Liquid solutions vapor pressure curve

When a solution freezes, crystals of pure solvent usually separate out flie solute molecules are not normally soluble in the solid phase of the solvent. When aqueous solutions are partially frozen, for example, tiie solid that separates out is almost always pure ice. As a result, the part of tiie phase diagram in Figure 13.22 that represents the vapor pressure of the solid is tiie same as that for tiie pure liquid. The vapor-pressure curves for the liquid and solid phases meet at tiie triple point. (Section 11.6) In Figure 13.22 we see tiiat tiie triple point of tiie solution must be at a lower temperature than tiiat in tiie pure liquid because tiie solution has a lower vapor pressure than tiie pure liquid. 

The Class I binary diagram is the simplest case (see Fig. 6a). The P—T diagram consists of a vapor—pressure curve (solid line) for each pure component, ending at the pure component critical point. The loci of critical points for the binary mixtures (shown by the dashed curve) are continuous from the critical point of component one, Ca , to the critical point of component two,Cp . Additional binary mixtures that exhibit Class I behavior are C02 -hexane and C02 benzene. More complicated behavior exists for other classes, including the appearance of upper critical solution temperature (UCST) lines, two-phase (liquid—liquid) immiscibility lines, and even three-phase (liquid—liquid—gas) immiscibility lines. More complete discussions are available (1,4,22). Additional simple binary system examples for Class III include C02—hexadecane and C02 H20 Class IV, C02 nitrobenzene Class V, ethane— -propanol and Class VI, H20— -butanol. 

KEY CONCEPT PROBLEM 11.17 The following phase diagram shows part of the vapor-pressure curves for a pure liquid (green curve) and a solution of the first liquid with a second volatile liquid (red curve). 

Liquid and vapor are at equilibrium along the vapor pressure curves shown for pure water (solid line) and an aqueous solution (dashed line). The vapor pressure is lower for the solution, in accord with Raoult s law, and thus the boiling point is increased (liquids boil at 1 atm)... 

Fig. 31. Boiling-point curve of a solution of two liquids, A and B, whose vapor-pressure curve shows a maximum. The boiling point curve shows a minimum.

The vapor pressure curve forms the basis for the description of vapor-liquid equilibrium for a pure fluid. As the temperature increases, the vapor pressure curve for the vapor-liquid situation ends at the critical pressure. In the case of a binary or multicomponent solution, the critical point is not necessarily a maximum with respect to either temperature or pressure. It is then possible for a vapor or liquid to exist at temperature or pressures higher than the critical pressure of the mixture. At constant temperature, it is then possible for condensation to take place as the pressure is decreased. At constant pressure, condensation may take place as the temperature is increased. Vaporization can take place at constant temperature as the pressure is increased and decreased. This unusual behavior can be useful in some process situations, for example, in the recovery of natural gas from deep wells. If the conditions are right, liquefaction of the product stream is possible. At the same time, the heavier components of the mixture may be separated from the lighter components. 

Vapor-pressure curves for a pure liquid, and for a solution of an involatile solid in the liquid, showing the elevation of the boiling point. 

Ti I Vapor-pressure curves for a solvent in its solid and liquid states, and a solution of an involatile solute. The diagram shows how... 

The vapor-pressure curves for the liquid and solid phases meet at the triple point, a." (Section 11.6) In FIGURE 13.24 we see that the triple-point temperature of the solution is lower than the triple-point temperature of pure liquid because the. solution has a lower vapor pressure than the pure liquid. 

An increase in the boiling point or a decrease in the freezing point of a solution containing a nonvolatile component is compared to pure solvent caused by a reduction in the vapor pressure. The reduction of the freezing point AT of a solution is T — T, as shown in Fig. 1-42 where T is the freezing point (or melting point) of the pure solvent. At Tg the vapor pressure is the same for the liquid and solid phases of the solvent. Tg is defined by the intersection A of the vapor pressure curve VC and the sublimation pressure curve SC of the solvent. If only pure solvent freezes, the freezing point T of the solution occurs at the intersection B of the vapor pressure curve of the solution VCS and the sublimation pressure curve of the solvent SC. 

The freezing behavior of a solution can also be considered in terms of lowered vapor pressure. Figure 14.8b shows the vapor pressure relationships of ice, water, and a solution containing 1 mol of solute per kilogram of water. The freezing point of water is at the intersection of the liquid and solid vapor pressure curves (i.e., at the point where water and ice have the same vapor pressure). Because the vapor pressure of the liquid is lowered by the solute, the vapor pressure curve of the solution does not intersect the vapor pressure curve of the solid until the solution has been cooled below the freezing point of pure water. So the solution must be cooled below 0°C in order for it to freeze. 

The normal boiling point of a liquid is the temperature at which its vapor pressure equals 1 atm. Because the addition of a nonvolatile solute to a liquid reduces its vapor pressure, the temperature must be increased to a value greater than the normal boiling point to achieve a vapor pressure of 1 atm. Figure 12.20 shows the vapor-pressure curve of a solution. The curve is below the vapor-pressure curve of the pure liquid solvent. The boilii -point elevation, AT, is a colligative property of a solution equal to the boiling point of the solution minus the boiling point of the pure solvent. 

This is called the activity coefficient, and it is analogous to the fugacity coefficient for gases. For a species that exists as a liquid in its standard state at the temperature of the solution, the activity coefficient approaches 1 as the mole fraction of the substance approaches unity. This is simply a statement that Raoult s law is at its best as an approximation in the regions close to the end points on the vapor pressure curve. 

Vapor Pressures of Solutions—The vapor pressure of a solution depends on the vapor pressures of its pure components. If the solution is ideal, Raoult s law (equation 14.3) can be used to calculate the solution vapor pressure. Liquid-vapor equilibrium curves showing either solution vapor pressures (Fig. 14-16) or solution boiling points (Fig. 14-17) as a function of solution composition... 

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