intermolecular forces
Summary
Intermolecular forces are attractions between separate molecules (or between ions and molecules), much weaker than the covalent bonds within a molecule. For molecules of similar size, strength usually rises from London dispersion to dipole–dipole to hydrogen bonding, and stronger forces mean a higher boiling point.
Detail
Meaning: These are attractions between neighboring molecules, far weaker than covalent bonds. London dispersion forces come from temporary dipoles, act between all molecules, and grow with more electrons and more surface contact. Dipole–dipole forces act between permanently polar molecules. A hydrogen bond forms when an H bonded to N, O, or F is attracted to a lone pair on N, O, or F of a neighbor. Ion–dipole forces, between an ion and a polar molecule, explain why salts dissolve in water.
On the MCAT: Passages ask you to rank compounds by boiling point, melting point, vapor pressure, viscosity, or surface tension, or to predict solubility by "like dissolves like." Stronger forces give a higher boiling point, lower vapor pressure, and higher viscosity. Typical pairs: ethanol boils higher than dimethyl ether (same formula, but only ethanol hydrogen-bonds), and a straight-chain alkane boils higher than its branched isomer because of greater surface contact.
Common confusion: Boiling water breaks intermolecular hydrogen bonds, not the covalent O–H bonds. Also, hydrogen bonding does not automatically win: a very large nonpolar molecule can have stronger total dispersion forces than a small polar one, so compare size as well as polarity.
Sources
- Chemistry: The Central Science
- General Chemistry: Principles and Modern Applications
- Chemistry (Zumdahl)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.