Question

The correct order of melting points of chlorides of alkali metals is:
(A) $$\text{LiCl > NaCl > KCl > CsCl}$$
(B) $$\text{LiCl > NaCl > CsCl > KCl}$$
(C) $$\text{NaCl > KCl > CsCl > LiCl}$$
(D) $$\text{LiCl > NaCl > CsCl > KCl}$$

Answer 1

Hint: Lattice energy represents the energy needed for the separation of one mole of ionic solid into its gaseous ions. With increase in the lattice energy, the melting point increases. Also, covalent solids have lower melting point than ionic solids.

Complete answer:
In the given alkali metal halides, the halogen atom is the same for all. The halogen atom is chlorine. The alkali metal is different for each halide.
When energy is supplied to an ionic solid, the ionic solid is transformed into gaseous ions. The amount of energy needed for this process is called lattice energy. A solid having greater value of the lattice energy will have a higher melting point than the solid having smaller value of lattice energy.
Lattice energy decreases with increase in the size of the cation. Thus, larger potassium cation has smaller lattice energy than smaller sodium cation. Hence, potassium chloride has lower boiling point than sodium chloride.
Similarly, larger cesium cation has smaller lattice energy than smaller potassium cation. Hence, cesium chloride has lower boiling point than potassium chloride.
Hence the correct order is $$\text{NaCl > KCl > CsCl}$$.
Thus, on moving down the group of alkali metals, with increase in the size of the alkali metal cation, the melting point of alkali metal chlorides decreases.
Lithium chloride has the lowest melting point among the alkali metal chlorides. This is because lithium chloride is covalent in nature whereas other alkali metal chlorides are ionic in nature.
Hence the correct order is $$\text{NaCl > KCl > CsCl > LiCl}$$.

Thus, the option (C) is the correct option.

Note: Lithium halides are covalent compounds. This is because lithium cation has a large charge density. So lithium cation can distort the electron cloud of neighbouring halide ions.