Question

The hydroxides and carbonates of sodium and potassium are easily soluble in water while the corresponding salts of magnesium and calcium are sparingly soluble in water. Explain.

Answer 1

Hint: To understand the solubility of the compounds in water, there are two important terms which should be known. They are lattice energy or enthalpy and hydration enthalpy. When the hydration energy of the compound is greater than that of lattice energy of that compound ,that compound becomes soluble.

Complete step by step solution:
Lattice enthalpy is an amount of energy which is required to break a bond. On the other hand we can say that the energy released when two atoms make bonds. Higher the lattice energy higher will be the bond energy of that bond as well as the bond strength.
Hydration enthalpy is an amount of energy which is released on hydration of one mole number of ions. When the solvent is not water is known as a solvation energy.
Now if the hydration energy of any particular ion overcomes the amount of lattice energy the bond dissociates and solubility occurs.
In case of hydroxides and carbonates of sodium and potassium get dissociated into water and get solubilized because their hydration energy is greater than that of lattice energy. On the other hand, the hydroxides and carbonates of magnesium and calcium are sparingly soluble in water because their hydration energy is less than that of lattice energy.

Note: Lattice energy is the force of attraction between the ions in a solid. lattice energy is directly proportional with the charge of the ions, higher the charge of the ions higher will be the lattice energy and vice-versa. Lattice energy also depends upon the size of the ions, lower the size of ions higher will be the lattice energy and vice-versa.