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

Answer: (C)

alkaline earth metal cations are more strongly held to the anions than the corresponding alkali metal cations

AIkaline earth metal ions $\left(M^{2+}\right)$ are smaller in size and have greater charge than the corresponding alkali metal cations $\left(M^{+}\right)$. Consequently, the lattice energies of $M^{2+}$ ions of alkaline earth metals are higher than those of corresponding alkali metal cations $\left(M^{+}\right)$. Hence, $\left(M^{2+}\right)$ salts are less soluble than $M^{+}$ salts.
Also, as ionic radii of alkaline earth metals are smaller, they have higher hydration energy than alkali metals and generally, lattice energy of alkaline earth metal salts are higher than their hydration energy due to smaller size.

Hence, option C is correct.