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
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.
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.
Pressure vs. Temperature
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.
11. 22. Vapor pressure of four common liquids, shown as a function
The temperature at which the vapor pressure is 760
torr is the normal boiling point of each liquid.
- Choose a liquid for
each flask from the drop-down menus.
- 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.
||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 80.1°C. 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
||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, >100°C, 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.
to evaporate readily.
pressure: The pressure exerted by a vapor in equilibrium with its liquid
or solid phase.
equilibrium: state of balance in which opposing processes occur at the
boiling point: The boiling point at 1 atm pressure.