Question

LiF is insoluble in ${{\text{H}}_{\text{2}}}{\text{O}}$ and LiCl is soluble in ${{\text{H}}_{\text{2}}}{\text{O}}$. Explain

Answer 1

Hint: Lattice energy is inversely proportional to the hydration energy. If the lattice energy is high then the hydration energy, the compound will be insoluble. If the lattice energy is low then the hydration energy, the compound will be soluble.

Complete answer
The energy released by the dissolution of an ionic compound in water is known as hydration energy.
The energy released by the formation of the lattice of an ionic compound is known as lattice energy.
More lattice energy means high stability which will correspond to the low solubility of an ionic compound in water means low hydration energy.

- Lattice energy depends upon the charge and size. On increasing the charge of the metal lattice energy increases. On increasing the size of the negative ion the lattice energy decreases.
- Lattice energy $ \propto $Charge of the metal.
- Lattice energy $ \propto $ $\dfrac{1}{Size\; of\; the\; anion}$.
- The size of fluoride ions is smaller than the size of chloride ions, so the lattice energy is high for lithium fluoride.

Lattice energy and hydration energy have inverse relation so, the hydration energy of lithium fluoride is less than the hydration energy of lithium chloride so, lithium chloride is insoluble in water whereas due to small hydration energy lithium chloride is soluble in water.

Note: Solubility can also be determined by determining the covalent character in the ionic compound. Fajan’s rule determines the covalent character in the ionic compound. The covalent character is directly proportional to the solubility in non-polar solvents and inversely proportional to the solubility in the polar solvent. The covalent character is directly proportional to the polarizing power of the cation and inversely proportional to the polarizability of the anion. Due to the large size of chloride ion, lithium chloride also has a covalent character so it is also soluble in a nonpolar solvent like acetone.