Question

• Stronger intermolecular forces result in higher boiling point.
• Strength of London forces increases with the number of electrons in the molecule.
• Boiling point of $HF,HCl,HBr$ and $HI$ are $293K,189K,206K$ and $238K$ respectively.

A Which type of intermolecular forces are present in the molecules $HF,HCl,HBr$ and $HI$?

B Looking at the trend of boiling points of $HCl,HBr$ and $HI$, explain out of dipole-dipole interactions and London interaction, which one is predominant here.

C Why is the boiling point of hydrogen fluoride highest while that of hydrogen chloride lowest?

Answer 1

All the given molecules (i.e., $HF,HCl,HBr$ and $HI$) have permanent dipoles. Hence, all of them possess dipole-dipole and London forces. $HF$ in addition to dipole-dipole and London forces also have hydrogen bonding.

• Electronegativity of chlorine, bromine and iodine decreases in the order: $Cl>Br>1$
• As here electronegativity is the more dominating factor than separation distance between dipoles , the dipole moment should decrease from $HCl$ to $HI$. Thus, dipole-dipole interactions should decrease from $HCl$ to $HI$.
• On the other hand, London forces increases as the number of electrons in a molecule increases and in this case the number of electrons is increasing from $HCl$ towards $HI$. So, the London forces increases from $HCl$ to $HI$.

Since, as per the given data, the boiling point increases on moving from $HCl$ to $HI$.

This means that, London interactions (or forces) are predominant than dipole-dipole interaction.


Hydrogen fluoride have the highest dipole moment, which led to strong dipole-dipole interactions. It also possess strong hydrogen bonding interactions. Therefore, the boiling point of $HF$ is highest.


Generally, the boiling point of covalent compounds increases with increase in size. It occurs because increase in size led to increase in number of electrons which increases the London forces. Hence, $HCl$ have the lowest boiling point because of the small size of $HCl$ molecule due to which it have weak London forces of interaction.