Question

The compound having lowest melting point is:
(A) LiCl
(B) CsCl
(C) RbCl
(D) KCl

Answer 1

Melting point is affected by the size and charge of the ions present. Since in the given options, anion is the same and cation is different, melting point of the compound will be told on the basis of the cation present.

Complete step by step answer:
The melting point of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depends on pressure and is usually specified at a standard pressure such as 1 atmosphere or 100 kPa.
When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point.
Comparison of melting points of ionic compounds is generally done by considering the following two factors:
(1) Charge of the cation/anion: More the charge of the cation or anion, stronger will be the melting points.
(2) ionic Radii: More the distance between ions, lesser will be the strength of the bond giving rise to lesser melting point.
Going by the above rules, The order should have been: LiCl> NaCl > KCl > RbCl
(Since charges of the ions are same for each molecule and cationic radius increases down the group)
But LiCl, due to excessive polarization, exhibits high covalent character and is placed last in the order. Hence the order will be: NaCl > KCl > RbCl > LiCl
So, the correct answer is “Option A”.

Note: Ionic compounds have strong electrostatic attractions between oppositely charged ions in a regular array. Higher lattice energies typically result in higher melting point and increased hardness because more thermal energy is needed to overcome the forces that hold the ions together.