0.15 Phase transitions and phase equilibrium  (Page 6/7)

The only reasonable conclusion is that the molecules in the liquid were always evaporating, even before the volume of the container was increased. There must be a constant movement of molecules from the liquid phase into the gas phase at all times, whether we move the piston or not. When we fix the volume, the pressure of the gas above the liquid remains constant, so there must be a constant number of molecules in the gas phase even though evaporation is constantly occurring. For this to be true, condensation must also always be occurring, as molecules in the gas must constantly be entering into the liquid phase. If the pressure remains constant in a fixed volume, then the number of molecules entering into the gas from the liquid must be exactly offset by the number of molecules entering the liquid from the gas. The rate of evaporation must be equal to the rate of condensation at equilibrium!

At equilibrium, therefore, even though the pressure and temperature inside the container are unchanging, there is constant movement of molecules between the phases. This is called “dynamic equilibrium.” The situation is “equilibrium” in that the observable properties of the liquid and gas in the container are not changing, but the situation is “dynamic” in that there is constant movement of molecules between phases. The dynamic processes that take place offset each other exactly, so that the macroscopic properties of the liquid and gas do not change.

What are the factors that are important in this dynamic equilibrium? To find out, we examine what happens when we increase the volume of the container to a larger fixed volume. We know that the pressure at equilibrium returns to the same vapor pressure and that there are therefore more molecules in the vapor phase. How did they get there? It must be the case that when the volume is increased, evaporation initially occurs more rapidly than condensation until equilibrium is achieved. Which changed: the rate of evaporation or the rate of condensation?

In order for a molecule to leave the gas phase and enter the liquid phase, the molecule must strike the surface of the liquid. So, the rate of condensation must depend on the frequency of molecules striking the liquid surface. We can recall our calculations in the Kinetic Molecular Theory, where we found that the frequency of collisions of gas molecules with the walls of a container depends in large part on the density of the gas molecules. This means that the frequency of molecules in the gas striking the liquid surface must decrease when the volume is increased, because the density of molecules in the gas decreases when the volume increases. Increasing the volume decreases the rate of condensation, which becomes smaller than the rate of evaporation. This means that there is a net flow of molecules from liquid to gas. This continues until the density of molecules in the gas is restored from the evaporation process to its original value, at which the rate of evaporation is matched by the rate of condensation. When this happens, the gas pressure stops increasing and the pressure must be the same as it was before the volume was increased. This explains why, at a given temperature, the vapor pressure must always be the same value at equilibrium.