Question

The right order of solubility of sulphates of alkaline earth metals in water is
A.${\text{Be > Ca > Mg > Ba > Sr}}$
B.${\text{Mg > Be > Ba > Ca > Sr}}$
C.${\text{Be > Mg > Ca > Sr > Ba}}$
D.${\text{Mg > Ca > Ba > Be > Sr}}$

Answer 1

Hint - We must note that the elements in the question are Group 2 alkaline earth metals –Beryllium${\text{(Be)}}$, Magnesium${\text{(Mg)}}$, Calcium${\text{(Ca)}}$, Barium${\text{(Ca)}}$, Strontium ${\text{(Sr)}}$ and Radium${\text{(Ra)}}$.

Beryllium${\text{(Be)}}$Z=4	Magnesium${\text{(Mg)}}$Z=12	Calcium${\text{(Ca)}}$Z=20	Barium${\text{(Ca)}}$Z=38	Strontium ${\text{(Sr)}}$Z=56	Radium${\text{(Ra)}}$Z=88

In order to answer this question, we need to understand two concepts clearly
(i) Lattice enthalpy and
(ii) The hydration enthalpy.

Complete step by step solution:
We can define the lattice enthalpy is the energy released when one mole of a compound is formed from its constituent gaseous ions under standard conditions. Now, the lattice enthalpy depends on the length of the ionic bond between the cation and the anion in the compound. A shorter bond would require more energy to be broken, while a longer bond could be easily broken. Hence, the shorter the bond, the higher will be it’s the lattice enthalpy. Shorter bonds are formed by cation with smaller size.
Hydration enthalpy is the energy released when one mole of gaseous ions is completely hydrated by water molecules under standard conditions. Let’s consider that two ions of different sizes were surrounded by water molecules, the smaller ion would be more hydrated when compared to the larger one. Therefore, small ions will have higher hydration enthalpy. Hence, larger the size of the ion lesser will be the lattice enthalpy and hydration enthalpy.
Now, the solubility of compounds decreases as we go down the group. Because both lattice enthalpy and hydration enthalpy both decrease as you go down the group. Therefore, they are related to each other.
We know that, as we move down the periodic table, the size of the nucleus increases because the number of energy levels increases. Therefore, greater the distance between nucleus and the electron in the outermost orbital, larger is the size of the atom. Therefore, in group 2 the order of size of elements will be in the order ${\text{Be > Mg > Ca > Sr > Ba}}$.
So the order of solubility of sulphate is ${\text{Be > Mg > Ca > Sr > Ba}}$. Hence, the option C is correct.

Note : We must know that lattice energy and hydration energy are two important properties of ionic solids on which their melting points, boiling points and solubility depend. In Group 2, all the elements have two electrons in their valence shells. All elements give +2 oxidation states. This enables the metals to easily lose electrons, which increases their stability and allows them to form compounds via ionic bonds.