Empirical Properties of Gases

Of the three states of aggregation, only the gaseous state allows a comparatively simple quantitative description. For the present we shall restrict this description to the relations among such properties as mass, pressure, volume, and temperature. We shall assume that the system is in equilibrium so that the values of the properties do not change with time, so long as the external constraints on the system are not altered. 

A system is in a definite state or condition when all of the properties of the system have definite values, which are determined by the state of the system. Thus the state of the system is described by specifying the values of some or all of its properties. The important question is whether it is necessary to give values of fifty different properties (or twenty or five) to ensure that the state of the system is completely described. The answer depends to a certain extent upon how accurate a description is required. If we were in the habit of measuring the values of properties to twenty significant figures, and thank heaven we are not, then quite a long list of properties would be required. Fortunately, even in experiments of great refinement, only f our properties—mass, volume, temperature, and pressure—are ordinarily required. 

The equation of state of the system is the mathematical relationship between the values of these four properties. Only three of these must be specified to describe the state the fourth can be calculated from the equation of state, which is obtained from knowledge of the experimental behavior of the system. 

Later experiments by Charles showed that the constant C is a function of temperature. This is a rough statement of Charles s law. 

Gay-Lussac made measurements of the volume of a fixed mass of gas under a fixed pressure and found that the volume was a linear function of the temperature. This is expressed by the equation 

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

When trying to understand and to manipulate matter and materials, chemistry does not start by looking at the natural world in all its complexity. Rather, it seeks to establish what have been termed exemplar phenomena ideal or simplified examples that are capable of investigation with the tools available at the time (Gilbert, Borrlter, Elmer, 2000). This level consists of representatiorrs of the empirical properties of solids, liquids (taken to include solutions, especially aqueous solutiorts), colloids, gases and aerosols. These properties are perceptible in chemistry laboratories and in everyday life and are therefore able to be meastrred. Examples of such properties are mass, density, concentration, pH, temperatrrre and osmotic presstrre. 

Data of densities of liquids are empirical in nature, but the effects of temperature, pressure, and composition can be estimated suitable methods are described by Reid et al. (Properties of Gases and Liquids, McGraw Hill, New York, 1977), the API Refining Data Book (American Petroleum Institute, Washington, DC, 1983), and the AIChE Data Prediction Manual (1984-date). The densities of gases are represented by equations of state of which the simplest is that of ideal gases from this the density is given by ... 

So far, we have empirically deduced several relationships between properties of gases. From Boyle s law,... 

The heat of vaporization can be obtained from various sources. It can often be found in tables directly, or calculated from tabulated values of the enthalpy of the saturated vapor and liquid. Several empirical equations have been developed for the heat of vaporization, and these are reviewed in The Properties of Gases and Liquids by Polling, Prausnitz, and O Connell (5th ed., 2007). Here we mention one that is based... 

Estimation of the corresponding parameters for liquid-phase systems is more uncertain. Equation 5.284 is valid for liquid-filled catalyst pores. The molecular liquid-phase diffusion coefficients can be estimated from, that is, the Wilke-Chang equation [20]. Different estimation methods are discussed further in the book The Properties of Gases and Liquids [20]. Eor the liquid phase, the film coefficient, ki, is always used. Different empirical correlations for the liquid film coefficient are presented and compared, for example, in Ref [23]. Methods for estimating liquid-phase diffusion coefficients and liquid film coefficients are described in Appendices 6 and 7. 

The values of r and e, such as those listed in Table 2,2, can be calculated from other properties of gases, such as viscosity. If necessary, they can be estimated for each component empirically [25]... 

The fluid properties of formation water may be looked up on correlation charts, as may most of the properties of oil and gas so far discussed. Many of these correlations are also available as computer programmes. It is always worth checking the range of applicability of the correlations, which are often based on empirical measurements and are grouped into fluid types (e.g. California light gases). 

Hea.t Ca.pa.cities. The heat capacities of real gases are functions of temperature and pressure, and this functionaHty must be known to calculate other thermodynamic properties such as internal energy and enthalpy. The heat capacity in the ideal-gas state is different for each gas. Constant pressure heat capacities, (U, for the ideal-gas state are independent of pressure and depend only on temperature. An accurate temperature correlation is often an empirical equation of the form ... 

In physics, fluid dynamics is a sub-discipline of fluid mechanics that deals with fluid flow —the natural science of fluids (liquids and gases) in motion. It has several subdisciplines itself, including aerodynamics (the study of air and other gases in motion) and hydrodynamics (the study of liquids in motion). Fluid dynamics offers a systematic structure that underlies these practical disciplines, that embraces empirical and semi-empirical laws derived from flow measurement and used to solve practical problems. The solution to a fluid dynamics problem typically involves calculating various properties of the fluid, such as velocity, pressure, density, viscosity and temperature, as functions of space and time. 

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