Lecture Notes, Wednesday, August 29, 2001

On several occasions up to this point I have used the term 'intermolecular attractive forces'. I first used it when we looked at the animation of the container of gas as it was cooled. (See the phase transition animation at the particulate level.) Recall in this animation a container of gas particles was cooled. As the temperature of a collection of particles was lower we observed the particles slowing down. At the lower velocities colliding particles appeared to stick together forming groups of particles. As the temperature continued to drop the number of particles in these groups increased. Eventually the groups of particles are of sufficient size that they fall to the bottom of the container as a result of force of gravity, forming a liquid. As the tmperature continues to drop the particles become more ordered, and their translational energy drops to a very small value and a solid forms. Condensation occurs when the intermolecular attraction between a pair of particles exceeds the kinetic energy of the collision.

The 'stickyness' exhibited by particles at the lower temperatures, which result in the formation of liquids and eventually solids is due to intermolecular attractive forces. Intermolecular means between molecules. Intramolecular means between atoms. Intramolecular forces are what we call covalent bonds and are very strong (100 - 1000 kJ/mol) (see also Table 9.2 in Silberberg). Intermolecular forces are between molecules and are weak (0.1 - 40 kJ/mol). Intermolecular forces are less directional compared to covalent bonds and operate over a longer range compared to covalent bonds. It is intermolecular forces which explain the formation of liquids and solids in covalent compounds. Intermolecular attractive forces are electrostatic in nature.

Intermolecular forces are classified into the following categories;

* ion-dipole (we will discuss when we cover Chapter 13)

* dipole-dipole

* induced dipole-induced dipole (London dispersion forces)

* hydrogen-bonding

To determine which type(s) of intermolecular attractive forces occur for a particular substance the most important characteristic to determine for the substance is its polarity. A polar molecule possess a permanent dipole moment (see pages 384 - 386 in Silberberg) as a result of its molecular shape and from the unequal sharing of electrons in chemical bonds which produces the separation of charge which produces the dipole.

To recognize a polar substance (you have to do this on the review problem set) you must draw the Lewis structure (see pages 361 - 371 in Silberberg, CHEM 1314 lecture notes). After drawing a Lewis structure look at the central atom.

Rules for predicting whether a molecule is polar (has a permanent dipole) or is nonpolar

A simple example is HCl. The Lewis structure for HCl is;

The pair of electrons in the covalent bond between hydrogen and chlorine is unequally shared due to the difference in electronegativity between hydrogen and chlorine. Chlorine has a greater electronegativity compared to hydrogen and as a result the electrons in the covalent bond spend a greater proportion of the time closer to the chlorine nucleus. So for chlorine the electrons in the covalent bond spend more time nearer its nucleus producing a small partial negative charge in the molecule. Since the hydrogen atom 'sees' the electron in the covalent bond less frequently it has a partial positive charge. This permanent separation of some small amount of charge on the HCl molecule produces a permanent dipole. Here is a picture of the charge on a space-filling model of the HCl molecule;

A collections of HCl molecules will align themselves such that the negative end of one HCl molecule is attracted to the positive end of an adjacent HCl molecule.

The ion-dipole force results from the attraction of an ion of positive or negative charge and the oppositely charged end of the dipole on the polar molecule. The strength of the attraction depends on the magnitude of the charge on the ion, the magnitude of the dipole moment and the distance between center of the ion and the midpoint of the polar molecule. Ion-dipole forces are important for ionic compounds which dissolve in water.

Dipole-dipole forces of attraction occur between polar molecules. This type of attractive intermolecular force occurs between the polar molecules of a pure substance, such as HCl, or between two different polar molecules. Here is an animation depicting the attraction when two HCl molecule approach each other. The attraction arises because of the permanent dipole in the polar molecule. Dipole-dipole attractions are relatively weak compared to ion-ion attractions, because the charges on polar molecules are generally quite small. In liquids the molecules are free to move relative to each other, but will do so such that sometimes orientations of adjacent molecules are attractive and sometimes they are repulsive. The overall average effect is attractive.