Steps, elementary series-parallel

Consider the following mechanism for step-change polymerization of monomer M (Px) to P2, P3,..., Pr,. The mechanism corresponds to a complex series-parallel scheme series with respect to the growing polymer, and parallel with respect to M. Each step is a second-order elementary reaction, and the rate constant k (defined for each step)1 is the same for all steps. 

Regarding the chemical and electrochemical processes taking place in the inner layer, they are modelled as series-parallel elementary kinetic steps (see, for example, Franco " ). In this approach, for a given metallic element M (Pt or a transition metal, in the case of a bimetallic) at the catalyst level... 

Complex reactions can be broken into a number of series and parallel elementary steps, possibly involving short-lived intermediates such as free radicals. These individual reactions collectively constitute the mechanism of the complex reaction. The individual reactions are usually second order, and the number of reactions needed to explain an observed, complex reaction can be surprisingly large. For example, a good model for... 

This reaction cannot be elementary. We can hardly expect three nitric acid molecules to react at all three toluene sites (these are the ortho and para sites meta substitution is not favored) in a glorious, four-body collision. Thus, the fourth-order rate expression 01 = kab is implausible. Instead, the mechanism of the TNT reaction involves at least seven steps (two reactions leading to ortho- or /mra-nitrotoluene, three reactions leading to 2,4- or 2,6-dinitrotoluene, and two reactions leading to 2,4,6-trinitrotoluene). Each step would require only a two-body collision, could be elementary, and could be governed by a second-order rate equation. Chapter 2 shows how the component balance equations can be solved for multiple reactions so that an assumed mechanism can be tested experimentally. For the toluene nitration, even the set of seven series and parallel reactions may not constitute an adequate mechanism since an experimental study found the reaction to be 1.3 order in toluene and 1.2 order in nitric acid for an overall order of 2.5 rather than the expected value of 2. 

As an example of a system with a series of reactions, we may look at methane oxidation under conditions of excess oxygen. Following the carbon atom, this process would typically involve the steps CH4 — CH3 — CH2O — HCO — CO —> CO2. We note that each of these steps may involve a number of parallel elementary reactions, but we assume that they do not affect the oxidation pathway. 

When a reaction rate is measured in a chemical reactor, the reaction is generally a composite reaction comprised of a sequence of elementary reactions. An elementary reaction is a reaction that occurs at the molecular level exactly as written (Laidler, 1987). The mechanism of the reaction is the sequence of elementary reactions that comprise the overall or composite reaction. For example, mineral dissolution reactions generally include transport of reactant to the surface, adsorption of reactant, surface dilfusion of the adsorbate, reaction of the surface complex and release into solution, and transport of product species away from the surface. These reactions occur as sequential steps. Reaction of surface complexes and release to solution may happen simultaneously at many sites on a surface, and each site can react at a different rate depending upon its free energy (e.g., Schott et al., 1989). Simultaneous reactions occurring at different rates are known as parallel reactions. In a series of sequential reactions, the ratedetermining step is the step which occurs most slowly at the onset of the reaction, whereas for parallel steps, the rate-determining step is the fastest reaction. 

Composite reactions consist of multiple elementary reaction steps that occur in series, in parallel, or both. Many geochemical reactions are composites of several elementary reaction steps. This makes elucidating their reaction mechanisms very challenging because their reaction order and molecularity are not related in a simple way. Marin and Yablonsky (2011) offer extensive guidance about dealing with composite reactions. 

The hydrocarbon oxidation kinetics is extremely complex because it includes many consequent-parallel steps. Thus, the full mechanism description is problematic. In such cases researchers confine themselves to model descriptions. Each of the model steps may represent a series of elementary stages, and each of the model symbols may correspond to a whole set of compounds playing the same kinetic function. 

It is possible to represent the whole scheme in Fig. 6.46 approximately in the form of a network of elementary steps in parallel and series. If the rate constants... 

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