The Foundations of Chemistry

A molecule in the vapor may strike the liquid surface and be captured there. This

process, the reverse of evaporation, is called condensation.As evaporation occurs in a

closed container, the volume of liquid decreases and the number of gas molecules above

the surface increases. Because more gas phase molecules can collide with the surface, the

rate of condensation increases. The system composed of the liquid and gas molecules of

the same substance eventually achieves a dynamic equilibriumin which the rate of evap-

oration equals the rate of condensation in the closed container.

evaporation

liquid3:::::::::4vapor

condensation

The two opposing rates are not zero, but are equal to each other—hence we call this

“dynamic,” rather than “static,” equilibrium. Even though evaporation and condensation

are both continuously occurring, no net change occursbecause the rates are equal.

However, if the vessel were left open to the air, this equilibrium could not be reached.

Molecules would diffuse away, and slight air currents would also sweep some gas mole-

cules away from the liquid surface. This would allow more evaporation to occur to replace

the lost vapor molecules. Consequently, a liquid can eventually evaporate entirely if it is

left uncovered. This situation illustrates LeChatelier’s Principle:

A system at equilibrium, or changing toward equilibrium, responds in the way that

tends to relieve or “undo” any stress placed on it.

In this example the stress is the removal of molecules in the vapor phase. The response

is the continued evaporation of the liquid.

VAPOR PRESSURE

Vapor molecules cannot escape when vaporization of a liquid occurs in a closed container.

As more molecules leave the liquid, more gaseous molecules collide with the walls of the

container, with one another, and with the liquid surface, so more condensation occurs.

This is responsible for the formation of liquid droplets that adhere to the sides of the

vessel above a liquid surface and for the eventual establishment of equilibrium between

liquid and vapor (see Figure 13-10b and c).

The partial pressure of vapor molecules above the surface of a liquid at equilibrium

at a given temperature is the vapor pressure (vp)of the liquid at that temperature.

Because the rate of evaporation increases with increasing temperature, vapor pres-

sures of liquids alwaysincrease as temperature increases.

13-7

As an analogy, suppose that 30
students per minute leave a classroom,
moving into the closed hallway
outside, and 30 students per minute
enter it. The total number of students
in the room would remain constant, as
would the total number of students
outside the room.

496 CHAPTER 13: Liquids and Solids

TABLE 13-4 Vapor Pressures (in torr) of Some Liquids

0°C 25°C 50°C 75°C 100°C 125°C

water 4.6 23.8 92.5 300 760 1741

benzene 27.1 94.4 271 644 1360

methyl alcohol 29.7 122 404 1126

diethyl ether 185 470 1325 2680 4859

See the Saunders Interactive
General Chemistry CD-ROM,
Screen 13.9, Properties of Liquids (2):
Vapor Pressure; this screen contains an
animation of the vaporization process.

As long as some liquid remains in
contact with the vapor, the pressure
does not depend on the volume or
surface area of the liquid.

This is one of the guiding principles
that allows us to understand chemical
equilibrium. It is discussed further in
Chapter 17.