Intermolecular forces
Also known as: IMFs
Intermolecular forces are the attractive forces between molecules, as opposed to the bonds within them. From weakest to strongest, the main types are London dispersion forces, dipole-dipole interactions, and hydrogen bonds.
Intermolecular forces (IMFs) are the attractions between separate molecules. They are distinct from — and far weaker than — the covalent and ionic bonds that hold atoms together within a molecule. Weak as they are, IMFs govern whether a substance is a gas, liquid, or solid at a given temperature and how molecules behave in mixtures.
The main types form a strength ladder. London dispersion forces, present in every molecule, arise from momentary shifts in electron distribution that create temporary dipoles; they strengthen as molecules get larger and more polarizable. Dipole-dipole interactions occur between polar molecules, whose permanent partial charges attract one another. Hydrogen bonds — the strongest common IMF — form when hydrogen bonded to nitrogen, oxygen, or fluorine is attracted to a lone pair on N, O, or F in another molecule. In solutions, ion-dipole forces between dissolved ions and polar solvent molecules are stronger still.
IMF strength predicts physical properties. Stronger intermolecular attractions mean higher boiling and melting points, higher viscosity and surface tension, and lower vapor pressure. Water is the showcase example: its extensive hydrogen bonding explains its unusually high boiling point for such a small molecule, and "like dissolves like" — polar solvents dissolving polar solutes — is IMF logic applied to solubility.
The MCAT tests intermolecular forces in its chemical and physical foundations section, particularly in explaining liquid-phase behavior. Expect questions ranking compounds by boiling point, identifying which IMFs a given molecule can form, and recognizing hydrogen bonding's role in the properties of water and biological molecules.
Key takeaways
- Intermolecular forces act between molecules and are much weaker than the bonds within molecules.
- The strength order of the common IMFs is London dispersion < dipole-dipole < hydrogen bonding.
- Hydrogen bonds require hydrogen attached to N, O, or F attracted to a lone pair on N, O, or F.
- Stronger IMFs raise boiling point, melting point, and surface tension while lowering vapor pressure.
- The MCAT tests IMFs through boiling point rankings, molecular comparisons, and water's hydrogen bonding.