EVIDENCE FOR REACTION STEPS

The search for realistic reaction schemes for chemical processes is an active field of research with a rapidly growing arsenal of methods. In this section we summarize several kinds of evidence that have proved useful in such investigations. 

Identification of the participating chemical species is most important. The completeness of reaction models is often limited by the difficulty of this step. Indirect evidence is commonly used to infer the existence of trace intermediates, but direct observations are becoming more frequent with modern spectroscopic methods. 

Some reactions may be representable as quasi-equilibria over the range of the experiments. For such a step, Eq. (2.5-2b) gives a linear relation 

Kinetic steps are best identified by measuring the initial products formed from individual species (including postulated intermediates) or from simple mixtures. Isotopically labeled species have proved useful in such experiments. Initial products of homogeneous processes are observable in batch reactors at sufficiently short times or in flow reactors at points sufficiently near the inlet. The most advanced systems for initial product detection are molecular beam reactors (Herschbach 1976 Levine and Bernstein 1987) in which specific collisions are observed. Each of these techniques restricts the number of contributing reactions in a given experiment, so that their stoichiometry and rates can often be inferred. 

The rank. NK. of the stoichiometric matrix i/ can be estimated by fitting Eq. (2.2-3) to measurements of the species production rates. This value is the minimum number of reactions needed to model the stoichiometry however, additional reactions may be needed to model the kinetics. This approach can be used also for complex mixtures, where the production rates may be available only as lumped values for groups of related species. 

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Chemical kinetics is an enormous subject just a few basic principles will be treated in this chapter. Section 2.1 deals with reaction stoichiometry, the algebraic link between rates of reaction and of species production. Section 2.2 considers the computability of reaction rates from measurements of species production the stoichiometric constraints on production rates are also treated there. The equilibrium and rate of a single reaction step are analyzed in Sections 2.3 and 2.4 then simple systems of reactions are considered in Section 2.5. Various kinds of evidence for reaction steps are discussed in Section 2.6 some of these will be analyzed statistically in Chapters 6 and 7. 

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