Question

The oxide, which is best soluble in water is
A. \[{\rm{Ba}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\]
B. \[{\rm{Mg}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\]
C. \[{\rm{Sr}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\]
D. \[{\rm{Ca}}{\left( {{\rm{OH}}} \right)_{\rm{2}}}\]

Answer 1

Hint: A substance is soluble in water if hydration energy is more than lattice energy. Lattice energy is defined as the energy that is required to transform one mole of a compound into gaseous ions. Hydration energy is defined as the liberated energy during the hydration of a mole of ions.

Complete Step by Step Answer:
We know that hydration happens to the dissociated ions when they interact with water dipoles to form hydrated ions. This is an exothermic process as energy is liberated in this process. This liberated energy is hydration energy.

Hydration energy is the energy liberated when the dissolved ions of the compound in water interact with water dipoles establishing hydrated ions. So, a compound with high lattice energy maintains a considerable amount of intermolecular force of attraction and it is difficult to break down the compound into cations and anions in water. Therefore, for the dissolving of cations and anions hydration energy must be bigger than lattice energy.

The lattice energy of hydroxides is less because of the small size of the hydroxide ion. So, the size of hydroxide ions does not affect the lattice energy. It only relies on the size of the cation created by alkaline earth metal. The lattice energy decreases with the increase in metallic size from Beryllium to Barium. So, solubility increases as the intermolecular force of attraction is not that huge so dissolution happens easily. So, the solubility of hydroxides increases from Beryllium to Barium.
Therefore, the solubility of barium hydroxide is the highest.
So, option A is correct.

Note: It should be pointed out that the solubility of barium hydroxide is the highest. Hydration energy is reduced in the group because of an increase in size. The reason is that it is tough for the water dipoles to react with very big-sized cations. But the drop in lattice energy of the hydroxides counterbalances this drop in hydration energy.