Elementary reaction definition

Elementary reactions are characterized by their moiecuiarity, to be clearly distinguished from the reaction order. We distinguish uni- (or mono-), hi-, and trimoiecuiar reactions depending on the number of particles involved in the essential step of the reaction. There is some looseness in what is to be considered essential but in gas kinetics the definitions usually are clearcut through the number of particles involved in a reactive collision plus, perhaps, an additional convention as is customary in iinimolecular reactions. 

According to the definition given, this is a second-order reaction. Clearly, however, it is not bimolecular, illustrating that there is distinction between the order of a reaction and its molecularity. The former refers to exponents in the rate equation the latter, to the number of solute species in an elementary reaction. The order of a reaction is determined by kinetic experiments, which will be detailed in the chapters that follow. The term molecularity refers to a chemical reaction step, and it does not follow simply and unambiguously from the reaction order. In fact, the methods by which the mechanism (one feature of which is the molecularity of the participating reaction steps) is determined will be presented in Chapter 6 these steps are not always either simple or unambiguous. It is not very useful to try to define a molecularity for reaction (1-13), although the molecularity of the several individual steps of which it is comprised can be defined. 

This definition for reaction order is directly meaningful only for irreversible or forward reactions that have rate expressions in the form of Equation (1.20). Components A, B,... are consumed by the reaction and have negative stoichiometric coefficients so that m = —va, n = —vb,. .. are positive. For elementary reactions, m and n must be integers of 2 or less and must sum to 2 or less. 

Definitions for the variables and constants appearing in eqns. 1 and 2 are given in the nomenclature section at the end of this paper. The first of these equations represents a mass balance around the reactor, assuming that it operates in a differential manner. The second equation is a species balance written for the catalyst surface. The rate of elementary reaction j is represented by rj, and v j is the stoichiometric coefficient for component i in reaction j. The relationship of rj to the reactant partial pressures and surface species coverages are given by expressions of the form... 

The rate-controlling step is the elementary reaction that has the largest control factor (CF) of all the steps. The control factor for any rate constant in a sequence of reactions is the partial derivative of In V (where v is the overall velocity) with respect to In k in which all other rate constants (kj) and equilibrium constants (Kj) are held constant. Thus, CF = (5 In v/d In ki)K kg. This definition is useful in interpreting kinetic isotope effects. See Rate-Determining Step Kinetic Isotope Effects... 

A definition of the cycle rate is clear for steady states because stationary rates of all elementary reactions in cycle coincide. There is no common definition of the cycle rate for nonstationary regimes. In practice, one of steps is the step of product release (the "final" step of the catalytic transformation), and we can... 

This reaction appears to be an elementary bimolecular reaction involving a simple transfer of an oxygen atom from OH to CO. In accord with the definition of an elementary reaction, one can imagine that it occurs during one collision of an OH radical with a CO molecule. 

For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. 

There are conceivable a priori different functions determining the fraction of the total number of surface sites characterized by definite adsorption energy of a given substance, various combinations of these functions at simultaneous adsorption of two or more substances and, finally, various combinations of adsorption energy with kinetic characteristics of surface sites with respect to adsorption and elementary reactions. 

To reduce the number of parameters in the kinetic equations that are to be determined from experimental data, we used the following considerations. The values klt k2, and k4 that enter into the definition of the constant L, (236), are of analogous nature they indicate the fraction of the number of impacts of gas molecules upon a surface site resulting in the reaction. So the corresponding preexponential factors should be approximately the same (if these elementary reactions are adiabatic). Then, since k1, k2, and k4 are of the same order of magnitude, their activation energies should be almost identical. It follows that L can be considered temperature independent. 

Equations (287) are again used for the definition of, l2, and l3 in this equation via rate constants of elementary reactions of scheme (292). At lt = 0, (293) becomes (290). This corresponds to stage 2 of scheme (292), being at equilibrium. Thus the reaction kinetics does not provide a possibility for giving preference to the Eidus-Zelinskii theory or the carbide theory. 

For chemical kinetics to be operational and thus Eqs. (2.5) and (2.6) to be valid, Eq. (2.4) must be an elementary reaction. To definitively determine this, one must prove experimentally that Eq. (2.4) and the rate law are valid. 

The simple relationship between the rate law and stoichiometry in elementary reactions allows one to derive a rate law for any multistep mechanistic scheme. The agreement between the derived rate law and that determined experimentally provides support for the proposed mechanism, although it does not prove it. The lack of agreement, on the other hand, definitely rules out the proposed scheme. 

The entire complex situation is untypical of hydrogen peroxide. Decomposition of H202 starts from dissociation by O—O-bond to hydroxyl radicals, which then in gas-phase (refer to Chapter 4) and liquid-phase (refer to Chapter 6) decomposition lead to final products H20 and 02. Here one deals with a single complex reaction with a definite set of subsequently proceeding elementary reactions, but not with several sets as for decomposition of organic peroxides. 

An elementary reaction is defined as a reaction that takes place as written in the reaction scheme. We will here distinguish between a truly elementary reaction, where the reaction takes place in isolation without any secondary collisions, and the traditional definition of an elementary reaction, where inelastic collisions among the molecules in the reaction scheme (or with container walls) can take place. 

By definition, multi-component reactions (MCRs) belong to the class of domino reactions [7]. Generally, domino reactions are regarded as sequences of uni- or bi-molecular elementary reactions that proceed without intermediate isolation or workup as a consequence of the reactive functionality that has been formed in the previous step. In addition to uni- and bimolecular domino reactions that are generally referred to as domino reactions , a third class is called multimole-cular domino reactions or MCRs. Whereas uni- and bimolecular domino reac-... 

A chemical transformation that takes place via exactly one transition state is called an elementary reaction. This holds regardless of whether it leads to a short-lived intermediate or to a product that can be isolated. According to the definition, an n-step reaction consists of a sequence of n elementary reactions. It takes place via n transition states and (n 1) intermediates. 

Do you remember the definition of an elementary reaction (Section 1.7.1) The SN2 reaction is such an elementary reaction. Recognizing this is a prerequisite for deriving the rate law for the Sn2 reaction, because the rate law for any elementary reaction can be obtained directly from the reaction equation. 

Chemical reaction — A process that results in the interconversion of chemical species. Chemical reactions maybe elementary reactions or stepwise reactions. (This definition includes experimentally observable interconversions of conformers.) Detectable chemical reactions normally involve molecular entities, as indicated by this definition, but it is often conceptually convenient to use the term also for changes involving single molecular entities (i.e., microscopic chemical events ). See also... 

Table 1 Elementary reactions of vinyl acetate emulsion polymerization and definition of their rates...

In some circles when a reaction has an elementary rate law it is referred to as an elementary reaction. A more restrictive definition of an elementary... 

A group of researchers in Budapest continued the line of Yoneda [16-21] but avoided the combinatorial explosion of the number of products by the preliminary definition of acceptable reaction products. Thus, the species to be included in the mechanism were fixed a priori, and the program provided the list of reactions. They used the matrix technique of Yoneda for the representation of reactions and species structures, but the number of generated reactions was limited by applying certain restrictions. The most important restriction was that bimolecular reactions were considered only with a maximum of three products. The number of generated reactions was kept low based on reaction complexity and thermochemical considerations. The mechanism obtained was reduced by qualitative and quantitative comparisons with experimental results, including contributions of elementary reactions to measured rates. The method proposed 538 reactions for the liquid phase oxidation of ethylbenzene. The reaction-complexity investigation approved only 272 reactions and the reaction heats were feasible in the cases of 168 reactions. This mechanism was reduced to a 31-step final mechanism. 

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