Vapor Pressure

10 Key excerpts on "Vapor Pressure"

• 

  eBook - ePub

  Liquid Matter, Revised Edition

  • Joseph Angelo(Author)
  • 2020(Publication Date)
  • Facts On File

    (Publisher)

  The term evaporation is also used. In the reverse process, called condensation, a substance in the gas (vapor) state becomes a liquid. When the evaporation rate of a liquid equals the condensation rate of the vapor, scientists say the liquid and vapor (gas) states of the particular substance are in equilibrium. They define the boiling point as the temperature, at a specified pressure, at which a substance in the liquid state experiences a change into the gas state. When a substance such as oxygen or nitrogen that is normally encountered as a gas on Earth experiences a change to the liquid state, scientists call the process liquefaction. Sometimes a solid substance is at a temperature and pressure that allows it to transition directly into the gas (vapor) state. Scientists call this process sublimation. A frozen block of carbon dioxide (CO 2) at room temperature and one atmosphere pressure will transition (sublime) directly into gaseous carbon dioxide. Scientists refer to the reverse process as deposition. Under certain environmental conditions, water vapor in Earth's atmosphere transitions directly into the familiar solid known as snow. Once formed, snowflakes gently deposit themselves on the planet's surface. Vapor Pressure is an important thermodynamic property of a liquid substance. Engineers regard it as a useful measure of a liquid's inclination to evaporate. Volatile substances are those with high Vapor Pressures at normal temperatures and pressures. Chemists use the term volatility to describe the tendency of a liquid to vaporize. Consider a sealed flask of liquid with a sufficient unfilled volume (space) above the liquid's free surface. Under equilibrium conditions, a number of molecules in the liquid will escape across the free surface and occupy the open space as gas molecules. Scientists define the Vapor Pressure of a liquid as the partial pressure of the vapor over the liquid under equilibrium conditions at a specified temperature

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Physico-Chemical Analysis of Molten Electrolytes

  • Vladimir Danek †(Author)
  • 2006(Publication Date)
  • Elsevier Science

    (Publisher)

  Chapter 7

  Vapor Pressure

  Publisher Summary

  This chapter discusses the thermodynamic principles and the measurement methods of Vapor Pressure. Vapor Pressure is an important part of investigation on molten salts. The mass spectroscopy method determines the composition of the gas phase and also determines the activities of components and the composition of both the liquid and vapor phases. Formation of vapor includes all the processes in which gas is created from a system of condensed phases. The measurement of Vapor Pressure is connected to the determination of equilibrium between the gaseous and liquid or solid phases. Pressure is one of the basic variables in thermodynamics. It is the force exerted by the system on the unit area of its wall. Two major experimental methods are used for the measurement of Vapor Pressure at high temperatures: the boiling point method and the transpiration method. Pressure equalization occurs very rapidly in the boiling point method, whereas thermal transport is rather slow. The transpiration method is a simple and versatile method for Vapor Pressure measurement at high temperatures. Hence, results from the transpiration method are also used to calculate the average molar mass of the vapor.

  The study of Vapor Pressure is an important part of investigation on molten salts. In connection with the mass spectroscopy method, which enables us to determine the composition of the gas phase, it is a useful tool to determine the activities of components and thus the composition of both the liquid and vapor phases.

  Formation of vapor includes all the processes in which gas is created from a system of condensed phases. Measurement of Vapor Pressure is closely connected to the determination of equilibrium between the gaseous and liquid or solid phases.

  7.1 THERMODYNAMIC PRINCIPLES

  Pressure is one of the basic variables in thermodynamics. It is the force exerted by the system on the unit area of its wall. In the SI system, the basic unit of pressure is Pascal (Pa). One Pascal is equal to the force of one Newton exerted on one square meter, Pa = N · m−2

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Physical and Chemical Equilibrium for Chemical Engineers

  • Noel de Nevers(Author)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  Chapter 5 Vapor Pressure, the Clapeyron Equation, and Single Pure Chemical Species Phase Equilibrium The Vapor Pressure is crucially important in a wide variety of physical and chemical equilibrium situations, such as those in Chapter 3. This short chapter discusses Vapor Pressure and related topics. 5.1 Measurement of Vapor Pressure The Vapor Pressure for a pure species is that pressure exerted by a pure sample of the liquid at a fixed temperature. The experimental procedure to measure Vapor Pressure is sketched in Figure 5.1. A sample of the material to be tested is placed in a closed container, with the amount chosen so that there will be both vapor and liquid present. Then the temperature is made constant over the whole container, normally by placing the whole container in a constant-temperature bath, with circulating water or some other heat transfer fluid. When the temperature and pressure readings have reached constant values, these are recorded, and the constant temperature bath is set for a new temperature. When the temperature and pressure readings are again unchanging, the values are again recorded, and a new temperature is chosen, continuing until the desired range of temperatures has been tested. Figure 5.1 Simplified schematic of the device for measuring the Vapor Pressure. Real vapor-pressure measuring devices are more refined versions of that shown in Figure 5.1, with special attention directed to getting very accurate measurements of the temperature and pressure, and to making sure that all the air and other possible contaminants are removed from the container, so that the sample being measured is as pure as possible. In principle, the measurement of the Vapor Pressure of solids is conducted exactly the same way as that of liquids; in practice, the pressure measurements are difficult because the pressures are very low

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Chemical Property Estimation

  Theory and Application

  • Edward Baum(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  CHAPTER 6 Vapor Pressure 6.1 INTRODUCTION

  This chapter describes methods of estimating the Vapor Pressure of a pure chemical and of chemicals in mixtures. The (saturation) Vapor Pressure, PS (Pa), is the pressure of a pure chemical vapor that is in equilibrium with the pure liquid or solid. It is an important control of a chemical’s partitioning between air, water, and soil and of its volatilization rate. For vapor-solid equilibrium, PS is sometimes called the sublimation pressure. The Vapor Pressure of a chemical in a mixture of volatile chemicals is its partial Vapor Pressure.

  Many different units of pressure are widely used in addition to the pascal, the SI unit. They include the standard atmosphere (1 atm= 101.325 kPa), the bar (1 bar = 100 kPa), the torr (1 atm = 760 torr), and the millimeter of mercury (1 mmHg ≈ 1 torr). The mmHg and the torr are almost exactly equal and can be used interchangeably.

  The reported saturation Vapor Pressures of chemicals at ordinary temperatures range from 760 to below 1 × 10−9 torr. Many hazardous chemicals exhibit Vapor Pressures of less than 1 torr in the normal ambient temperature range of −40 to +40°C. Vapor Pressures below 1 torr are difficult to measure, and reliable values are hard to find in the literature. Computational methods that offer reasonably accurate estimates of such low Vapor Pressures are particularly useful, therefore, and of particular interest to environmental specialists. The subject was reviewed by Burkhard et al. (1985) and Reid et al. (1987). The literature prior to 1981 was reviewed by Mackay et al. (1982) and by Grain (1990a).

  6.2 A Vapor Pressure MODEL

  Saturation Vapor Pressure is a sensitive function of molecular structure and ambient temperature. Molecular structure determines the type and strength of the attractive intermolecular forces that a chemical exhibits. The attractive forces are, in order of increasing strength: the London dispersion forces exhibited by all molecules, polar interactions exhibited by all asymmetric molecules, and hydrogen bonding exhibited by molecules containing O–H, N–H, and F–H bonds. The energy required to escape the liquid phase and the Vapor Pressure depends on the sum of all forces exhibited by a chemical.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Handbook of Property Estimation Methods for Chemicals

  Environmental Health Sciences

  • Donald Mackay, Robert S. Boethling, Donald Mackay, Robert S. Boethling(Authors)
  • 2000(Publication Date)
  • CRC Press

    (Publisher)

  A substance’s vapor pres- 54 Handbook of Property Estimation Methods for Chemicals sure determines whether it will occur as a free molecule in the atmosphere or will be asso-ciated with particulate matter (see Chapter 10). This chapter provides a simple procedure for estimating the Vapor Pressure of a sub-stance at normal environmental temperatures. For volatile substances that boil at or below 100°C, the Vapor Pressure is likely to be known, but, for many high-boiling substances with low Vapor Pressure, the value may be unknown or poorly known. An estimation procedure may be needed to help convert the known Vapor Pressure at the normal boiling point (i.e., 1 atmosphere) to the Vapor Pressure at the lower temperatures of environmental impor-tance. For some of these high-boiling compounds, the actual boiling point may also be unknown, since the substance may decompose before it boils. In that case, the boiling point must be estimated using one of the procedures discussed in Chapter 2. The Vapor Pressure of a chemical substance increases rapidly with temperature. Many equations have described this temperature dependence so that the Vapor Pressure can be calculated for a temperature of interest. The Antoine (1888) equation is most familiar. Riddick, Bunger, and Sakano (1996) provide an extensive discussion of many of the empirical equations used to describe the temperature dependence of the Vapor Pressure. To apply a particular equation to a chemical, the necessary parameters must be available from experimental data. If no such information exists, some method is needed to estimate the parameters. Many compilations of Vapor Pressure are available, some for certain classes of chemicals and others for organic chemicals in general. In older compilations, the units of vapor pres-sure generally are given in mm Hg or torr, atmospheres (for more volatile substances), psi, or mbars. Most recent compilations report Vapor Pressures in Pa or kPa.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Water Vapor Measurement

  Methods and Instrumentation

  • Pieter R. Wiederhold(Author)
  • 2012(Publication Date)
  • CRC Press

    (Publisher)

  2 DEFINITIONS AND FUNDAMENTALS Water vapor measurements are closely related to--and dependent on-temperature and pressure. Water vapor is a form of gas and therefore follows the traditional gas laws. This chapter discusses definitions, equations and formulas showing the interrelationships. Much of the information presented in this chapter was derived from General Eastern's Handbook 4 • I. Temperature, Pressure, Humidity and Gases Every substance can exist as a solid, liquid or gas, depending on the temperature and pressure that it experiences. At low temperatures all substances become solids. At some intermediate temperature these substances become liquids, and at sufficiently high temperatures they become gaseous. Temperatures needed to achieve a given physical state in one substance do not necessarily yield the same state for another. Melting point and boiling point tables have been tabulated for many substances and can be found in chemistry handbooks. A substance in the solid state has a definite shape, which is rigid and tends to resist change. Solids form a crystalline lattice where each molecule is fixed in space relative to each other. A liquid substance will conform to any confining volume, flows easily, and is rela-tively incompressible. It is the state of matter intermediate between solid and gas. Liquids can also possess a free surface. Gaseous substances can expand readily and without limit to fill any confining volume. They have the lowest density of all the physical states. The following discussion will concentrate specifically on the gaseous state of matter. A. Temperature Temperature is defined as: • The degree of hotness or coldness of a body or environment • A specific degree of hotness or coldness as indicated on, or referred to, a standard scale. A scale must be independent of the size of the system and must determine the direction of heat flow between any two systems that are in thermal contact. 7

  Sign up to read

  Learn more about book
• 

  eBook - ePub

  Environmental Organic Chemistry

  • René P. Schwarzenbach, Philip M. Gschwend, Dieter M. Imboden, Rene P. Schwarzenbach(Authors)
  • 2016(Publication Date)
  • Wiley

    (Publisher)

  rate of evaporation of a compound from its pure phase or a mixture.

  By simple observation, we know that at ambient temperatures (e.g., 25°C) and pressures (e.g., 1 bar), some organic chemicals in their pure form are present as gases, some as liquids, and others as solids. We should recall that when we talk about a pure chemical, we mean that only molecules of that particular compound are present in the phase considered. Hence, in a pure gas, the partial pressure of the compound is equal to the total pressure. As already addressed in Chapter 4 , a pure compound will be a liquid or a solid at ambient conditions if the forces between the molecules in the condensed phase are strong enough to overcome the tendency of the molecules to “fly” apart. In other words, if the enthalpy terms (which reflect the “glue” among the molecules in the liquid) outweigh the entropy terms (which are measures of “freedom” gained when going from the liquid phase to the gas phase), then the free energy term is positive, and the material will exist as a liquid or solid. Conversely, if this free energy term is negative, then the compound is a gas at the given conditions. This variation in phase is illustrated by the series of n-alkanes, where the C1 – to C4 –compounds are gases (p*

  i

  > 1 bar), the C5 – to C17 –compounds are liquids, and the compounds with more than 18 carbon atoms are solids at 25°C and 1 bar total pressure (Fig. 8.1 ) This series of hydrocarbons exhibits a Vapor Pressure range of more than fifteen orders of magnitude ranging from 40.7 bar or 4.07×106 Pa (C2 H6 ) down to about 10−14 bar or 10−9 Pa (n-C30 H62

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  The Characterization of Chemical Purity

  Organic Compounds

  • L. A. K. Staveley(Author)
  • 2016(Publication Date)
  • Butterworth-Heinemann

    (Publisher)

  Synthetic materials usually contain only a few by-products, and generally the nature of these is known. In such cases, the probability that the system conforms to condition (9) is very small and an organic chemist need not fear that every one of his carefully rectified samples is a mixture. THE PRINCIPLES OF VAPOUR PRESSURE AND BOILING TEMPERATURE MEASUREMENTS The method and the device used for liquid-vapour equilibrium measure-ments must fulfil at least the following requirements : (/) The liquid and the vapour must be kept in thermodynamic equilibrium during the determination of the equilibrium parameters P, V, T, x. (ii) The parameters must be kept constant, and measured with adequate precision. (in) The effect of gravity (hydrostatic pressure) on the equilibrium under the given conditions must be known so that suitable corrections can be made. (iv) The consumption of the liquid must be as low as possible. The last condition is imposed by the increasing demands for high purity of the liquids used in scientific work. There is a general trend to use costly standard samples, available in small amounts. We shall not discuss methods which certainly do not fulfil conditions (/'), as for instance, the determination of vapour pressure or vapour composition by bubbling an inert gas through the liquid followed by analysis of the 'saturated' gas. Let us examine more closely whether and under what circumstances the above conditions are satisfied in the use of the two most common techniques 56 TEMPERATURE MEASUREMENTS for studying the equilibrium, briefly called the dynamic method and the static method. The dynamic method The dynamic method consists in determining the boiling temperature of a liquid brought into circulation by being heated in one part of the apparatus and cooled in another, with return of the vapour which has been liquefied in a reflux condenser. The device is called an equilibrium still or an ebulliometer.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Physics and Chemistry of Clouds

  • Dennis Lamb, Johannes Verlinde(Authors)
  • 2011(Publication Date)
  • Cambridge University Press

    (Publisher)

  3.8. The vertical axis is the vapor pres- sure calculated from Eq. (3.61) for the temperature and solute mole fraction specified on 3.5 Multicomponent systems 149 Solute mole fraction 1 0 0.2 0.4 0.6 0.8 Vapor Pressure, hPa 500 0 1000 Temperature, deg C 75 50 25 –25 0 100 Figure 3.8 Effects of solute and temperature on the equilibrium Vapor Pressure of liquid water. the abscissas. The Vapor Pressure over a solution is now seen to be a surface because it depends on two independent variables (T and x s ). Along the “back” side of the graph, where x s = 0, we see the exponential rise of Vapor Pressure with temperature for pure water. In this limiting case, the system contains only water, so the function reverts back to a curve (our familiar Clausius–Clapeyron relationship). So, too, whenever the solute amount is fixed, but the temperature is allowed to vary, the dependence of Vapor Pressure on temperature over a solution maintains the same one-dimensional form, but all values are reduced by the factor (1 − x s = x w ). Thus, as shown by the dashed curve in Fig. 3.8, if half the molecules in the solution were solute, the equilibrium Vapor Pressure would be just half of its pure-water value. At any given temperature, on the other hand, the Vapor Pressure follows the linear trend shown by the straight line (at about 60 ◦ C). The greater the solute concentration (larger x s ), the more the Vapor Pressure over the solution is suppressed. The mathematical relationship (Eq. (3.61)) and the graphical depiction both provide the same information, but each offers its own viewpoint. 3.5.2 Effect of solute on melting The same solute that lowers the equilibrium Vapor Pressure of a solution also lowers the melting point of ice. We can see how the two phenomena are related by consider- ing the phase diagram in the vicinity of the nominal melting point of ice, that is, near T = T 0 = 0 ◦ C.

  Sign up to read

  Learn more about book
• 

  eBook - PDF

  Physical Chemistry

  • Robert J. Silbey, Robert A. Alberty, George A. Papadantonakis, Moungi G. Bawendi(Authors)
  • 2021(Publication Date)
  • Wiley

    (Publisher)

  Another deficiency in this simple equation is that the vapor has been assumed to be an ideal gas. Over narrow ranges of temperatures, the enthalpy of vaporization can be taken to be a linear func- tion of temperature (see equation 5.43). However, in calculating Vapor Pressures over a wider range of temperature, we have to recognize that the enthalpy of vaporization approaches zero as the temperature approaches the critical temperature, as shown in Fig. 6.5. log (P/Pa) 6 5 10 3 K/T 4 3 2 2 3 4 Std BP FP FIGURE 6.4 Vapor Pressure of water (FP = freezing point, BP = boiling point). 186 CHAPTER 6 Phase Equilibrium Table 6.1 Vapor Pressures of Ice and Water t / ∘ C P/kPa −40 0.013 −30 0.038 −20 0.103 −10 0.260 0 0.611 10 1.228 20 2.338 30 4.245 40 7.381 60 19.933 100 101.325 140 361.21 180 1001.9 FIGURE 6.5 The enthalpy of vaporization approaches zero as the temperature approaches the critical temperature. Δ vap H T c T 0 The dependence of the heat of vaporization on temperature can be approximately represented by Δ vap H = A + BT + CT 2 (6.15) Thus equation 6.9 can be written dP vap P vap = Δ vap H RT 2 dT = 1 R ( A T 2 + B T + C ) dT (6.16) Integration yields ln P vap = 1 R ( − A T + B ln T + CT + D ) (6.17) where D is the integration constant. Thus Vapor Pressures determined experimentally over a range of tem- peratures can be represented by values of A, B, C, and D determined by curve fitting to minimize the sum of the squares of the deviations from the experimental values. Since the enthalpy of vaporization is given by Δ vap H = RT 2 ( d ln P vap dT ) P = −R [ d ln P vap d(1∕T) ] P (6.18) where P is the total pressure on the surface of the liquid, the heat of vaporization as a function of tem- perature can be obtained by differentiating ln P vap with respect to T or 1/T. The use of these equations is

  Sign up to read

  Learn more about book