Molecular elementary reaction rate data

The pre-exponents of reactions have been chosen to have no activation entropy for most of the surface reactions steps [4], but large activation entropy differences are included for molecular adsorption or desorption reaction steps. Table 16.1 collects the basic elementary reaction rate data used. Figure 16.5 presents reaction... 

It is impossible to understand all the complicated kinetic paths and to determine the elementary reaction rate constants without a detailed quantitative investigation of all the donor-acceptor interactions in the reaction system. Strictly speaking, at present there are no data on the elementary reaction rate constants even in low-molecular model systems. 

How well does this simple theory agree with experiment. By fitting data for gas-phase elementary reaction rates to the Arrhenius form, we can obtain the activation energy and the factor A. The value of A can be compared with the theory, once we estimate the molecular diameter. For the elementary reaction... 

Although reaction rate expressions and reaction stoichiometry are the experimental data most often used as a basis for the postulation of reaction mechanisms, there are many other experimental techniques that can contribute to the elucidation of these molecular processes. The conscientious investigator of reaction mechanisms will draw on a wide variety of experimental and theoretical methods in his or her research program in an attempt to obtain information about the elementary reactions taking... 

Power-law expressions are found at all hierarchical levels of organization from the molecular level of elementary chemical reactions to the organismal level of growth and allometric morphogenesis. This recurrence of the power law at different levels of organization is reminiscent of fractal phenomena. In the case of fractal phenomena, it has been shown that this self-similar property is intimately associated with the power-law expression [28]. The reverse is also true if a power function of time describes the observed kinetic data or a reaction rate higher than 2 is revealed, the reaction takes place in fractal physical support. 

The main goals of dynamical studies are to develop theories for chemical reactions and gain an insight into the molecular mechanisms of elementary reactions. The value of a validated and workable theory for reaction kinetics is obvious, rate coefficients would no longer have to be measured but could be calculated for any set of conditions. More specifically, good models of a specific elementary reaction, validated against dynamical data, allows rate coefficients to be calculated at any temperature. 

A computer model has been developed which can generate realistic concentration versus time profiles of the chemical species formed during photooxidation of hydrocarbon polymers using as input data a set of elementary reactions with corresponding rate constants and initial conditions. Simulation of different mechanisms for stabilization of clear, amorphous linear polyethylene as a prototype suggests that the optimum stabilizer would be a molecularly dispersed additive in very low concentration which can trap peroxy radicals and also decompose hydroperoxides. 

Microkinetic modeling assembles molecular-level information obtained from quantum chemical calculations, atomistic simulations and experiments to quantify the kinetic behavior at given reaction conditions on a particular catalyst surface. In a postulated reaction mechanism the rate parameters are specified for each elementary reaction. For instance adsorption preexponential terms, which are in units of cm3 mol"1 s"1, have been typically assigned the values of the standard collision number (1013 cm3 mol"1 s 1). The pre-exponential term (cm 2 mol s 1) of the bimolecular surface reaction in case of immobile or moble transition state is 1021. The same number holds for the bimolecular surface reaction between one mobile and one immobile adsorbate producing an immobile transition state. However, often parameters must still be fitted to experimental data, and this limits the predictive capability that microkinetic modeling inherently offers. A detailed account of microkinetic modelling is provided by P. Stoltze, Progress in Surface Science, 65 (2000) 65-150. 

The twentieth century saw an enormous amount of experimental and theoretical research on elementary chemical reactions, an effort which continues today. The fruits of this work are extensive kinetics databases, and molecular theories from which to make estimates when experimental data are not available. Equally important are parallel developments in thermochemistry. All of this information makes possible the development of detailed chemical kinetics models of overall chemical reactions. Models have been constructed and applied to such diverse topics as atmospheric chemistry, combustion, low temperature oxidation, chemical vapor deposition, and reactions in traditional chemical process industries. The rate of each elementary reaction in a model is expressed as... 

With kinetics assumed to be of the mass action type, two main characteristics remain to be determined by the kinetic analysis 1) the rate coefficient, k, 2) the reaction order, global a + b oi partial a with respect to A h with respect to B. The order of a reaction is preferably determined from experimental data. It only coincides with the molecularity for elementary processes that actually occur as described by the stoichiometric equation (1.1.1-1). When the latter is only an "overall" equation for a process that really consists of several steps, the order cannot be predicted on the basis of this equation. Only for elementary reactions does the order have to be 1, 2, or 3. The order may be a fraction or even a negative number. In Section 1.6 examples will be given of reactions whose rate cannot be expressed as a simple product like (1.2.1-1). 

The ordo = molecularity property of elementary reactions is not a two-way street. If a reaction is elementary, then the form of the rate equation can be written directly, as in the above example. However, we carmot conclude that a reaction is elementary just because the form of its rate equation, as detmnined from experimental data, is identical to the form that results from assuming an elementary reaction. For example, if expoimental data show that the rate equation for eth)iene hydrogenation... 

Subsequent investigations211-214 have confirmed this finding and have shown that at least up to 750 °C the reaction is completely homogeneous and the rate is independent of the pressure of added inert gases. The interpretation of the kinetic data in terms of the elementary molecular process... 

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. ... 

In molecular reaction schemes, only stable molecular reactants and products appear short-lived intermediates, such as free radicals, are not mentioned. Nearly all the reactions written are considered as pseudo-elementary processes, so that the reaction orders are equal to the mol-ecularities. For some special reactions (such as cocking) first order or an arbitrary order is assumed. Pseudo-rate coefficients are written in Arrhenius form. A systematic use of equilibrium constants, calculated from thermochemical data, is made for relating the rate coefficients of direct and reverse reactions. Generally, the net rate of the reversible reaction... 

The kinetics of the reaction between OFj and CO have been studied in shock tubes in the temperature range of 800-1400 K (at about 133-267 kPa) [943], Significant amounts of COFj are produced, in addition to CO, Oj and F. COF is understood to be formed from [COF] as a result of the following elementary steps, the rate constants of which were estimated from thermodynamic and molecular data ... 

---