Elementary chemical reaction types

Table 2-2. Rate Laws for Three Common Types of Elementary Chemical Reactions ... 

Formula (2-213) does not include any detailed information on the dynamics of the elementary chemical reaction and type of excitation. For this reason we can expect good results from applying the formula to the wide range of different chemical reactions of different excited species. 

The unimolecular decay and isomerization reactions are the simplest type of elementary chemical reactions in the gas phase. Consider, for instance, a decay reaction... 

Changes in the concentration of chemically interacting reaction partners may arise from two types of elementary chemical reactions intramolecular (or monomolecular) and bimolecular elementary steps. 

Within the solar system the observable changes are of a different kind, best described as chemical change. The most striking common feature of those chemical reactions driven by solar energy is their cyclic nature, linked to planetary motion. All phenomena, collectively known as life, or growth, are of this type. Their essential characteristic is a state far from equilibrium. For a life process, equilibrium is synonymous with death and chemical change after death is a rapid slide towards equilibrium. The most advanced chemical theories deal with these posthumous effects and related reactions only, albeit rather superficially. A fundamental theory to predict conditions for the onset of elementary chemical change is not available. 

We shall also encounter situations where a chemical reaction influences the concentrations. For a volume reaction of the type j - i, the reaction rate for the production of i is given by k(Cj (if we assume it to be an elementary reaction), where k is the forward reaction rate constant, and the rate of consumption of i by a term where kb is the rate constant of the backward... 

Electronic to vibrational and rotational (E-V-R) energy transfer certainly belongs to the elementary processes in chemical reaction dynamics and, in particular, in photochemistry. A fairly general type of photochemical reaction may be written as follows ... 

All the work just mentioned is rather empirical and there is no general theory of chemical reactions under plasma conditions. The reason for this is, quite obviously, that the ordinary theoretical tools of the chemist, — chemical thermodynamics and Arrhenius-type kinetics - are only applicable to systems near thermodynamic and thermal equilibrium respectively. However, the plasma is far away from thermodynamic equilibrium, and the energy distribution is quite different from the Boltzmann distribution. As a consequence, the chemical reactions can be theoretically considered only as a multichannel transport process between various energy levels of educts and products with a nonequilibrium population20,21. Such a treatment is extremely complicated and - because of the lack of data on the rate constants of elementary processes — is only very rarely feasible at all. Recent calculations of discharge parameters of molecular gas lasers may be recalled as an illustration of the theoretical and the experimental labor required in such a treatment22,23. ... 

For linear mechanisms we have obtained structurized forms of steady-state kinetic equations (Chap. 4). These forms make possible a rapid derivation of steady-state kinetic equations on the basis of a reaction scheme without laborious intermediate calculations. The advantage of these forms is, however, not so much in the simplicity of derivation as in the fact that, on their basis, various physico-chemical conclusions can be drawn, in particular those concerning the relation between the characteristics of detailed mechanisms and the observable kinetic parameters. An interesting and important property of the structurized forms is that they vividly show in what way a complex chemical reaction is assembled from simple ones. Thus, for a single-route linear mechanism, the numerator of a steady-state kinetic equation always corresponds to the kinetic law of the overall reaction as if it were simple and obeyed the law of mass action. This type of numerator is absolutely independent of the number of steps (a thousand, a million) involved in a single-route mechanism. The denominator, however, characterizes the "non-elementary character accounting for the retardation of the complex catalytic reaction by the initial substances and products. 

Mechanisms for complex chemical reactions can be represented by graphs having nodes of two types [4]. One corresponds to elementary reactions and the other accounts for substances. 

If initial phases are chemical compounds, not elementary substances, the growth of the layers of two new chemical compounds in a quasibinary system takes place as a result of counter diffusion of the same-type ions or atoms of smaller size. The common ion usually does not take active part in the layer-growth process. This does not mean, however, that its presence has no effect on the mechanism of formation of the layers. The Rb2AgI3 and RbAg4J5 layers are known to form in the Rbl-AgI system. 5 5 Their formation is due to the following partial chemical reactions ... 

If a chemical reaction is operated in a flow reactor under fixed external conditions (temperature, partial pressures, flow rate etc.), usually also a steady-state (i.e., time-independent) rate of reaction will result. Quite frequently, however, a different response may result The rate varies more or less periodically with time. Oscillatory kinetics have been reported for quite different types of reactions, such as with the famous Belousov-Zha-botinsky reaction in homogeneous solutions (/) or with a series of electrochemical reactions (2). In heterogeneous catalysis, phenomena of this type were observed for the first time about 20 years ago by Wicke and coworkers (3, 4) with the oxidation of carbon monoxide at supported platinum catalysts, and have since then been investigated quite extensively with various reactions and catalysts (5-7). Parallel to these experimental studies, a number of mathematical models were also developed these were intended to describe the kinetics of the underlying elementary processes and their solutions revealed indeed quite often oscillatory behavior. In view of the fact that these models usually consist of a set of coupled nonlinear differential equations, this result is, however, by no means surprising, as will become evident later, and in particular it cannot be considered as a proof for the assumed underlying reaction mechanism. 

Lee, L.-S., and Sinanoglu, O., Reaction mechanisms and chemical networks—Types of elementary steps and generation of laminar mechanisms. Z. Phys. Chem. 124, 129-160 (1981). 

As a whole, the approach based on the decomposition of kinetic graphs into linear and nonlinear subgraphs provides for a complete description of chemical reactions, and at the same time reflects the type and stoichiometry of the nonlinear elementary steps. 

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