The Relationship Between Vapor Pressure and Intermolecular Forces

There is an important connection between vapor pressure and the strength of intermolecular forces. In this article I will describe what vapor pressure is, and how it relates to the intermolecular forces. This is an intuitively significant topic for anyone studying general chemistry.

First we must understand what vapor pressure is. To give an example, suppose that you have a closed container with a liquid at a fixed temperature. Parts of this liquid continuously undergoes a change of state to vapor (vaporization). Simultaneously, some of the molecules in the vapor phase changes back to a liquid. You then reach what is called a dynamic equilibrium. Think of this as something that is changing at the same rate (speed) back and forth.

Once equilibrium is reached there is a constant pressure being exerted down on the liquid from the gas molecules. As the gas molecules move in random motion in the space above the liquid, some of these molecules are bound to hit the surface of liquid. In doing so, they push down on the liquid and thus exerts a force in the downwards direction. The strength of this force is called vapor pressure.

The stronger the intermolecular forces between the molecules, the harder it is for vaporization to occur since more energy is required to break the bonds holding the molecules together. The vapor pressure is therefore lower because there are less gas molecules pushing down at the liquid. This is the first important concept check: strong intermolecular forces correspond to a lower vapor pressure. Weak intermolecular forces means vaporization occurs more easily, and you get a stronger vapor pressure. You should notice that there is a negative correlation between vapor pressure and intermolecular forces.

It might take some reflection to get an intuitive feeling of the connection above but it should help to draw diagrams and picture the molecules going from one state to another at the same time as some other molecules bounce off the surface of the liquid and exert a pressure downwards.

On a general chemistry test you might be asked to compare the vapor pressure of two compounds. Your general strategy should then be to consider the intermolecular forces of both compounds (dispersion, dipole/dipole and H-bonding) and whichever has the stronger intermolecular force will have the lower vapor pressure.