Example of each of the three types of intermolecular forces.

The three types of intermolecular forces.

Intermolecular forces are electrostatic interactions that operate between molecules. They are of primarilly three types: dipole-dipole, hydrogen bonding, and Van der Waals (or London disperson forces). The type of intermolecular force a molecule exert depends on the electronegativity difference between its bonded atoms and its molecular geometry. Polar covalent bonds make it possible for a molecule to engage in dipole-dipole interactions.

Furthermore, in the special case where a hydrogen atom is covalently bonded to a highly electronegative atom such as nitrogen or oxygen, the type of intermolecular force that results is called hydrogen bonding. Because the hydrogen end is a bare proton that can partially share electrons from the negative pole of the adjacent molecule, hydrogen bonds are an order of magnitude stronger than ordinary dipole-dipole interactions. Lastly, nonpolar molecules interact by London dispersion forces, which are weaker than dipole-dipole interactions or hydrogen bonds.

It is especially essential when you go into the MCAT to have good sense of the types of intermolecular forces exerted by the organic functional groups. Intermolecular force is a major conceptual foundation for understanding many phenomena including the physical properties and solution properties of substances.

Intermolecular forces are fundamentally electrostatic interactions (although London dispersion forces have a magnetic component also). Applying the classical electrostatic description, we understand that the greater the forces of attraction between molecules, the more the energy of the system must be increased as molecules are separated from each other, as in a phase change such as vaporization.

Phase change is an intermolecular event. Let us apply our understanding of electrostatics to the phenomenon of vaporization to help us begin to develop a sense of the thermochemistry of the process.

We are at a stage in our MCAT preparation where we want to create a bridge between basic electrostatics that runs through our understanding of the types of interactions that occur at the particle level in chemistry to the next stage of chemistry, which is thermodynamics.

The internal energy change associated with vaporization is the electrostatic potential energy increase of separating mutually attracting molecules. The greater the strength of intermolecular force, the greater the internal energy increase required for vaporization, so substances for which the mode of intermolecular force is hydrogen bonding are higher boiling than similarly sized molecules taking part in dipole-dipole interactions. A higher temperature must be reached for the kinetic energy of the molecules to be sufficient to allow them to escape from each other.

The lowest boiling substances only have weak Van der Waals attractions between molecules, so a simplified picture shows them moving only slowly and having sufficient energy to become released from their mutual potential energy well, to become vapor.

Molecular weight and the strength of intermolecular force are the primary determinants of the melting point, vapor pressure, and boiling point.