Topic 4.3: Intermolecular Forces

4.3.1 Describe the types of intermolecular forces (attractions between molecules that have temporary dipoles, permanent dipoles or hydrogen bonding) and explain how they arise from the structural features of the molecule

Van der Waal's forces

Electron behave somewhat like mobile clouds of negative charge, the density of this cloud may at any one moment be greater over one atom that the other. This is known as a temporary dipole or instantaneous dipole. The strength increases as number of electron increases.

Dipole-Dipole

Molecules which have a polarity has a permanent dipole. It results in opposite charges on neighboring molecules attracting each other, generating a force known as the dipole-dipole.

Hydrogen Bond

When a molecule contains hydrogen covalently bonded to a very electronegative atom (F, N, O), these molecules are attracted to each other by a particularly strong type of intermolecular force called a hydrogen bond. The hydrogen bond is in essence a particular case of dipole-dipole. Given its small size, hydrogen can only exert a strong attractive force on a lone pair in the electronegative atom of a neighboring molecule.

4.3.2 Describe and explain how intermolecular forces affect the boiling points of substances.

The strength of intermolecular forces will play a particular important role indetermining the volatility of a substance. Change state from solid to liquid and from liquid to gas both involve seperating particles by overcoming the forces between them. It follows that the stronger the intermolecular forces, the more energy will be required to do this and so the higher will be the substance's melting and boiling points.