Question

Give reasons for low solubility of $LiF$ and $CsI$ in water.

Answer 1

Solution:-

The solubility of a salt in water depends upon two factors-

(a) Lattice enthalpy

(b) Hydration enthalpy

If the hydration enthalpy is greater than the lattice enthalpy, the salt is soluble in water. However if the lattice enthalpy is greater than the hydration enthalpy, the salt is insoluble water.

In the case of $CsI$, both the ions are larger in size. As a result, both the ions in $CsI$ are less hydrated and have smaller hydration enthalpy, due to which, it is less soluble in water.

On the other hand, $LiF$ is almost insoluble in water due to its high lattice enthalpy. Both the ions in $LiF$ are small and heavily hydrated. However because of high charge density on both the ions, the lattice enthalpy is also very high. So in the case of $LiF$, the lattice enthalpy exceeds the hydration enthalpy, due to which, it is almost insoluble in water.

Lithium chloride $LiCl$ is soluble in pyridine because solvation energy of lithium is higher than other salts of the same group.