Q. 34

Discuss the trend of the following:

(i) Thermal stability of carbonates of Group 2 elements.

(ii) The solubility and the nature of oxides of Group 2 elements.

Answer :

(i) All Group 2 elements form carbonates. All carbonates of alkaline earth metals are insoluble in water. These carbonates decompose on heating to give carbon dioxide and the metal oxide. The thermal stability of these carbonates increases down the group, as the solubility reduces. The order is BeCO3 < MgCO3 < CaCO3 < SrCO3 < BaCO3. Beryllium carbonate is unstable and requires to be in an atmosphere of CO2. The thermal stability of the carbonates increases with increases with increasing cationic size. The more stable the oxide of an alkaline earth metal, the less stable is the carbonate of the same. Hence BeCO3 is highly unstable as BeO is stable.

(ii) All alkaline earth metals form oxides with oxygen in the formula MO where M is the metal. The alkaline earth metals burn in oxygen to give metal oxides. The compounds are ionic in nature except BeO which is covalent. The oxides are basic in nature except BeO which is amphoteric in nature and behaves as both acid and base. The basic nature increases down the group due to the increase in ionic nature. The enthalpy of formation of these oxides are high and hence they are thermally stable. They also react with water to form sparingly soluble hydroxides. As the size of the cations increase, the lattice energy decreases. BeO and MgO have the highest lattice energy and they are insoluble in water.

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