Finishing Up Intermolecular Forces

11.4 Phase Equilibrium

•       A dynamic equilibrium exists when particles are constantly moving between two or more phases yet no net change in the amount of substance in either phase takes place.

•        When you cap a bottle of rubbing alcohol, the liquid and gas are at equilibrium.

•        That is, the rate of evaporation equals the rate of condensation.

•        For an enclosed gas and liquid in equilibrium, the gas particles above the liquid exert pressure when they strike the walls of the container.

•        The pressure exerted by the molecules of a gas, or vapor, phase in equilibrium with a liquid is called the vapor pressure.

•       The boiling point is the temperature at which the vapor pressure equals the external pressure.

•        As you increase the temperature of a closed system, more liquid particles escape into the gas phase.

•        Thus, as you increase temperature, the vapor pressure of the substance also increases.

•        When you increase the temperature of a system to the point at which the vapor pressure of a substance is equal to standard atmospheric pressure has reached the substance’s boiling point.

Phase Diagrams

•        A substance’s state depends on temperature and that pressure affects state changes.

A phase diagram is a graph that shows the relationship between the physical state of a substance and the temperature and pressure of the substance.

•        A phase diagram has three lines:

•        One line shows the liquid-gas equilibrium.

•        Another line shows the liquid-solid equilibrium

•        A third line shows the solid-gas equilibrium.

The three lines meet at the triple point, the temperature and pressure at which the three states of a substance coexist at equilibrium.

•        For any given point (x, y) on the phase diagram for water, you can see in what state water will be.

•        For example, at the coordinates 363 K (x = 90°C) and standard pressure (y = 101.3 kPa), the point falls in the region labeled “Liquid.”

•        Line AD shows liquid-vapor equilibrium.

•        As you follow line AD upwards, the vapor pressure is increasing, so the density of the vapor increases and the density of the liquid decreases.

•       At a temperature and pressure called the critical point the liquid and vapor phases are identical.

•        Above this point, the substance is called a supercritical fluid.

•        Line AC shows solid-liquid equilibrium.

•        Only solid is present to the left of AC.

•        Water is an unusual substance: the solid is less dense than the liquid.

•        If the pressure is increased at point F, at constant temperature, water will melt. Thus, the line AC has a slightly negative slope, which is very rare in phase diagrams of other substances.

•        Line AB shows solid-vapor equilibrium.

•        If the pressure is decreased below the line AB, the solid will sublime.