Question

Select the compound having the lowest boiling point.
A. ${\text{NaCl}}$
B. ${\text{CuCl}}$
C. ${\text{CuC}}{{\text{l}}_2}$.
D. ${\text{CsCl}}$

Answer 1

Hint: The boiling point of ionic compound depends upon the covalent character in ionic compound. The boiling point is inversely proportional to the covalent character. As the Covalent character increases the boiling point decreases.

Complete step by step answer:
 The temperature at which the vapour pressure of the liquid equals the vapour pressure of the surrounding then the liquid converts into vapour is known as boiling point.

$\,{\text{Boiling}}\,{\text{point}} \propto \dfrac{{\text{1}}}{{{\text{Covalent}}\,{\text{character}}}}$

The covalent character of an ionic compound is determined by using Fajan’s rule.

According to Fajan's rule, as the polarizing power of the cation increases the covalent character increases and as the polarizability of anion increases the covalent character increases.

The polarizing power of cation is inversely proportional to the size of cation and directly proportional to the charge of the ion.

The polarizability is directly proportional to the size of anion.

${\text{Covalent}}\,{\text{character}} \propto \dfrac{{\text{1}}}{{{\text{Size}}\,{\text{of}}\,{\text{cation}}}}$

$\,{\text{Covalent}}\,{\text{character}} \propto {\text{Charge}}\,{\text{of}}\,{\text{ion}}$

${\text{Covalent}}\,{\text{character}} \propto \dfrac{{\text{1}}}{{{\text{Size}}\,{\text{of}}\,{\text{anion}}}}$

Determine the covalent character in each of the compounds as follows:

Since the anion that is chloride is the same in all four compounds, compare the covalent character based on the polarizing power of the cations.

In ${\text{NaCl}}$, the cation has a +1 charge. In ${\text{CuCl}}$, the cation has a +1 charge. In ${\text{CuC}}{{\text{l}}_2}$, the cation has a +2 charge. In ${\text{CsCl}}$, the cation has a +1 charge.
Covalent character is directly proportional to the polarizing power or charge of cation so, the covalent character is maximum in ${\text{CuC}}{{\text{l}}_2}$ .

The boiling point is inversely proportional to the covalent character so, ${\text{CuC}}{{\text{l}}_2}$has the minimum boiling point.

So, the correct answer is “Option C”.

Note:
If the given compounds have common cation then the covalent character is compared on the basis of the polarizability of the anion. As the polarizability of the anion increases, covalent character increases hence boiling point decreases.