General characteristics of compounds of the Alkaline earth metals
The oxides MO are obtained either by heating the metals in oxygen or by thermal decomposition of their carbonates.
2M + O2 ———→ 2MO ( M = Be, Mg, Ca)
MCO3 ———→ MO + CO2 ( M = Be, Mg, Ca, Sr, Ba)
Expect BeO all other oxides are extremely stable ionic solids due to their high lattice energies.
These have high melting point, have very low vapour pressure, are very good conducts of heat, are chemically inert and act as electrical insulators. Therefore, these oxides are used for lining furnaces and hence used as refractory materials.
Due to small size of beryllium ion, BeO is covalent but still has high melting point because of its polymeric nature.
The hydroxides of Ca, Sr & Ba are obtained either by treating the metal with cold water or by reacting the corresponding oxides with water. The reaction of these oxides with H2O is also sometimes called as slaking.
M + 2H2O ——→ M (OH)2 + H2 ( M = Ca, Sr, Ba)
MO + H2O ——→ M (OH)2 (M = Ca, Sr, Ba)
Be(OH)2 and Mg(OH)2 being insoluble are obtained from suitable metal ion solutions by precipitation with OH- ions.
BeCI2 + 2NaOH ——→ Be (OH)2 ↓ + 2NaCI
MgSO4 + 2NaOH ——→ Mg (OH)2 ↓ Na2SO4
(i) Basic Character
All the alkaline earth metal hydroxides are bases except Be (OH)2 which is amphoteric. This basic strength increases as we move down the group. This is because of increase in size which results in decrease of ionization energy which weakens the strength of M – O bonds in MOH and thus increases the basic strength. However, these hydroxides are less basic than the corresponding alkali metal hydroxides because of higher ionization energies, smaller ionic sizes and greater lattice energies.
(ii) Solubility in Water
Alkaline earth metals hydroxides are less soluble in water as compared to alkali metals.
The solubility of the alkaline earth metal hydroxides in water increases with increase in atomic number down the group. This is due to the fact that the lattice energy decreases down the group due to increase in size of the alkaline earth metals cation whereas the hydration energy of the cation remains almost unchanged. The resultant of two effects i.e.
ΔHsolution = ΔHlattice - ΔHHydration
Becomes more negative as we move from Be(OH)2 to Ba(OH)2 which accounts for increase in solubility.