Liquids equilibrium vapor pressure

TABLE 26-15 Liquids Having Equilibrium Vapor Pressure near the... 

The equilibrium vapor pressure above a confined liquid depends only on temperature. The fraction of the total pressure exerted by vapor pressure determines the composition of the vapor-air mixture. Thus when the total pressure is reduced, for example at high elevations or in vacuum tmcks, the vapor concentration in air increases. Since flash points are reported at a... 

The vapor pressure (P ) of a pure liquid at a given temperature (T) is the pressure exerted by its vapor in equilibrium with the liquid phase in a closed system. All liquids and solids exhibit unique vapor pressure-temperature curves. For instance, in Figure 2-79, lines BA and AC represent the equilibrium vapor pressure curves of the solid and liquid phases, respectively. 

Vapor—Liquid Systems. The vapor-liquid region of a pure substance is contained within the phase or saturation envelope on a P-V diagram (see Figure 2-80), A vapor, whether it exists alone or in a mixture of gases, is said to be saturated if its partial pressure (P.) equals its equilibrium vapor pressure (P, ) at the system temperature T. This temperature is called the saturation temperature or dew point T ... 

It is important to realize that so long as both liquid and vapor are present the pressure exerted by the vapor is independent of the volume of the container. Ifa small amount ofliquid is introduced into a closed container, some of it will vaporize, establishing its equilibrium vapor pressure. The greater the volume of the container, the greater will be the amount ofliquid that vaporizes to establish that pressure. The ratio nIV stays constant, so P = nRTIV does not change. Only if all the liquid vaporizes will the pressure drop below the equilibrium value. 

Consider an experiment in which liquid carbon dioxide is introduced into an otherwise evacuated glass tube, which is then sealed (Figure 9.4, p. 232). At 0°C, the pressure above the liquid is 34 atm, the equilibrium vapor pressure of C02(Z) at that temperature. As the tube is heated, some of the liquid is converted to vapor, and the pressure rises, to 44 atm at 10°C and 56 atm at 20°C. Nothing spectacular happens (unless there happens to be a weak spot in the tube) until 31°C is reached, where the vapor pressure is 73 atm. Suddenly, as the temperature goes above 31°C, the meniscus between the liquid and vapor disappears The tube now contains only one phase. 

Curve AB is a portion of the vapor pressure-temperature curve of liquid water. At any temperature and pressure along this line, liquid water is in equilibrium with water vapor. At point A on the curve, these two phases are in equilibrium at 0°C and about 5 mm Hg (more exactly, 0.01°C and 4.56 mm Hg). At B, corresponding to 100°C, the pressure exerted by the vapor in equilibrium with liquid water is 1 atm this is the normal boiling point of water. The extension of line AB beyond point B gives the equilibrium vapor pressure of the liquid above the normal boiling point. The line ends at 374°C, the critical temperature of water, where the pressure is 218 atm. 

Figure 5.5 Change of gas pressure with time as a liquid evaporates into a closed container. The pressure p is the equilibrium vapor pressure.

Fugacity in Liquid Mixtures Raoult s Law and Henry s Law Each component in a liquid mixture has an equilibrium vapor pressure, and hence, a vapor fugacity. These fugacities are functions of the composition and the nature of the components, with the total vapor fugacity equal to the sum of the fugacities of the components, That is,... 

Binary (vapor + liquid) equilibria studies involve the determination of / as a function of composition. the mole fraction in the liquid phase. Of special interest is the dependence of/ on composition in the limit of infinite dilution. In the examples which follow, equilibrium vapor pressures, p,. are measured and described. These vapor pressures can be corrected to vapor fugacities using the techniques described in the previous section. As stated earlier, at the low pressures involved in most experiments, the difference between p, and / is very small, and we will ignore it unless a specific application requires a differentiation between the two. 

The liquid line and vapor line together constitute a binary (vapor + liquid) phase diagram, in which the equilibrium (vapor) pressure is expressed as a function of mole fraction at constant temperature. At pressures less than the vapor (lower) curve, the mixture is all vapor. Two degrees of freedom are present in that region so that p and y2 can be varied independently. At pressures above the liquid (upper) curve, the mixture is all liquid. Again, two degrees of freedom are present so that p and. v can be varied independently/... 

In porous media, liquid-gas phase equilibrium depends upon the nature of the adsorbate and adsorbent, gas pressure and temperature [24]. Overlapping attractive potentials of the pore walls readily overcome the translational energy of the adsorbate, leading to enhanced adsorption of gas molecules at low pressures. In addition, condensation of gas in very small pores may occur at a lower pressure than that normally required on a plane surface, as expressed by the Kelvin equation, which relates the radius of a curved surface to the equilibrium vapor pressure [25],... 

Most measurements of densities of liquids below their normal boiling points are made in the presence of air. Densities reported here refer to liquids in equilibrium with a gas phase consisting of a mixture or air and vapor at a total pressure of one atmosphere below the normal boiling point and of vapor at the equilibrium vapor pressure above the boiling point. Thus air is not regarded as an impurity. 

Henry s law constants for most of the compounds of interest can be found in the literature.54 Figure 18.11 shows Henry s law constants for TCE, EDC and several gasoline compounds.19 These data are derived from water solubility data and the equilibrium vapor pressure of pure liquids at certain temperatures, and may be extrapolated correctly to field design work. Temperature has a major effect on Henry s constant and on stripper performance. Each rise of 10°C in temperature... 

When the standard states for the solid and liquid species correspond to the pure species at 1 atm pressure or at a low equilibrium vapor pressure of the condensed phase, the activities of the pure species at equilibrium are taken as unity at all moderate pressures. Consequently, the gas phase composition at equilibrium will not be... 

Kelvin s equation determines the equilibrium vapor pressure over a curved meniscus of liquid ... 

If a liquid is placed into a sealed container, molecules will evaporate from the surface of the liquid and will eventually establish a gas phase over the liquid that is in equilibrium with the liquid phase. This is the vapor pressure of the liquid. This vapor pressure is temperature dependent, the higher the temperature the higher the vapor pressure. If a solution is prepared, then the solvent contribution to the vapor pressure of the solution depends upon the vapor pressure of the pure solvent, P°soivenb and the mole fraction of the solvent. We can find the contribution of solvent to the vapor pressure of the solution by the following relationship ... 

The equilibrium vapor pressure of organic compounds is comparable to solubility in that it is a measure of the volatilization tendency from liquid or solid phases. The equilibrium vapor pressure of a gas can be conceived as its solubility in air. The vapor pressure of a liquid or solid is the pressure of the gas in equilibrium with the liquid or solid at a given temperature. The thermodynamic Clausius-Clapeyron expression (Eq. 1, [31]) describing this equilibrium is... 

Figure 11.6. Entropy of liquid He under its equilibrium vapor pressure. Data below 1.90 K from H. C. Kramers, J. D. Wasscber, and C. J. Gorter, Physica 18, 329 (1952). Data from 1.90 K to 4.00 K from R. W. Hill and O. V. Lounasmaa, Phil. Mag. Ser. 8, 2, 143 (1957).

Solvent in Solution. We shall use the pure substance at the same temperature as the solution and at its equilibrium vapor pressure as the reference state for the component of a solution designated as the solvent. This choice of standard state is consistent with the limiting law for the activity of solvent given in Equation (16.2), where the limiting process leads to the solvent at its equilibrium vapor pressure. To relate the standard chemical potential of solvent in solution to the state that we defined for the pure liquid solvent, we need to use the relationship... 

The data in Table 17.3 are for vapor pressure and vapor and liquid composition of solutions of methyl tert-butyl ether (1) and acetonitrile (2), (9). The symbol Xi represents the mole fraction of (1) in the liquid phase, and yi represents the mole fraction of (1) in the vapor phase. P is the equilibrium vapor pressure of the solution. The temperature is 313.15 K. 

As progressively higher pressures are used during N2 adsorption, capillary condensation will occur in pores that are increasingly larger. The Kelvin equation illustrates that the equilibrium vapor pressure is lowered over a concave meniscus of liquid, which is why N2 is able to condense in catalyst pores at pressures lower than the saturahon pressure ... 

Vapor pressure is exerted by a solid or liquid in equilibrium with its own vapor. All liquids have vapor pressures. Vapor pressure depends on temperature and is characteristic of each substance. The higher the vapor pressure at ambient temperature, the more volatile the substance. Vapor pressure of water at 20 0 is 17.535 torr. 

To understand vapor pressure, let s consider an empty jar that is partially filled with water and then covered with a lid. We will assume the space above the water in the jar contains only air when we screw on the jar s lid. After the lid is place on the jar, water molecules leave the liquid and enter the air above the liquid s surface. This process is known as vaporization. As time goes by, more water molecules fill the air space above the liquid, but at the same time, some gaseous water molecules condense back into the liquid state. Eventually, a point is reached where the amount of water vapor above the liquid remains constant. At this point, the rates of vaporization and condensation are equal, and equilibrium is reached. The partial pressure exerted by the water at this point is known as the equilibrium vapor pressure or just vapor pressure. Vapor pressure is directly related to the temperature, that is, the higher the temperature, the higher the vapor pressure. Table 9.4 gives... 

The term supersaturation, S, defined as (PA/PA - 1) is often expressed in the form of percent supersaturation, i.e., as 100(PA/PA - 1), where PA and P are defined in Box 14.2. The relationship between the equilibrium vapor pressure over the droplet and that over the bulk liquid [Eq. (HH)] is often expressed in a simplified form using the supersaturation ... 

Adsorption studies leading to measurements of pore size and pore-size distributions generally make use of the Kelvin equation which relates the equilibrium vapor pressure of a curved surface, such as that of a liquid in a capillary or pore, to the equilibrium pressure of the same liquid on a plane surface. Equation (8.1) is a convenient form of the Kelvin equation ... 

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