Q. 293.7( 15 Votes )

How would you explain the following observations?

(i) BeO is almost insoluble but BeSO4 is soluble in water,

(ii) BaO is soluble but BaSO4 is insoluble in water,

(iii) LiI is more soluble than KI in ethanol.

Answer :

(i) BeO is almost insoluble but BeSO4 is soluble in water due to the following reasons:


Because of the smaller size , higher ionization enthalpy and higher electronegativity, BeO is essentially covalent and hence is insoluble in water.


On the other hand, BeSO4 is ionic.


As we know that, the solubility of a ion in water depends on the balance between lattice energy and hydration energy. The hydration energy of BeSO4 is much higher than its lattice energy and BeSO4 is soluble in water.


(ii) BaO is soluble but BaSO4 is insoluble in water because of the following reasons:


Both BaO and BaSO4 are ionic compounds. However, the size of O2- ion is much smaller than that of SO42- ion.


Since a bigger anion stabilizes a bigger cation more than a smaller anion stabilizes a bigger cation, therefore, the lattice energy of BaO is much smaller than that of BaSO4.


As we know that smaller the lattice energy, greater will be the solubility. Hence, BaO is soluble while BaSO4 is insoluble in water.


(iii) LiI is more soluble than KI in ethanol because of the following reasons:


Li+ is much smaller than K+ ion.


Therefore, according to the Fajan’s rule, Li+ ion can polarise bigger I- ion to a greater extent than K+ ion.


As a result, LiI is more covalent than KI and hence more soluble in organic solvents like ethanol.


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