11.5 Vapor Pressure

At a given temperature the molecules of a liquid have a certain tendency to escape from the liquid surface into the vapor phase. In a closed, evacuated system the evaporation of liquid molecules will result in a vapor pressure being established above the liquid. As evaporation occurs, so does the reverse process, condensation. Both processes will continue to occur until a dynamic equilibrium is established whereby both forward and reverse processes continue to occur at equal rates, and the vapor pressure above the liquid remains constant. The vapor pressure over the liquid when this equilibrium has been established is the equilibrium vapor pressure. A substance with relatively weak intermolecular forces will have a greater tendency to escape into the vapor phase and will exhibit a higher vapor pressure then a substance with stronger intermolecular forces. A substance with a high vapor pressure is said to be volatile.

As temperature increases, the number of liquid molecules with enough energy to escape from the liquid surface increases, resulting in a higher equilibrium vapor pressure.

Figure 11. 21. Distribution of kinetic energies of surface molecules of a hypothetical liquid at two temperatures. Only the fastest molecules have sufficient kinetic energy to escape the liquid and enter the vapor, as shown by the shaded areas. The higher the temperature, the larger the fraction of molecules with enough energy to escape.


Vapor Pressure vs. Temperature


A volatile substance is one that:
explodes easily
vaporizes readily
has very strong intermolecular forces
exists only as a gas

One measure of a compound's intermolecular forces is its boiling point. A liquid boils when its vapor pressure equals the external pressure acting on the liquid's surface. The boiling point of a liquid at 1 atm pressure is called its normal boiling point. This explains why cooking can take longer at high elevations; the atmospheric pressure is lower at higher altitudes, so water boils at a lower temperature. When the atmospheric pressure is less than 760 torr, water boils at less than 100°C.

Figure 11. 22. Vapor pressure of four common liquids, shown as a function of temperature.
The temperature at which the vapor pressure is 760 torr is the normal boiling point of each liquid.


Equilibrium Vapor Pressure

  1. Choose a liquid for each flask from the drop-down menus.
  2. Starting with the temperature at 0°C, dial up the temperature gradually to 100°C to plot the vapor pressure versus temperature for the two liquids.


Based on data from the Equilibrium Vapor Pressure simulation, estimate the boiling point of benzene.

Ethanol and acetic acid have very similar molar masses and both exhibit hydrogen bonding. In which of these two compounds is hydrogen bonding a more significant component of the overall intermolecular forces?
Acetic acid

No. Although some volatile substances are explosive, that's not what the term volatile means.
Right. The equilibrium vapor pressure of a substance is a measure of its volatility.
No. The weaker the intermolecular forces, the more volatile a substance is.
No. Volatility refers to the ease with which a solid or liquid becomes a gas.

No. That's the highest temperature on the graph, and it's the boiling point of water but not the boiling point of benzene. Try again.
Right. In fact, the boiling point of benzene is 80.1C. That's the temperature at which its vapor pressure is equal to atmospheric pressure (~760 torr).
No. Look again. Remember that the boiling point is the point at which a liquid's vapor pressure equals atmospheric pressure.
No. The pressure on the graph is given in torr, not atmospheres. Try again.

No. Ethanol has a significantly lower boiling point than acetic acid. The lower boiling point of ethanol is evidence of weaker overall intermolecular forces. Because they are approximately the same size, we expect their dispersion forces to be comparable, so we attribute the difference to differences in hydrogen bonding.
Right. The higher boiling point of acetic acid, >100C, means that it has stronger overall intermolecular forces. Because they are approximately the same size, we expect their dispersion forces to be comparable, so we attribute the difference to differences in hydrogen bonding.

volatile: Tending to evaporate readily.
vapor pressure: The pressure exerted by a vapor in equilibrium with its liquid or solid phase.
dynamic equilibrium: state of balance in which opposing processes occur at the same rate.
normal boiling point: The boiling point at 1 atm pressure.