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8.23: Untitled Page 212

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  • Chapter 9

    Reaction Kinetics

    In Chapter 6 we introduced stoichiometry as the concept that atomic species are neither created nor destroyed by chemical reactions, and this concept was stated explicitly by Axiom II. In Chapters 7 and 8 we applied Axioms I and II to the analysis of systems with reactors, separators, and recycle streams. The pivot theorem (see Sec. 6.4) is an essential part of the analysis of chemical reactors; however, the design of chemical reactors requires that the size be determined. To design a chemical reactor (Whitaker and Cassano, 1986), we need information about the rates of chemical reaction in terms of the concentration of the reacting species. In this chapter we introduce students to this process with a study of chemical kinetic rate equations and mass action kinetics (Horn and Jackson, 1972).

    9.1 Chemical Kinetics

    In order to predict the concentration changes that occur in reactors, we need to make use of Axiom I (see Eq. 6‐4) and Axiom II (see Eq. 6‐20) in addition to chemical reaction rate equations that allow us to express the net rates of production, R , R , etc., in terms of the concentrations, c , c , etc. The subject of chemical A




    kinetics brings us in contact with the chemical kinetic schemata that are used to illustrate reaction mechanisms. To be useful these schemata must be translated to equations and we will illustrate how this is done in the following paragraphs.

    Hydrogen bromide reaction

    As an example of both stoichiometry and chemical kinetics, we consider the reaction of hydrogen with bromine to produce hydrogen bromide. Keeping in mind the principle of stoichiometric skepticism (see Sec. 6.1.1) one could assume that the molecular species involved are H , Br and HBr , and this idea is 2


    illustrated in Figure 9‐1. There we have suggested that the reaction does not go 395