Explain whether phosphine or ammonia has a higher boiling point
Background knowledge Boiling point reflects the strength of intermolecular forces. The more attracted molecules are to each other, the more energy is needed to separate them and turn them into gases. Three main categories of intermolecular forces are (from weakest to strongest): - London dispersion forces result from the constant motion of electrons and their unsymmetrical distribution in adjacent atoms, which leads to the formation of a temporary (or in other words instantaneous) dipole. London dispersion forces exist between any type of molecules, compounds, ions etc. The strength of intermolecular forces increases with an increasing mass of molecules. - dipole-dipole interactions occur when two polar molecules interact with each other. The partially negative portion of one molecule is attracted to the partially positive portion of another molecule. - hydrogen bonding is an electrostatic attraction between molecules which have a hydrogen atom covalently bonded to a highly electronegative atom possessing a lone pair such as oxygen, nitrogen or fluorine. Answer Both phosphine (PH3) and ammonia (NH3) have London dispersion forces. Phosphine with a molecular mass of 34g/mole is a larger and heavier molecule than ammonia with its molecular mass of 17g/mole. Hence, phosphine has stronger London dispersion forces. However, ammonia, unlike phosphine, has hydrogen bonds due to the presence of a covalent bond between hydrogen and highly electronegative nitrogen with a lone pair. Hydrogen bonds are stronger than London dispersion forces, therefore ammonia has stronger intermolecular forces and higher boiling point.
