Why is the boiling point of hydrogen fluoride higher than that of hydrogen chloride?

The boiling point of hydrogen fluoride is higher than that of hydrogen chloride due to stronger hydrogen bonding in HF.

Hydrogen fluoride (HF) and hydrogen chloride (HCl) are both simple covalent molecules, but they exhibit different types of intermolecular forces which significantly affect their boiling points. The key difference lies in the presence of hydrogen bonding in HF, which is a stronger intermolecular force than the dipole-dipole interactions found in HCl.

Hydrogen bonding is a special type of dipole-dipole interaction that occurs when a hydrogen atom is bonded to a highly electronegative atom (such as fluorine, oxygen or nitrogen) and is attracted to another electronegative atom in a different molecule. In the case of HF, the hydrogen atom is bonded to a fluorine atom, which is the most electronegative element in the periodic table. This results in a very polar bond, creating a large dipole moment. The positive end of this dipole (the hydrogen atom) is then strongly attracted to the negative end of the dipole in another HF molecule (the fluorine atom), forming a hydrogen bond.

On the other hand, while HCl also has a polar bond due to the difference in electronegativity between hydrogen and chlorine, chlorine is less electronegative than fluorine. This results in a smaller dipole moment and weaker dipole-dipole interactions between HCl molecules. Furthermore, hydrogen bonds cannot form in HCl because chlorine is not one of the highly electronegative atoms that can participate in hydrogen bonding.

Therefore, due to the stronger hydrogen bonding in HF, more energy is required to overcome these intermolecular forces and convert the substance from a liquid to a gas. This results in a higher boiling point for HF compared to HCl. This is a key concept in understanding the properties of covalent substances and the effects of different types of intermolecular forces.