Thermodynamics of Chemical Reaction Equilibria

The problems solved in Chapters 5 and 6 are simple problems with many numerical parameters specified. You may have wondered where those numbers came from. In a real case, of course, you will have to make design choices and discover their impact. In chemical engineering, as in real life, these choices have consequences. Thus, you must make mass and energy balances that take into account the thermodynamics of chemical reaction equilibria and vapor-liquid equilibria as well as heat transfer, mass transfer, and fluid flow. To do this properly requires lots of data, and the process simulators provide excellent databases. Chapters 2-4 discussed some of the ways in which thermodynamic properties are calculated. This chapter uses Aspen Plus exclusively. You will have to make choices of thermodynamic models and operating parameters, but this will help you learn the field of chemical engineering. When you complete this chapter, you may not be a certified expert in using Aspen Plus , but you will be capable of actually simulating a process that could make money. 

The treatment of chemical reaction equilibria outlined above can be generalized to cover the situation where multiple reactions occur simultaneously. In theory one can take all conceivable reactions into account in computing the composition of a gas mixture at equilibrium. However, because of kinetic limitations on the rate of approach to equilibrium of certain reactions, one can treat many systems as if equilibrium is achieved in some reactions, but not in others. In many cases reactions that are thermodynamically possible do not, in fact, occur at appreciable rates. 

The first question is one of thermodynamics the second is one of kinetics. The prediction of chemical reaction equilibria is one of the most useful aspects of thermodynamics. It is possible to calculate the equilibrium conversion of a given reaction from data taken on other reactions or from thermal data on the individual substances involved. 

Chapter 12 gives an extensive coverage on the thermodynamics of chemical reactions, which emphasizes the importance of the real mbcture behavior on the description of reaction equilibria and the enthalpies of reaction as well as solvent effects on chemical equilibrium conversion. 

From a thermodynamic point of view a solid containing point defects constitutes a solid solution where the point defects are dissolved in the solid. In analogy with liquid solutions, the solid may be considered to be the solvent and the point defects the solute. Similarly, the defect equilibria may be treated in terms of the thermodynamics of chemical reactions and solutions. 

As a basis for the subsequent considerations of thermodynamics of defect equilibria the following section provides a brief review of some general aspects of thermodynamics of chemical reactions. 

There is hardly a discipline in either physical or biological science, or in engineering, that is not to some extent dependent on thermodynamics. Many significant developments in chemistry and materials science could not have taken place without the guidance of thermodynamics. Our understanding of chemical-reaction equilibria, the co-existence of phases, solution properties, electrochemical cells and surface phenomena would have hardly advanced beyond cataloging without the aid of thermodynamics. (1 will follow the custom of referring to the theory without the adjective classical , except when a distinction has to be made with statistical theory.)... 

Chemistry can be divided (somewhat arbitrarily) into the study of structures, equilibria, and rates. Chemical structure is ultimately described by the methods of quantum mechanics equilibrium phenomena are studied by statistical mechanics and thermodynamics and the study of rates constitutes the subject of kinetics. Kinetics can be subdivided into physical kinetics, dealing with physical phenomena such as diffusion and viscosity, and chemical kinetics, which deals with the rates of chemical reactions (including both covalent and noncovalent bond changes). Students of thermodynamics learn that quantities such as changes in enthalpy and entropy depend only upon the initial and hnal states of a system consequently thermodynamics cannot yield any information about intervening states of the system. It is precisely these intermediate states that constitute the subject matter of chemical kinetics. A thorough study of any chemical reaction must therefore include structural, equilibrium, and kinetic investigations. 

The importance of the Gibbs free energy and the chemical potential is very great in chemical thermodynamics. Any thermodynamic discussion of chemical equilibria involves the properties of these quantities. It is therefore worthwhile considering the derivation of equation 20.180 in some detail, since it forms a prime link between the thermodynamics of a reaction (AG and AG ) and its chemistry. 

Knud 0stergaard, Gas-Liquid-Particle Operations in Chemical Reaction Engineering J. M. Prausnitz, Thermodynamics of Fluid-Phase Equilibria at High Pressures Robert V. Macbeth, The Burn-Out Phenomenon in Forced-Convection Boiling William Resnick and Benjamin Gal-Or, Gas-Liquid Dispersions... 

In chemical equilibria, the energy relations between the reactants and the products are governed by thermodynamics without concerning the intermediate states or time. In chemical kinetics, the time variable is introduced and rate of change of concentration of reactants or products with respect to time is followed. The chemical kinetics is thus, concerned with the quantitative determination of rate of chemical reactions and of the factors upon which the rates depend. With the knowledge of effect of various factors, such as concentration, pressure, temperature, medium, effect of catalyst etc., on reaction rate, one can consider an interpretation of the empirical laws in terms of reaction mechanism. Let us first define the terms such as rate, rate constant, order, molecularity etc. before going into detail. 

PHYSICAL CHEMISTRY. Application of the concepts and laws of physics to chemical phenomena in order to describe in quantitative (mathematical) terms a vast amount of empirical (observational) information. A selection of only the most important concepts of physical chemistiy would include the electron wave equation and the quantum mechanical interpretation of atomic and molecular structure, the study of the subatomic fundamental particles of matter. Application of thermodynamics to heats of formation of compounds and the heats of chemical reaction, the theory of rate processes and chemical equilibria, orbital theory and chemical bonding. surface chemistry (including catalysis and finely divided particles) die principles of electrochemistry and ionization. Although physical chemistry is closely related to both inorganic and organic chemistry, it is considered a separate discipline. See also Inorganic Chemistry and Organic Chemistry. 

Chemical kinetics deals with the speed and mechanisms of chemical reactions. It is a recent development in chemistry and a difficult one—one in which it is easy to raise more questions than answers. It is more difficult than the prediction of chemical equilibria and the applications of thermodynamics, for these are concerned merely with the initial and final states and not with time nor with intermediate steps. 

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