A) solubility of their hydroxides in water
B) solubility of their sulphates in water
C) ionisation energy
D) electro negativity
Correct Answer: A
Solution :
[a] Electro negativity as well as ionisation energy both usually decrease on moving downward a group with increase in atomic number. The hydroxides and sulphates of alkaline earth metals are ionic solids and the solubility of ionic solids is governed by two factors, viz, lattice energy and hydration energy. For solubility, hydration energy > lattice energy. Hydration energy varies inversely with size; ie, decreases with increase in size. However, lattice energy in case of sulphates, remains almost same with increase in the atomic number of alkaline earth metals, due to large size of sulphate ion. Hence, hydration energy only governs the solubility in this case. Thus, solubility of alkaline earth metal sulphates decreases as the hydration energy decreases on moving downward the II A group. On the other hand, in case of hydroxides, the lattice energies are different because of medium size of hydroxide ions, and decreases on moving from Be to Ba. This tends to increase the solubility and to overcome the counter-effect produced by the decrease in hydration energy. Hence, the solubility of alkaline earth metal hydroxides increases with increase in the atomic number of alkaline earth metals.
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