Attractions between molecules that are weaker than bonds and control physical properties.
Practise Bonding & Molecular Structure All Bonding & Molecular Structure topics
Intermolecular forces are attractions between separate molecules. They are considerably weaker than covalent bonds, typically about 1 to 100 kJ/mol compared with several hundred kJ/mol for bonds. They control physical properties such as melting point, boiling point, vapour pressure, solubility and viscosity. The three main types are London dispersion forces, which exist in all molecules, dipole-dipole forces between polar molecules and hydrogen bonding, a particularly strong dipole interaction. Substances with stronger intermolecular forces require more energy to separate their molecules.
Key ideas: Rank for similar size: ion-dipole greater than hydrogen bonding greater than dipole-dipole greater than dispersion. Decide by asking if a molecule is polar, has H on N, O or F, and how large it is. Boiling points trend with total attraction: for nonpolar molecules use molar mass and surface area. Total force is the sum of all kinds that are present.
Common mistakes and connections: Students confuse intermolecular with intramolecular forces, saying that boiling water breaks O-H bonds, while it only breaks hydrogen bonds between molecules. Another error is saying dispersion forces are always weakest, whereas for large molecules they can exceed dipole-dipole interactions. Links to phase changes, vapour pressure, solubility and viscosity.
Approximate strengths: intermolecular attractions are far weaker than covalent bonds, which is why substances melt and boil without breaking molecules. The interactive lesson shows this as a live animation or chart.
Pentane, with a molar mass of 72 g/mol, boils at 36 degrees Celsius, whereas ethanol, with a molar mass of 46 g/mol, boils at 78 degrees Celsius because of hydrogen bonding.
Attractions between molecules that are weaker than bonds and control physical properties.
Intermolecular Forces is a beginner topic in Bonding & Molecular Structure. It needs no earlier chemistry lessons and is a good starting point.
Study these earlier topics in Bonding & Molecular Structure first: Delocalization, Benzene Bonding, Aromaticity and Huckel's Rule.
The next topics in Bonding & Molecular Structure are London Dispersion Forces and Dipole-Dipole Forces.
Yes. The interactive lesson includes a self-check question on Intermolecular Forces, and the Bonding & Molecular Structure quiz draws on every topic in this area.
Intermolecular Forces is topic 70 of 104 in Bonding & Molecular Structure.