Ingeniero químico

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The Properties of Gases and Liquids, Fifth Edition Bruce E. Poling, John M. Prausnitz, John P. O’Connell


Printed from Digital Engineering Library @ McGraw-Hill ( Copyright ©2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as givenat the website.


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The structural engineer cannot design a bridge without knowing the properties of steel and concrete. Similarly, scientists and engineers often require the properties of gases and liquids. The chemical or processengineer, in particular, finds knowledge of physical properties of fluids essential to the design of many kinds of products, processes, and industrial equipment. Even the theoretical physicist must occasionally compare theory with measured properties. The physical properties of every substance depend directly on the nature of the molecules of the substance. Therefore, the ultimate generalization of physicalproperties of fluids will require a complete understanding of molecular behavior, which we do not yet have. Though its origins are ancient, the molecular theory was not generally accepted until about the beginning of the nineteenth century, and even then there were setbacks until experimental evidence vindicated the theory early in the twentieth century. Many pieces of the puzzle of molecularbehavior have now fallen into place and computer simulation can now describe more and more complex systems, but as yet it has not been possible to develop a complete generalization. In the nineteenth century, the observations of Charles and Gay-Lussac were NRT, which combined with Avogadro’s hypothesis to form the gas ‘‘law,’’ PV was perhaps the first important correlation of properties. Deviationsfrom the idealgas law, though often small, were finally tied to the fundamental nature of the molecules. The equation of van der Waals, the virial equation, and other equations of state express these quantitatively. Such extensions of the ideal-gas law have not only facilitated progress in the development of a molecular theory but, more important for our purposes here, have provided a framework forcorrelating physical properties of fluids. The original ‘‘hard-sphere’’ kinetic theory of gases was a significant contribution to progress in understanding the statistical behavior of a system containing a large number of molecules. Thermodynamic and transport properties were related quantitatively to molecular size and speed. Deviations from the hard-sphere kinetic theory led to studies of theinteractions of molecules based on the realization that molecules attract at intermediate separations and repel when they come very close. The semiempirical potential functions of Lennard-Jones and others describe attraction and repulsion in approximately quantitative fashion. More recent potential functions allow for the shapes of molecules and for asymmetric charge distribution in polar molecules.1.1 Downloaded from Digital Engineering Library @ McGraw-Hill ( Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website.


Although allowance for the forces of attraction and repulsion between molecules is primarily a development of thetwentieth century, the concept is not new. In about 1750, Boscovich suggested that molecules (which he referred to as atoms) are ‘‘endowed with potential force, that any two atoms attract or repel each other with a force depending on their distance apart. At large distances the attraction varies as the inverse square of the distance. The ultimate force is a repulsion which increases without...
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