Types of Elementary Reactions

The following list of elementary reactions, divided into various categories, allows us to understand and build rate laws for a wide variety of chemical systems. 

1 Elementary Reactions Involving Neutral Species (Homogeneous Gas or Liquid Phase) 

This is the most common category of elementary reactions and can be illustrated by unimolecular, bimolecular, and termolecular steps. 

The asterisk denotes an excited state-a molecule with excess energy (more than enough energy to enable it to undergo a specific reaction step). 

3 Elementary Reactions Involving Charged Particles (Ions, Electrons) 

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The two simple theories SCT and TST have been developed in the context of neutral gas-phase reactions. In this section, we consider other types of elementary reactions listed in Section 6.2.1, and include reactions in condensed phases. The rates of this diverse set of reactions, including photochemistry, can be understood with the concepts developed for gas-phase reactions. 

Thermochemical quantities can also be related to kinetic data in solution, and the bridges are still provided by the TST. We consider the same types of elementary reactions addressed in the previous section, namely,... 

Insertion and -elimination. A catalytic cycle that involves only one type of elementary reaction must be a very facile process. Isomerisation is such a process since only migratory insertion and its counterpart P-elimination are required. Hence the metal complex can be optimised to do exactly this reaction as fast as possible. The actual situation is slightly more complex due to the necessity of vacant sites, which have to be created for alkene complexation and for P-elimination. 

In the balance of this section we shall review the types of elementary reactions believed to be responsible for plasma polymerization. For purposes of clarity these reactions have been listed in Table 1 and grouped into five generic types-initiation, adsorption, propagation, termination, and reinitiation. The factors controlling the rate of each type of reaction will also be discussed. 

It is easy to understand that every type of reactive chemical function causing an irritation or a corrosion corresponds to a type of elementary reaction (Sect. 3.2). 

Elementary reactions are reactions that directly express basic chemical events, that is, the making or breaking of chemical bonds. In the gas phase, there are only two types of elementary reactions 2... 

In Chapter 7 we turn to the other basic type of elementary reaction, i.e., uni-molecular reactions, and discuss detailed reaction dynamics as well as transition-state theory for unimolecular reactions. In this chapter we also touch upon the question of the atomic-level detection and control of molecular dynamics. In the final chapter dealing with gas-phase reactions, Chapter 8, we consider unimolecular as well as bimolecular reactions and summarize the insights obtained concerning the microscopic interpretation of the Arrhenius parameters, i.e., the pre-exponential factor and the activation energy of the Arrhenius equation. 

Finally, it should be stressed that organic electron transfers only rarely occur as isolated steps because of the high chemical reactivity of odd-electron species. Normally, they are part of multi-step mechanisms together with other types of elementary reaction, such as bond forming and breaking. In organic electrochemistry a useful shorthand nomenclature for electrode mechanisms denotes electrochemical (= electron transfer) steps by E and chemical ones by C, and it is appropriate to use the same notation for homogeneous electron-transfer mechanisms too. Thus, an example of a very common mechanism would be the ECEC sequence illustrated below by the Ce(IV) oxidation of an alkylaromatic compound (14-17) (Baciocchi et al., 1976,... 

Thanks to this work, and subsequent work on pyrolysis mechanisms, it has become clear that three different types of elementary reactions are involved (see Chapter 2, Volume 2 for a discussion of chain reactions). 

The most common type of elementary reaction involves the collision of two atoms, ions, or molecules and is called bimolecular. An example is the reaction... 

As we have mentioned above, if the catalyst pores are sufficiently narrow, i.e. the species diffuse through them in Knudsen or transitional regimes, the contribution of heterogeneous reactions of free radicals to the overall reaction rate and selectivity may be predominant. The main types of elementary reactions between radicals and surface sites proposed elsewhere [ 15] are the following ... 

For complex reactions such as the oxidation of methane no mathematically simple rate laws can be formulated. The rates of any of the species involved are complicated functions of the concentrations of all the other participating reactants and intermediates, temperature, pressure, sometimes also wall conditions of the vessel, and other parameters. Isolated elementary reactions, in contrast, obey comparatively simple rate laws. Table 2-2 summarizes rate expressions for several basic types of homogeneous elementary reactions. Let us single out for the purpose of illustration the bimolecular reaction A+B— C+D. On the left-hand side of the rate expression one has equality between the rates of consumption of each reactant and the rates of formation of each product, in accordance with the requirements of stoichiometry. On the right-hand side, the product of reactant concentrations expresses the notion that the rate of the reaction at any instant is proportional to the number of encounters between reactant molecules of type A and B occurring within unit time and volume. The rate coefficient kbltn is still a function of temperature, but it is independent of the concentrations. The same is assumed to hold for the rate coefficients of the other types of elementary reactions in Table 2-2. At a constant temperature the rate coefficients are constants and the equations can be integrated to yield the concentrations of reactants and products as a function of time. 

Temperature and pressure represent external variables that can be controlled in experimental studies of reaction rates. It has been found that the rate coefficients associated with the three basic types of elementary reactions in Tables 2-2 behave quite differently with regard to both variables, so that it is convenient to treat each reaction type separately. Bimolecular reactions will be discussed first, and decomposition and recombination reactions subsequently. 

There are only two types of elementary reactions monomolecular and bimolecular. The easier of the two to understand is the bimolecular reaction. 

This section, which consists only of Volume 18, deals with certain types of elementary reactions, occurring in the gas and liquid phases, which either have not been covered at all in the previous sections or have appeared fragmentarily or scattered among various volumes and chapters. Much of the information is summarized in tabular or graphical form and, as far as possible, the data have been assessed critically and correlated. 

Electrochemical processes at the ITIES involve two basic types of elementary reactions ion transfer and electron tunneling across the liquid liquid boundary. Depending on the properties of the ionic species and the solvents, these two processes can be accompanied by a variety of phenomena such as solvent exchange, interfacial complexation, adsorption, photoexcitation, acid-base dissociation, etc. There are conceptual as well as practical... 

Radicals are generated, consumed, or propagated by a relatively few types of elementary reactions. Radical generation usually involves the homolytic dissociation of some covalent bond. 

Essentially, two broad types of elementary reactions occur in homogeneous catalysis coordination and addition. Although these are reactions and we designate them so, they should more appropriately be viewed as steps in a catalytic cycle that activate the substrate prior to reaction. 

Wigner classifies the elementary reactions in three groups. Only the second type of elementary reactions can be treated with transition state theory, hence, only (WE2) is considered in the theory. 

Summaiy of molecularity of elementary reactions Type of Elementary Reaction Molecularity... 

Another characterization of composite mechanisms is in terms of the types of elementary reactions occurring. Sometimes reactions occur in parallel, such as... 

The most common type of elementary reaction results from bimolecular collisions ... 

Equations similar to (5.36) and (5.37) can be derived using assumption (4) instead of (4) in this case we might call the equations the combinatorial model. Generally these equations are called the master equations. The structure of (5.37) is clear. Two types of elementary reactions are taken into consideration. The effect of the first class of reaction is that the state j is available from / (/ can denote different possible states). The second class describes all of the possible transitions from the state J. Therefore we can write ... 

Note that the reaction profiles can differ for diffraent types of elementary reactions (Fig. 2.8). As can be seen in Fig. 2.8, the reaction profiles can have a potential barrier (a), can be without a potential barrio- (c) or with two potential barriers b, d). This distinction can be used for the classification of elonoitary reactions. 

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