Advanced Placement (AP) Chemistry Practice Exam

Which of the following is considered a strong intermolecular force?

1. Dipole-dipole forces

2. Hydrogen bonding

3. London dispersion forces

4. Van der Waals forces

The correct answer is: Hydrogen bonding

Hydrogen bonding is considered a strong intermolecular force due to the specific interactions between molecules that have hydrogen covalently bonded to highly electronegative atoms such as nitrogen, oxygen, or fluorine. The reason hydrogen bonds are stronger than typical dipole-dipole interactions is that the large difference in electronegativity creates a significant partial positive charge on the hydrogen, leading to a strong attraction with the lone pair of electrons on an electronegative atom in a neighboring molecule. Hydrogen bonds can significantly impact the boiling and melting points of substances. For example, the presence of hydrogen bonding in water is responsible for its relatively high boiling point compared to other similar-sized molecules, contributing to water's unique properties, such as high heat capacity and surface tension. Other intermolecular forces, such as dipole-dipole forces, London dispersion forces, and Van der Waals forces, are generally weaker compared to hydrogen bonds. Dipole-dipole forces occur between polar molecules due to their permanent dipoles, while London dispersion forces arise from temporary fluctuations in electron density, leading to momentary dipoles in nonpolar molecules. Van der Waals forces encompass both dipole-dipole and London dispersion interactions but are still weaker than hydrogen bonds.