The vapor pressure of a liquid is the partial pressure of the vapor over the liquid,
measured at equilibrium. In molecular terms, vapor pressure involves molecules of a
liquid vaporizing from the liquid phase. The liquid molecules gain enough kinetic energy
through collisions with other molecules to vaporize. They become a gas and exert a
pressure in the usual way. The equilibrium is a dynamic one because molecules of the
liquid are continually leaving the liquid phase and returning to it from the vapor phase. At
equilibrium, the rate of vaporization and the rate of condensation are exactly equal.
As explained in 11.3, only the fastest, most energetic liquid-phase molecules can escape
into the gas phase. Thus when they leave, the average energy of the remaining liquid-
phase molecules decreases, as does the temperature. In other words, vaporization is like
breaking bonds, which are holding the molecules together in the liquid; so vaporization is
The pressure in the cylinder of nitrogen at room temperature (above its critical
temperature of -147
C) decreases continuously as gas is released because the number
of molecules in the vapor phase, which governs the pressure, decreases continuously.
The pressure in the cylinder of propane at room temperature (below its critical
temperature) is constant because liquid propane and gaseous propane exist at
equilibrium in the cylinder. The pressure will remain constant at the vapor pressure of
propane until only gaseous propane remains. At that point, the pressure will decrease
until all of the propane is gone. The phase diagram shown for the solution of Question
11.101 (see below) may help: propane would have the properties of species (a) and
nitrogen the properties of species (c) in that question.
Hydrogen bonding is a weak to moderately-strong attractive force that exists between a
hydrogen atom covalently bonded to a very electronegative atom, X (N, O, or F), and a
lone pair of electrons on another small, electronegative atom, Y. (X and Y may be the
same or different elements.) Hydrogen bonding in water involves a hydrogen atom of one
water molecule bonding to a lone pair of electrons on the oxygen atom of another water
molecule. Usually, the covalent bond to H and its hydrogen bond are in a collinear
arrangement, as shown below.
The answer is e, cooling the H
O sample from 105
C to 84
For each of processes a, c, d, and e, the temperature change is approximately the same,
so m x s x ΔT is the same (assuming the specific heats of water and steam are the
same). However, processes (a) and (e) also involve an exothermic phase change. The
ΔH of condensation (e) is much larger than that for freezing (a), so the largest amount of
heat is released in (e).
As shown in lecture, the condensation of steam releases much more energy than cooling