http://www.ck12.org Chapter 13. States of Matter
A dynamic equilibrium can be illustrated by an equation with a double arrow, meaning that the reaction is occurring
in both directions and at the same rate.
H 2 O(l)⇀↽H 2 O(g)The forward direction represents the evaporation process, while the reverse direction represents the condensation
process.
Because they cannot escape the container, the vapor molecules above the surface of the liquid exert a pressure on
the walls of the container. Thevapor pressureis a measure of the pressure exerted by the vapor that forms above
its liquid form in a sealed container. Vapor pressure is considered a property of the liquid and is constant for a
given substance at a set temperature. The vapor pressure of a substance at a given temperature is based on the
strength of its intermolecular forces. A liquid with weak intermolecular forces evaporates more easily and has a high
vapor pressure. A liquid with stronger intermolecular forces does not evaporate easily and, thus, has a lower vapor
pressure. For example, diethyl ether is a nonpolar liquid with weak dispersion forces. Its vapor pressure at 20°C
is 58.96 kPa. Water is a polar liquid whose molecules are attracted to one another by relatively strong hydrogen
bonding. The vapor pressure of water at 20°C is only 2.33 kPa, far less than that of diethyl ether.
Vapor pressure can be measured by the use of a manometer (Figure13.11).
FIGURE 13.11
In the top picture, the flask of liquid ethanol has just been sealed and no vapor has accumulated, so the pressure
inside is equal to the external atmospheric pressure. This is seen by the equal levels of the mercury in the U-tube. In
the bottom picture, the system has been allowed to reach a dynamic equilibrium and the ethanol vapor is exerting a
pressure equal to its vapor pressure. The vapor pressure can be measured by the height difference between the levels
of mercury on each side of the U-tube.
Vapor Pressure and Temperature
Vapor pressure is dependent upon temperature. When the liquid in a closed container is heated, more molecules
escape the liquid phase and evaporate. The greater number of vapor molecules strike the container walls more
frequently, resulting in an increase in pressure. The table below (Table13.1) shows the relationship between
temperature and vapor pressure for three different liquids.
TABLE13.1: Vapor Pressure (in kPa) of Three Liquids at Different Temperatures
0°C 20°C 40°C 60°C 80°C 100°C
Water 0.61 2.33 7.37 19.92 47.34 101.33
Ethanol 1.63 5.85 18.04 47.02 108.34 225.75
Diethyl ether 24.70 58.96 122.80 230.65 399.11 647.87