Chapter 12 · Liquids, Solids, and Intermolecular Forces
12.6Types of Intermolecular Forces
12 min · two checks
Predict
Put these forces in order from weakest typical strength to strongest: hydrogen bonding, London dispersion, dipole–dipole.
The idea
- Distinguish dispersion, dipole–dipole, and hydrogen bonding.
- Predict which substance has the higher boiling point.
London dispersion forces exist in every particle. Electrons slosh, a temporary dipole appears, and it induces a dipole next door. Bigger, more spread-out electron clouds polarize more easily, so dispersion forces — and boiling points — rise with molar mass. That is why F₂ is a gas, Br₂ a liquid, and I₂ a solid, all nonpolar halogens. Dipole–dipole forces are extra attractions between permanent dipoles and show up in polar molecules of similar size.
Hydrogen bonding is a particularly strong dipole interaction that requires H bonded to N, O, or F, attracted to a lone pair on N, O, or F in a neighbor. It is not a covalent bond. Water, ammonia, and hydrogen fluoride have boiling points far above what their small masses suggest because of it. When comparing boiling points, name the strongest force each substance can use, and if those match, give the win to the larger molecule.
Keep these
- Dispersion: everything, stronger for larger electron clouds.
- Dipole–dipole: polar molecules.
- Hydrogen bonding: H attached to N, O, or F, attracted to a neighbor’s lone pair.