Question

The right order of the solubility of sulphates of alkaline earth metal is:
(A) $Be > Ca > Mg > Sr$
(B) $Mg > Be > Ba > Ca > Sr$
(C) $Be > Mg > Ca > Sr > Ba$
(D) $Mg > Ca > Ba > Be > Sr$

Answer 1

Hint- To solve this question we need to use the concept of lattice energy and hydration energy. As we know lattice energy is the measure of the strength of the forces between the ions in an ionic solid while the hydration energy is the amount of energy released when one mole of the ions will undergo hydration. Using this definition, we will proceed further to solve the question.

Complete step by step solution:
The solubility of the sulphates of alkaline earth metals in water decreases down the group in the periodic table. This is because as we move down the group the hydration enthalpy of the elements increases faster as compared to the relative decrease in their lattice enthalpy.
$Be > Mg > Ca > Sr > Ba$
As we know if there is increase in the release of hydration energy, that is amount of energy stored in the ionic solid is less than the energy released then the ion is soluble in the water while if the amount of energy stored in the ionic solid is greater than the energy released then the ion is less soluble in the water as compared to the other.
While if the lattice energy is larger than the energy of solvation of the ions, the ions will not dissociate in the water and the ions will be insoluble in the water.
Also, for a compound to dissolve in water, the hydration energy must be greater than or equal to the lattice energy of the compound.

Hence, the correct option is C.

Note- To solve questions related to solubility you need to have a brief idea about the hydration enthalpy and lattice enthalpy of the compound. The solubility of the compound depends upon these two energies. Also, in salts where the energy of hydration is higher than the energy of the lattice, the solvation of the ions occurs and the energy is released in the form of heat.