Question

Lattice energy and hydration energy are two important properties of ionic solids on which their melting point, boiling point and solubility depend. Lattice energy increases as the charge on the ion increases and the size decreases. Hydration energy increases as the size of the ion decreases and charges on it increases. The solubilities of carbonates, sulphates and phosphates of alkaline earth metals are much less than those of the corresponding alkali metals. This is because:
A) Alkaline earth salts have lattice energy which is almost the same as those of alkali metal salts.
B) Lattice energy of alkaline earth metal salts are lesser than their hydration energy.
C) Alkaline earth metal cations are more strongly held to the anions than the corresponding alkali metal cations.
D) Hydration energy of alkali metal salts are much lesser than those of corresponding alkaline earth metal salts.

Answer 1

Hint: We know that the energy required to convert one mole of an ionic solid into its gaseous ionic constituents is known as lattice energy. And the energy released when one mole of an ion undergoes hydration is known as hydration energy.

Complete answer :
We are given that the melting point, boiling point and solubility of ionic solids depend on the lattice energy and hydration energy.

The alkali metal ions are ${{\text{M}}^ + }$ and the alkaline earth metal cations are ${{\text{M}}^{2 + }}$. The alkaline earth metal cations ${{\text{M}}^{2 + }}$ have a greater charge and thus, the alkaline earth metal cations are smaller in size.

We are given that lattice energy increases as the charge on the ion increases and the size of the ion decreases. Thus, the lattice energies of the alkaline earth metal cations are higher than those of corresponding alkali metal cations. Also we are given that the hydration energy increases as the size of the ion decreases and charges on it increases. Thus, the hydration energies of the alkaline earth metal cations are higher than those of corresponding alkali metal cations.

Thus, the solubilities of carbonates, sulphates and phosphates of alkaline earth metals are much less than those of the corresponding alkali metals due to the smaller size of alkaline earth metal cations.
Due to the smaller size of alkaline earth metal cations there is a stronger interaction between the alkaline earth metal cations and the anions than the corresponding alkali metal cations.Thus, alkaline earth metal cations are more strongly held to the anions than the corresponding alkali metal cations.

Hence, the correct answer is option (C) alkaline earth metal cations are more strongly held to the anions than the corresponding alkali metal cations.

Note: Alkali metals belong to the group 1 of the periodic table. Alkali metals are lithium \[\left( {{\text{Li}}} \right)\], sodium \[\left( {{\text{Na}}} \right)\], potassium \[\left( {\text{K}} \right)\], rubidium \[\left( {{\text{Rb}}} \right)\], cesium \[\left( {{\text{Cs}}} \right)\] and francium \[\left( {{\text{Fr}}} \right)\]. The alkali metal cations are ${{\text{M}}^ + }$. Alkaline earth metals belong to the group 2 of the periodic table. Alkaline earth metals are beryllium \[\left( {{\text{Be}}} \right)\], magnesium \[\left( {{\text{Mg}}} \right)\], calcium \[\left( {{\text{Ca}}} \right)\], strontium \[\left( {{\text{Sr}}} \right)\], barium \[\left( {{\text{Ba}}} \right)\] and radium \[\left( {{\text{Ra}}} \right)\]. The alkaline earth metal cations are ${{\text{M}}^{2 + }}$. The main difference between alkaline and alkali is that alkali metals have one valence electron, whereas alkaline earth metals have two valence electrons.