0.17 Phase equilibrium and intermolecular interactions  (Page 7/7)

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

What happens when we increase the volume of the container to a larger fixed volume? We know that the pressureequilibrates at the same vapor pressure, and that therefore there are more molecules in the vapor phase. How did they get there? Itmust be the case that when the volume is increased, evaporation initially occurs more rapidly than condensation until equilibriumis achieved. The rate of evaporation must be determined by the number of molecules in the liquid which have sufficient kineticenergy to escape the intermolecular forces in the liquid, and according to the kinetic molecular theory, this number depends onlyon the temperature, not on the volume. However, the rate of condensation must depend on the frequency of molecules striking thesurface of the liquid. According to the Kinetic Molecular Theory, this frequency must decrease when the volume is increased, becausethe density of molecules in the gas decreases. Therefore, the rate of condensation becomes smaller than the rate of evaporation whenthe volume is increased, and therefore there is a net flow of molecules from liquid to gas. This continues until the density ofmolecules in the gas is restored to its original value, at which point the rate of evaporation is matched by the rate ofcondensation. At this point, this pressure stops increasing and is the same as it was before the volume was increased.

Review and discussion questions

1. In the phase diagram for water in [link] , start at the point where the temperature is 60°C and the pressure is 400 torr. Slowlyincrease the temperature with constant pressure until the temperature is 100°C. State what happens physically to thewater during this heating process.
2. In the phase diagram for water in [link] , start at the point where the temperature is 60°C and the pressure is 400 torr. Slowlylower the pressure at constant temperature until the pressure is 80 torr. State what happens physically to the water during thisprocess.
3. Explain why [link] is both a graph of the boiling point of liquid water as a function of applied pressure and a graph ofthe vapor pressure of liquid water as a function of temperature.
4. We observe that, when the applied pressure is less than the vapor pressure of a liquid, all of the liquid willspontaneously evaporate. In terms of dynamic equilibrium, explain why no liquid can be present under these conditions.
5. Using arguments from the Kinetic Molecular Theory and the concept of dynamic equilibrium, explain why, at agiven applied pressure, there can be one and only one temperature, the boiling point, at which a specific liquid and its vapor can bein equilibrium.
6. Using dynamic equilibrium arguments, explain why the vapor pressure of a liquid is independent of the amount ofliquid present.
7. Using dynamic equilibrium arguments, explain why the vapor pressure of a liquid is independent of the volumeavailable for the vapor above the liquid.
8. Using dynamic equilibrium arguments, explain why a substance with weaker intermolecular forces has a greatervapor pressure than one with stronger intermolecular forces.
9. According to [link] the vapor pressure of phenol is much less than the vapor pressure of dimethyl ether. Which of thesesubstances has the greater intermolecular attractions? Which substance has the higher boiling point? Explain the difference inthe intermolecular attractions in terms of molecular structure.
10. The text describes dynamic equilibrium between a liquid and its vapor at the boiling point. Describe the dynamicequilibrium between a liquid and its solid at the melting point. Using this description, explain why the melting point of a solidvaries very little as the pressure increases.