KINETICS OF ELEMENTARY STEPS

Elementary steps are molecular events in which reactants are transformed into products directly, i.e., without passing through an intermediate that is susceptible of isolation (Boudart 1968). We can represent such an event symbolically as 

Suppose, for the moment, that a postulated reaction scheme consists entirely of elementary steps with known coefficients Cji and e jj. Then it is natural to apply Eq. (2.1-1) to those steps, and set 

Probability considerations suggest that the frequency of the forward event j in Eq. (2.4-1) at a given temperature should be proportional to each of the concentrations bi raised to the power Cji, and a similar relation with exponents e jj should hold for the reverse event. (This method of reasoning is restricted to ideal solutions.) The resulting net rate expression. 

At thermodynamic equilibrium, Ttj must vanish for every reaction otherwise, the equilibrium could be shifted by adding catalysts or inhibitors to alter the nonzero rates TZj. Formal proofs of this detailed balance principle are presented by de Groot and Mazur (1962). Therefore, setting 7 = 0 at equilibrium and using Eq. (2.4-3), we get 

Equation (2.4-6) is rather awkw ard for representation of data, because Aj is an extrapolated value of kj at infinite temperature. Arrhenius (1889) actually used the form 

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Rate equations for simple reversible reactions are often developed from mechanistic models on the assumption that the kinetics of elementary steps can be described in terms of rate constants and surface concentrations of intermediates. An application of the Langmuir adsorption theory for such development was described in the classic text by Hougen and Watson (/ ), and was used for constructing rate equations for a number of heterogeneous catalytic reactions. In their treatment it was assumed that one step would be rate-controlling for a unique mechanism with the other steps at equilibrium. 

Returning to the mechanism of the CO + NO reaction, we can now list the kinetic parameters for several of the elementary steps, see Table 5.3. As the mechanism of any catalytic reaction is inevitably a sequence of several steps, the surface science approach for studying the kinetics of elementary steps is vitally important, because parameters such as those listed in Table 5.3 form the highly desirable input for the modeling of more complex reaction mechanisms. 

Chemical Kinetics of Elementary Steps Table 2.1.1—Cont. 

Thus, if the old book shows its age, it is primarily in the choice of symbols and abbreviations that antedate the current recommendations of lUPAC. Of course, this does not mean that nothing has happened in chemical kinetics since 1968. Enormous advances have been made in the past 20 years in the kinetics of elementary steps (Chapter 2), especially from the viewpoint of the detailed manner by which they take place from selected energy states of reactants to determined energy states of products. Yet this informative invasion into the private lives of reacting molecules does not detract from the classical treatment of transition state theory as presented in Chapter 2. 

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