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.
Carbonates of metal: Thermal stability
The carbonates of alkali metals except lithium carbonate are stable to heat. The carbonates of group-2 metals and that of lithium decompose on heating, forming an oxide and carbon dioxide . For example,
Li2CO3 +heat -> Li2O +CO2
MgCO3 +Heat -> MgO +CO2
Na2CO3 +heat -> no effect.
The stabilities of carbonates of alkaline earth metals increase on moving down the group.For example, BeCO3 decompose at 373K, MgCO3 at 813K, CaCO3 at 1173K, SrCO3 at 1563K .
solubilities in water:
carbonates of alkali metals, except Li2CO3, are soluble in water. Their solubilities increase on moving down the group. Carbonates of group-2 metals are almost insoluble in water and their solubilities further decrease on moving down the group. In fact, these metals can precipitate from their salt solutions as carbonates.
Nitrates: Thermal stability
Nitrates of alkali metals,except LiNO3, decompose on strong heating forming nitrites and oxygen. for example,
2KNO3 -> 2KNO2 +O2
Nitrates of alkaline-earth metals and LiNO3 decompose on heating to form oxides, nitrogen to form oxides, nitrogen dioxide and oxygen.
2LiNO3 +Heat -> Li2O +2NO2 +O2
2Ca(NO3)2 +Heat -> 2CaO +4NO2 +O2
Thermal stabilities of nitrates of group-1 and group-2 metals increase on moving down the group from top to bottom.
Nitrates of group -1 and group-2 metals are all soluble in water. The solubilities of these salts further increase on descending the group.
Sulphates: Thermal stability
The sulphates of group-1 and group-2 metals are all thermally stable.
sulphates of alkali metals are soluble in water. The sulphate of alkaline earth metals is less soluble. Their solubilities decrease on moving down the group from Be to Ba.