GENERAL CHEMISTRY TOPICS

Intermolecular forces

Types of intermolecular forces. Dispersion forces. Dipole-dipole interactions. Hydrogen bonding. Effects of intermolecular forces on physical properties. Types of interactions in solution. Ion-dipole interactions.

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Returning to a major theme of general chemistry, the correlation of the properties of substances that we observe on a macroscopic scale with structures and processes at the ultra-microscopic scale of atoms and molecules, the role of intermolecular forces is examined. Without any intermolecular forces, any molecular substance would behave like an ideal gas. The fact that there are intermolecular forces to varying degrees explains the existence of condensed phases. From there, developing a basic understanding of intermolecular forces can be used to rationalize macroscopic properties such as boiling point and melting points, as well as equilibrium vapor pressure and enthalpies of vaporization.

Types of intermolecular forces

• Dispersion forces (a.k.a. London forces)
• Dipole-dipole interactions
• Hydrogen bonding
• Ion-dipole interactions

A crucial distinction is made between chemical bonding and intermolecular forces. The distinction is analogous, on a macroscale, between chemical and physical changes. A physical change such as the condensation of the gaseous phase of a pure molecular substance to its liquid phase does not change the chemical bonding of the molecules, merely the closeness of their approach. A chemical change involves breaking and making chemical bonds. Hence intermolecular forces are also referred to as noncovalent forces.

At a very close range, on the nanoscale, there must be a weak attractive force between all atoms and molecules, since every substance, even substances consisting of purely monatomic species like He and Ne, forms a condensed phase when temperature is lowered sufficiently. We speak in general of these weak attractive forces as "intermolecular forces", or simply as "interactions", and although we further classify these as belonging to several different types, they are all significantly weaker than the forces holding atoms together in molecules (i.e. forces arising from covalent bonds, or "covalent forces") and all can be understood in terms of what we will loosely refer to as electrostatics: forces arising from the existence of charged and dipolar species.

The effect of dipole-dipole interactions can be seen in the pattern of boiling points of a number of compounds with similar molecular masses but different degrees of polarity (quantitatively expressed as the magnitude of the molecular dipole, which is called the dipole moment).

Hydrogen bonding will occur generally for species fitting the pattern X-H ... :Y, where the dots represent the hydrogen bond interaction (H-bond), and X and Y are the most electronegative atoms (N, O, F). Note that atom Y must donate a lone pair of electrons (:Y is called the hydrogen bond acceptor, the group X-H is the hydrogen bond donor). The existence of hydrogen bonds as the strongest type of intermolecular force of all (we can think of it as an especially strong dipole-dipole interaction that is enhanced because of the relatively strong dipole moments and the closeness of their approach). Hydrogen bonding explains anomalies in boiling point trends involving the compounds of Group 5A, 6A, and 7A elements with hydrogen. The anomalously low boiling points for ammonia, hydrogen fluoride, and water are due the special strength of the H-bond type of intermolecular force.

The weakest type of intermolecular forces are the dispersion forces (also called London forces). Although weak, these forces exist for all atoms and molecules (such forces account for the formation of liquid helium at extremely low temperatures); furthermore these forces can cumulatively amount to significant intermolecular attractions for large molecules that can make large intermolecular surface area contacts. This explains melting point trends for series of straight-chain hydrocarbon molecules, or similarly for fatty acids, in which the smaller molecules constitute the viscous liquids called oils, while increasing size increases melting temperatures, resulting in the greasy solids we call waxes or fats.

The factors that affect the strength of the dispersion force are somewhat interrelated. As noted, the forces increase for nonpolar molecules as they increase in size, allowing for greater intermolecular contact surface area. Also, larger atoms exhibit larger attractive dispersion forces due to greater polarizability. This can be considered to be the result of the valence electrons being further from the nucleus for atoms of higher period number, and thus held less tightly by attraction to the nucleus, hence more influenced by other nearby charges or dipoles. Comparison of the boiling points of the diatomic halogens (Group 7A) F₂, Cl₂, Br₂, and I₂ illustrates this effect.

Ion-dipole interactions

An important example is the interaction between polar solvent molecules with ions, or solvation.