Kaplan MCAT General Chemistry Review

When a compound is heated, the temperature rises until the melting or boiling points are reached. Then the temperature remains constant as the compound is converted to the next phase (i.e., liquid or gas, respectively). Once the entire sample is converted, then the temperature begins to rise again. See the heating curves depicted in Figure 8.4.

Figure 8.4

Phase Diagrams

Phase diagrams are graphs that show the temperatures and pressures at which a substance will be thermodynamically stable in a particular phase. They also show the temperatures and pressures at which phases will be in equilibrium.

Key Concept

Every pure substance has a characteristic phase diagram.

SINGLE COMPONENT

The phase diagram for a single compound is shown in Figure 8.5.

Figure 8.5

Real World

Because of H₂O’s unique properties, ice floats and ice skates flow smoothly over ice. This all “boils” down to the negative slope of the solid-liquid equilibrium line in its phase diagram. Because the density of ice is less than that of liquid H₂O, an increase in pressure (at a constant temperature) will actually melt ice (the opposite of the substance seen in Figure 8.5).

The lines on a phase diagram are called the lines of equilibrium or the phase boundaries and indicate the temperature and pressure values for the equilibria between phases. The lines of equilibrium divide the diagram into three regions corresponding to the three phases—solid, liquid, and gas—and they themselves represent the phase transformations. Line A represents crystallization/fusion, line B vaporization/ condensation, and line C sublimation/deposition. In general, the gas phase is found at high temperatures and low pressures, the solid phase is found at low temperatures and high pressures, and the liquid phase is found at moderate temperatures and moderate pressures. The point at which the three phase boundaries meet is called the triple point. This is the temperature and pressure at which the three phases exist in equilibrium. The phase boundary that separates the solid and the liquid phases extends indefinitely from the triple point. The phase boundary between the liquid and gas phases, however, terminates at a point called the critical point. This is the temperature and pressure above which there is no distinction between the phases. Although this may seem to be an impossibility—after all, it’s possible always to distinguish between the liquid and the solid phase—such “supercritical fluids” are perfectly logical. As a liquid is heated in a closed system its density decreases and the density of the vapor sitting above it increases. The critical point is the temperature and pressure at which the two densities become equal and there is no distinction between the two phases. The heat of vaporization at this point and for all temperatures and pressures above the critical point values is zero. So it’s certainly possible to create a supercritical fluid, but definitely not common (in everyday life). Rest assured that you have never come close to approaching, say, the critical point temperature and pressure for water, no matter how much “industrial strength” your high-pressure home espresso machine possesses: 647 K (374°C or 705°F) and 22.064 MPa (218 atm).

Figure 8.6

MULTIPLE COMPONENTS

The phase diagram for a mixture of two or more components is complicated by the requirement that the composition of the mixture, as well as the temperature and pressure, must be specified. For example, consider a solution of two liquids, A and B, shown in Figure 8.7. The vapor above the solution is a mixture of the vapors of A and B. The pressures exerted by vapor A and vapor B on the solution are the vapor pressures that each exerts above its individual liquid phase. Raoult’s law enables one to determine the relationship between the vapor pressure of gaseous A and the concentration of liquid A in the solution.

Figure 8.7