Dependence of Boiling Point on Pressure

Update of Dependence of Boiling Point on Pressure Section 16 Health and Safety Information... 

The relation between temperature and pressure in a system with two phases in equilibrium is shown by the coexistenee curve separating the two one-phase areas on the pressure-temperature diagram (see Fig. 8.6). Consider the liquid-gas curve. If we think of T as the independent variable, the curve is a vapor-pressure curve showing how the vapor pressure of the liquid varies with temperature. If, however, p is the independent variable, then the curve is a boiling-point curve showing the dependence of the boiling point on pressure. 

As with vapor-pressure lowering (Section 11.6), the actual amount of boiling-point elevation and freezing-point depression observed for solutions of ionic substances depends on the amount of dissociation, as given by a van t Hoff factor. The formulas for both boiling-point elevation and freezing-point depression can be modified to take dissociation into account ... 

In Section A.l, the general laws of thermodynamics are stated. The results of statistical mechanics of ideal gases are summarized in Section A.2. Chemical equilibrium conditions for phase transitions and for reactions in gases (real and ideal) and in condensed phases (real and ideal) are derived in Section A.3, where methods for computing equilibrium compositions are indicated. In Section A.4 heats of reaction are defined, methods for obtaining heats of reaction are outlined, and adiabatic flame-temperature calculations are discussed. In the final section (Section A.5), which is concerned with condensed phases, the phase rule is derived, dependences of the vapor pressure and of the boiling point on composition in binary mixtures are analyzed, and properties related to osmotic pressure are discussed. 

When a distillation is carried out in a system open to the air and the boiling point is thus dependent on existing air pressure, the prevailing barometric pressure should be noted and allowance made for appreciable deviations from the accepted boiling point temperature (see Table 1). Distillation can also be done at the lower pressures that can be achieved by an oil pump or an aspirator with substantial reduction of boiling point. 

Boiling points are pressure dependent because pressure has a large effect on the entropy of a gas. When a gas is expanded (pressure is decreased), its entropy increases because the degree of disorder of the molecules increases. At sea level, water boils at 100°C. In Denver, Colorado, where the elevation is 1.6 km, atmospheric pressure is about 0.84 times the pressure at sea level. At that elevation, water boils at about 95°C. On Pike s Peak, where the elevation is 4.3 km, water boils at about 85°C. People often use pressure cookers at that altitude to increase the boiling point of water. 

We have seen (Section 12-15) how the presence of strong attractive forces between gas molecules can cause gas behavior to become nonideal when the molecules get close together. In liquids and solids the molecules are much closer together than in gases. As a result, properties of liquids, such as boiling point, vapor pressure, viscosity, and heat of vaporization, depend markedly on the strengths of the intermolecular attractive forces. These forces are also directly related to the properties of solids, such as melting point and heat of fusion. Let us preface our study of these condensed phases with a discussion of the types of attractive forces that can exist between molecules and ions. 

The molality scale is useful for experiments in which physical measurements (as of freezing point, boiling point, vapor pressure, etc.) are made over a wide range of temperatures. The molality of a given solution, which is determined solely by the masses of the solution components, is independent of temperature. The molar concentration or the normality of a solution, on the other hand, being defined in terms of the volume, may vary appreciably as the temperature is changed, because of the temperature dependence of the volume. In dilute aqueous solutions (< 0.1 M) the molality is very close numerically to the molar concentration. 

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