Chapter 18, Problem 2TC

To determine

The order of inceasing boiling of the following compounds:

(a) Fluorine, ${\text{F}}_2$

(b) Hydrogen fluoride, $\text{HF}$

(c) Hydrogen chloride, $\text{HCl}$

Answer to Problem 2TC

The increasing order of boiling point is given as: ${\text{F}}_2<\text{HCl}<\text{HF}$.

Explanation of Solution

A non-polar bond is formed by the sharing of negatively charged species (called electrons) between two same atoms having no electronegativity difference whereas a polar bond is formed by the sharing of negatively charged species (electrons) between two different atoms having a slight electronegativity difference between them. In a polar bond, the atom with slightly higher electronegativity gains a negative charge while the atom with slightly lower electronegativity gains a positive charge.

The boiling point of a molecule depends on the nature of a bond as well on the type of intermolecular force of attraction. A non-polar molecule has a lower boiling point due to no extra intermolecular forces whereas a polar molecule shows a higher value of boiling point because of extra intermolecular forces.

Among the given molecules, ${\text{F}}_2$ has a very low value of a boiling point due to its nonpolar nature. The $\text{HCl}$ and $\text{HF}$ have comparatively higher boiling point due to the polar nature and also the tendency of having hydrogen bonding. The hydrogen bonding capacity of $\text{HF}$ is much higher than that of $\text{HCl}$ due to the high electronegativity of fluorine than that of chlorine.

Thus, considering all the consequences the increasing order of boiling point is given as: ${\text{F}}_2<\text{HCl}<\text{HF}$.

Conclusion:

Therefore, the increasing order of boiling point is given as: ${\text{F}}_2<\text{HCl}<\text{HF}$.