Molecular Interactions and Properties
What you must understand
Intermolecular forces explain nearly every physical property question in this course. Rank them: ion–dipole and hydrogen bonding are strongest among common interactions, then dipole–dipole, then dispersion forces. Dispersion forces strengthen with polarisability - larger electron clouds and more elongated shapes give stronger dispersion. Hydrogen bonding requires H bonded to N, O or F, and is responsible for water's anomalous boiling point, ice's low density, and the high boiling points of alcohols and carboxylic acids (which dimerise).
You must predict and explain trends in boiling point, melting point, vapour pressure, viscosity and solubility. "Like dissolves like" is a starting point, but exam answers reward the mechanism: solute–solvent interactions must be comparable in strength to the interactions they replace. Understand surfactants and emulsions, and how soap's amphiphilic structure allows dispersion of grease in water.
Chromatography and spectroscopy connect molecular structure to measurable behaviour: polarity governs retention in paper and thin-layer chromatography (more polar components travel less in a polar stationary phase), while infrared and mass spectra give functional group and molar mass evidence.
Traps to watch
- Molecular mass alone does not decide boiling point - shape and polarity can override it (e.g. neopentane vs pentane).
- Hydrogen bonding is not a covalent bond; do not describe it as bond breaking in boiling.
- Melting point vs boiling point trends differ: packing efficiency affects melting point, so symmetry matters.
- Solubility of alcohols in water decreases with chain length because the hydrophobic tail disrupts hydrogen bonding.
Worked example
Explain why ethanol (b.p. 78 °C) boils higher than dimethyl ether (b.p. −24 °C), though both are C₂H₆O. Ethanol has an O–H bond, so hydrogen bonding between molecules is possible; dimethyl ether has only dipole–dipole and dispersion forces. Stronger intermolecular forces mean more energy to separate molecules.
Rule of thumb: identify the strongest force present in each substance, then compare magnitudes.