Question

In alkaline earth metal sulphates, the value of hydration energy decreases down the group because of the:
A.) decrease in size
B.) increase in size
C.) greater lattice energy
D.) none of the above

Answer 1

Hint: The elements of Group 2 of the modern periodic table are called alkaline earth metals because their oxides are alkaline in nature and remain unaffected by the heat of earth crust.

Complete step by step solution:

Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba) and Radium (Ra) are the elements present in the Group 2 of the modern periodic table. All the elements have two electrons in their valence shells, which gives them an oxidation state of +2.
The energy released as a result of the formation of new bonds between ions and water molecules is called hydration energy.
The sulphates of alkaline earth metals ($MSO_{4}$) are prepared by the action of sulphuric acid on metals, metals oxides, hydroxides and carbonates.
The hydration enthalpy decreases appreciably as the size of the cation increases down the group.
$B{{e}^{+2}}\, > \,M{{g}^{+2}}\, > \,C{{a}^{+2}}\, > \,S{{r}^{+2}}\, > \,B{{a}^{+2}}$
Hence, the hydration energy of sulphates of alkaline earth metals decreases down the group mainly due to the decreasing size of the cations from $B{{e}^{+2}}$ to $B{{a}^{+2}}$.

Therefore, the correct answer is (a).

Note: Don’t get confused between enthalpy and energy. Essentially, both the terms have the same meaning. Also note that, high solubility of $BeS{{O}_{4}}$ and $MgS{{O}_{4}}$ is due to high hydration energies due to smaller $B{{e}^{+2}}$ and $M{{g}^{+2}}$ ions.