Properties of gases

The state of a gas is defined by the values of its volume (V), its absolute thermodynamic temperature (T), its absolute pressure (P) and the amount of substance or number of moles ( ). An equation of state is a mathematical relationship between these four physical quantities /= P,V,T,n). The equation is obtained from knowledge of the experimental behavior of a system. 

Ideal gases. The ideal gas assumption is an ideal state, where the size of the microscopic entities (i.e., atoms or molecules) constitutive of the gas is negligible and the interatomic or intermolecular forces existing between them are neglected in a first approximation. Therefore, the ideal gas assumption is suitable for assessing properties of common gases under low pressure or at high temperature. 

Real gases. In real gases, non-ideality arises from either atomic or molecular size or intermolecular interactions caused by electrostatic attraction or repulsion (i.e.. Coulomb s forces). The departure from ideal behavior of a gas is particularly noticeable under high pressure or at cryogenic temperatures. Under high pressure, the volume occupied by the atoms or molecules of gas is no longer negligible compared with the overall volume and electrostatic attractions are more important so the equation of state of the actual gas must take additional parameters into account. 

The pressure of a fluid (e.g., liquid, gas) is a scalar physical quantity, denoted P, and is expressed in the SI in pascals (Pa), corresponding to the force F expressed in newtons (N), exerted uniformly onto 

Under the tangential force, the hquid would move alone along the wall. 

Many molecular compounds, most often those with low molar masses, exist as gases at room temperature. Table 11.1 lists some gaseous compounds that may be familiar to you. 

TABLE 11.1 I Moiecuiar OunptJumis Thar Are Gjaos at Room TemperatU C 

Gases differ from the condensed phases (solids and liquids) in the following important ways  

A sample of gas assumes both the shape and volume of its container. Like a liquid, a gas consists of particles (molecules or atoms) that do not have fixed positions in the sample [ W Section 1.2]. As a result, both liquids and gases are able to flow. (Recall from Chapter 1 that we refer to liquids and gases collectively as fluids.) While a sample of liquid will assume the shape of the part of its container that it occupies, a sample of gas will expand to fill the entire volume of its container. 

Gases are compressible. Unlike a solid or a liquid, a gas consists of particles with relatively large distances between them that is, the distance between any two particles in a gas is much larger than the size of a molecule or atom. Because gas particles are far apart, it is possible to move them closer together by confining them to a smaller volume. 

Compared with liquids and solids, gases exhibit some very unique properties. Let us take a look at a few of these properties. 

FIGURE 9.1 Unlike liquids and solids, gases can be compressed, or forced to occupy a smaller volume. (From Kenkel, J., Kelter, P., and Hage, D., Chemistry An Industry-Based Introduction with CD-ROM, CRC Press, Boca Raton, FL, 2001. With Permission.) 

The air that surrounds us is a sea of mixed gases called the atmosphere. It is not necessary, then, to search very far to find a gas whose properties we may study. Some of the familiar characteristics of air—in fact, of all gases—are the following  

Compression and expansion properties of gases. The piston and cylinder show that gases may be compressed and that they expand to fill the volume available to them. 

From Eqn 5-3 it is evident that the two properties of a gas basically involved in its performance as a lubricating fluid are its pressure behavior and its viscosity. How they are involved depends on what aspect of gas lubrication is being examined. For most lubrication problems gases are regarded as obeying the ideal equation of state, 

The viscosity of an ideal gas by kinetic molecular theory is given by 

Equations 5-12 and 5-13 both indicate departure from ideal gas behavior, the one by intermolecular attraction and the other by an occupied volume effect. Such departure from ideal behavior has a direct influence on density and hence on the transformation of the generalized Reynolds equation. An equation of state for the gas such as the van der Waals equation, 

Pinkus and B. Sternlicht, Theory of Hydrodynamic Lubrication, McGraw-Hill, New York, 1961, pp. 178-196. 

MEASUREMENT OF FLUID FILM THICKNESS AND DETECTION OF FILM FAILURE 

Even in this brief overview, we see that a single pollutant—NO2 in this case—can have a complex impact on the air around us. We also see that the effects of various pollutants can be coupled in unexpected ways the simultaneous presence of NO2 and VOCs leads to the generation of O3, another of the criteria pollutants. Clearly, the chemistry of air pollution is quite complex, so we will not be able to delve into many aspects of it in great detail. But it should be clear that to improve our understanding of these important issues, we must first explore the properties of gases, gas mixtures, and chemical reactions involving gases. 

Several observable properties are common to all gases and distinguish them fi-om 

Some of these properties help explain why air pollution is difficult to combat. The fact that gases expand to fill the available volume, for example, means that exhaust gases released from a car or an industrial plant will readily spread throughout the surrounding area. Similarly, the fact that gases mix readily with one another means that once they are present, traces of gaseous pollutants will usually be difficult to separate from the air. 

As we work toward developing an accurate model for gas behavior, we will want to be sure that it can account for all of these observations. We ll want our model to be based on a molecular level understanding of what a gas is like, and we ll also want it to enable us to make accurate numerical predictions about how a gas will respond to different conditions. 

As it turns out, you probably already know an important quantitative part of the model we use most often to describe gases. If there is just one equation that you remember from your high school chemistry class, chances are it is the ideal gas law  

As a gas, water occupies 1 300 times as much space as it does as a liquid at 25 °C and atmospheric pressure 

To learn about atmospheric pressure and how barometers work To learn the units of pressure 

To understand how the pressure and volume of a gas are related To do calculations involving Boyle s law To learn about absolute zero 

Student Annotation Recall that oxygen also exists as the triatomic molecule ozone (O3) Section 2.6]. Diatomic O2, however, is the more stable allotrope at room temperature. 

Molecular Compounds That Arc Gases at Room Temperature 

Using Molar Mass as a Conversion Factor (7.2) Using Mole-Mole Factors (7.6) 

The behavior of gases is quite different from that of liquids and solids. Gas particles are far apart, whereas particles of both liquids and solids are held close together. A gas has no definite shape or volume and will completely fill any container. Because, there are great distances between gas particles, a gas is less dense than a solid or liquid, and easy to compress. A model for the behavior of a gas, called the kinetic molecular theory of gases, helps us understand gas behavior. 

Describe the kinetic molecular theory of gases and the units of measurement used for gases. 

A gas consists of small particles (atoms or molecules) that move randomly with high velocities. Gas molecules moving in random directions at high speeds cause a gas to fill the entire volume of a container. 

The attractive forces between the particies of a gas are usually very small. Gas 

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The third edition of "Properties of Gases and Liquids" by Reid et al. (1977) lists useful group contribution methods for predicting critical properties. Contributions to the second... 

Properties of Gases 4 4.1 Thermodynamic Properties of Gases 4.4.1.1 Gas Density... 

Calculation of thermophysical properties of gases relies on the principle of corresponding states. Viscosity and conductivity are expressed as the sum of the ideal gas property and a function of the reduced density ... 

R. C. Reid, J. M. Prausnitz, B.E. Polling, The Properties of Gases and Liquids, 4th edition, McGraw-Hill, New York, 1988. 

Typical Properties of Gases, Liquids, and Supercritical Fluids ... 

Table 2. Comparison of Properties of Gases, Supercritical Fluids, and Liquids...

B. D. Smith and R. Srivastava, Thermodynamic Data for Pure Compounds, Part A, Elsevier Science Pubhshers, Amsterdam, The Netherlands, 1986 DIPPR, Project 801, Data Compilation (July 1990) R. C. Reid, J. M. Prausnitz, and B. E. Poling, The Properties of Gases and Eiquids, 4th ed.,... 

L. H. Chen, Thermodynamic and Transport Properties of Gases, Eiquids and Solids, McGraw-HiU Book Co., Inc., New York, 1959, pp. 358—369. 

The idea of using reduced variables to correlate the pressure—volume—temperature properties of gases, was suggested by van der Waals in 1873. The... 

Numerous other methods have been used to predict properties of gases and Hquids. These include group contribution, reference substance, approaches, and many others. However, corresponding states theory has been one of the most thoroughly investigated methods and has become an important basis for the development of correlation and property estimation techniques. The methods derived from the corresponding states theory for Hquid and gas property estimation have proved invaluable for work such as process and equipment design. 

MYERS Aboveground Storage Tanks POWER Steam Jet Ejectors for the Process Industries REID, PRAUSNITZ, POLING Properties of Gases and Liquids, Fourth Edition... 

The most satisfactory calciilational procedure for thermodynamic properties of gases and vapors requires PVT data and ideal gas heat capacities. The primary equations are based on the concept of the ideal gas state and the definitions of residual enthalpy anci residual entropy ... 

Another property of gases which appears in the Reynolds and the Schmidt numbers is the viscosity, which results from momentum transfer across the volume of the gas when drere is relative bulk motion between successive layers of gas, and the coefficient, y], is given according to the kinetic theoty by the equation... 

The classical kinetic theoty of gases treats a system of non-interacting particles, but in real gases there is a short-range interaction which has an effect on the physical properties of gases. The most simple description of this interaction uses the Lennard-Jones potential which postulates a central force between molecules, giving an energy of interaction as a function of the inter-nuclear distance, r. 

Chapman and Enskog (see Chapman and Cowling, 1951) made a semi-empirical study of tire physical properties of gases using the Lennard-Jones... 

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