Kinetic Energy vs Intermolecular forces

There is an epic never-ending battle between the KE-thermal motion-of molecules and the intermolecular forces of attraction between the molecules.

At times the KE is much greater than any intermolecular forces attracting the molecules to each other. When this is the case the substance is a gas.

Other times the intermolecular forces are greater than the KE. When this is the case the substance is a solid.

When the intermolecular forces are similar in strength to the KE a liquid results.

Last semester we determined the average KE of a gas. KEave= 3/2RT Of course this does not mean that all the gas molecules have the same KE. Some of the molecules have more KE while others have less. The KE distribution would look something like the figure to the right.

While the KE distribution of a solid or a liquid would not be the same as a gas, the solid and the liquid contain high energy molecules just like gases have high energy and low energy molecules. Some of the high energy molecules can overcome the intermolecular forces of attraction; that is, some high energy molecules have enough energy to vaporize. The ability of some of the molecules to vaporize is why water evaporates and why moth-balls smell.