except Li) decompose to form metal nitrites with the evolution of oxygen.
`2MNO_3 overset(Delta)to 2MNO_2 + O_2` (M = Na , K , Rb or Cs)
Lithium nitrate decomposes to form `Li_2O, NO_2 and O_2`.
`4LiNO_3 overset(Delta)to 2Li_2 O + 4NO_2 + O_2`
All alkaline earth metal nitrates decompose to form metal oxide, `NO_2 and O_2`
`2M(NO_3)_2 overset(Delta)to 2MO + 4NO_2 + O_2 ` (M = Be, Mg, Ca, Sr or Ba)
(b) Carbonates of alkali metals (except Li) are stable and do not decompose even at high temperatures. Lithium carbonate decomposes to form lithium oxide and carbon dioxide.
`Li_2CO_3 overset(Delta)to Li_2O + CO_2`
Carbonates of all alkaline earth metals decompose on heating to form metal oxide and `CO_2`.
`MCO_3 overset(Delta)to M_2O + CO_2` (M = Be, Mg, Ca, S or Ba)
The decomposition temperature of alkaline earth metal carbonates increases on moving down the group. This is due to an increase in the stability on account of increasing electropositive character on moving down the group.
(c) Sulphates of alkali metals (except Li) are quite stable and do not decompose easily. Lithium sulphate decomposes as follows.
`Li_2SO_4 overset(Delta)to Li_2O + SO_3`
Sulphates of alkaline earth metals are also thermally stable and do not decompose easily on heating. This is due to their high lattice energies. However, they can be decomposed at very high temperatures.