Vapor Pressure

The Macroscopic View

The vapor pressure of a liquid is the equilibrium pressure of a vapor above its liquid (or solid); that is, the pressure of the vapor resulting from evaporation of a liquid (or solid) above a sample of the liquid (or solid) in a closed container. Examples:

 substance vapor pressure at 25oC diethyl ether 0.7 atm bromine 0.3 atm ethyl alcohol 0.08 atm water 0.03 atm

The vapor pressure of a liquid varies with its temperature, as the following graph shows for water. The line on the graph shows the boiling temperature for water.

As the temperature of a liquid or solid increases its vapor pressure also increases. Conversely, vapor pressure decreases as the temperature decreases.

The vapor pressure of a liquid can be measured in a variety of ways. A simple measurement involves injecting a little of the liquid into a closed flask connected to a manometer. Click here for an illustration.

The Microscopic View

• When a solid or a liquid evaporates to a gas in a closed container, the molecules cannot escape.
• Some of the gas molecules will eventually strike the condensed phase and condense back into it.
• When the rate of condensation of the gas becomes equal to the rate of evaporation of the liquid or solid, the amount of gas, liquid and/or solid no longer changes.
• The gas in the container is in equilibrium with the liquid or solid.

 Microscopic equilibrium between gas and liquid. Note that the rate of evaporation of the liquid is equal to the rate of condensation of the gas. Microscopic equilibrium between gas and solid. Note that the rate of evaporation of the solid is equal to the rate of condensation of the gas.

• The pressure exerted by the gas in equilibrium with a solid or liquid in a closed container at a given temperature is called the vapor pressure.
Factors That Affect Vapor Pressure

• Surface Area: the surface area of the solid or liquid in contact with the gas has no effect on the vapor pressure.
• Types of Molecules: the types of molecules that make up a solid or liquid determine its vapor pressure. If the intermolecular forces between molecules are:
• relatively strong, the vapor pressure will be relatively low.
• relatively weak, the vapor pressure will be relatively high.
 = C = O = H ethyl ether (C4H10O)Pvapor (25oC) = 520 torrThe relatively weak dipole-dipole forces and London dispersion forces between molecules results in a much higher vapor pressure compared to ethyl alcohol. ethyl alcohol (C2H6O)Pvapor (25oC) = 75 torrAlthough dipole-dipole forces and London dispersion forces also exist between ethyl alcohol molecules, the strong hydrogen bonding interactions are responsible for the much lower vapor pressure compared to ethyl ether.

• Temperature: at a higher temperature, more molecules have enough energy to escape from the liquid or solid. At a lower temperature, fewer molecules have sufficient energy to escape from the liquid or solid.

 Microscopic equilibrium between gas and liquid at low temperature. Note the small number of particles in the gas. Microscopic equilibrium between gas and liquid at high temperature. Note the large number of particles in the gas.