Steps, elementary trimolecular

Elementary reactions are individual reaction steps that are caused by collisions of molecules. The collision can occur in a more or less homogeneous reaction medium or at the reaction sites on a catalyst surface. Only three elementary kinetic processes exist mono-, bi-, and trimolecular processes. Of these, trimolecular processes are rarely found, because the chance of three molecules colliding at the same time is very small. Each elementary reaction consists of an activation of the reactants, followed by a transition state and decomposition of the latter into reaction products ... 

Molecularities. An equally elementary criterion is the fact that a great majority of reaction steps are uni- or bimolecular trimolecular steps are rare and slow, and steps of still higher molecularities are unheard of (see Section 2.1). A trimolecular forward or reverse step in a postulated mechanism calls for an explanation why its reactants are not consumed by bimolecular steps before they have a chance to undergo the trimolecular one (e.g., see the Example 7.8 farther below). No mechanism involving a forward or reverse step of even higher molecularity should ever be considered. 

Elementary reactions are characterized by their molecularity, to be clearly distinguished from the reaction order. We distinguish uni- (or mono-), bi-, and trimolecular 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 unimolecular reactions. 

The molecularity of an elementary reaction is the number of molecules that are to collide in order that the elementary reaction take place. In a bimolecular reaction the transformation is the result of the collision of two molecules. The collision of three or more molecules is highly improbable, a seemingly trimolecular reaction usually is the resultant of mono- and bimolecular elementary steps. In our example the molecularities of the chemical species A, B, C and D are a, b, c and d. 

Chemical kinetics studies the rate of chemical reactions and factors that influence them. A chemical process can be divided into a sequence of one or more elementary reactions that typically involve collisions between two molecules (bimolecular reaction) or dissociation/isomerization of a reagent molecule (uni-molecular reaction). A trimolecular reaction, which is the collision of three reactant molecules, occurs less frequently. Usually, the mechanisms are described in a single global step, but in reahty they occur in a series of elanentary steps [1]. 

The molecularity of a reaction is the number of reactant molecules participating in a simple reaction consisting of a single elementary step. Reae-tions can be unimolecular, bimolecular, and trimolecular. Unimolecular reactions can include isomeiizations (A —> B) and decompositions (A B + C). Bimolecular reactions include association (A - - B AB 2A A2) and exchange reactions (A -h B C -h D or 2A C -h D). The less eommon termolecular reactions can also take place (A -h B -h C —> P). 

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