Molecularity of an Elementary Reaction

Many reactions take place by a series of elementary reactions. The molecularity of an elementary reaction is the number of reactant particles that take part in the step. 

The molecularity of an elementary reaction refers to the number of particles in the reactants (left-hand side). If the molecularity is 1, the elementary reaction is said... 

The order of a reaction is derived from an empirical reaction rate equation the molecularity refers to a molecular mechanism and hence to a theoretical model of a certain elementary step in a reaction. For example, it appears that in the reaction between iodine vapour and hydrogen there is a single elementary step involving the collision of tioo molecules (Hg and Ig) and their emergence as two molecules of HI. This is accordingly a 6 molecular reaction. The molecularity of an elementary reaction is defined as the smallest number of molecules which must coalesce prior to the formation of the products. The term does not apply to processes which consist of a succession of elementary steps, such as chemical reactions very often are. Thus, the oxidation of an iron(II) salt by a permanganate,... 

The molecularity of an elementary reaction that is written in the reverse direction also follows from its stoichiometry. The products have become reactants. 

It will be clear from what has been said that the decision about the molecularity of an elementary reaction must involve not only a careful kinetic study in which as many factors as possible are varied, but also a consideration of other aspects of the reaction, including the nature of the products. ... 

An elementary reaction step is a reaction that converts reactants directly to products through a single transition state (see Chapter 5). The reaction order for an elementary reaction step usually reflects the molecularity of the reaction. The molecularity of an elementary reaction step is the number of species that come together to form the activated complex. 

For an elementary reaction, but not for a composite reaction, the term molecularity can be employed. The molecularity of an elementary reaction is the number of reactant particles (atoms, molecules, free radicals, or ions) that are involved in each individual chemical event. For example, the cyclopropane isomerization appears to be elementary, in that each chemical act involves a single cyclopropane molecule the molecularity is unity, and the... 

It is important to distinguish clearly between the molecularity and the order. The latter is a purely experimental quantity, which is concerned with how the rate depends on reactant concentrations the concept of order applies to some composite reactions. The molecularity of an elementary reaction, on the other hand, is arrived at by inference from all of the evidence available about the reaction. One such piece of evidence is the order. If a reaction in the gas phase appears to be elementary and has an order of one, it is reasonable to conclude that it is unimolecular. However, as will be seen in Section VI, unimolecular gas reactions become second order at low pressures, and it is therefore unsafe to conclude that a second-order gas reaction is bimolecular it may be a unimolecular reaction in its second-order region. 

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. 

The molecularity of an elementary reaction is defined as the number of molecules participating in the reaction. A unimolecular reaction describes the dissociation or rearrangement of a single reactant as in the reaction... 

Equally important is the molecularity of an elementary reaction. One of the fast steps in the hydrogen bromine kinetic sequence (4.5) is... 

Each step in a reaction mechanism represents an elementary reaction, one that occurs in a single collision of the reactant molecules. The molecularity of an elementary reaction is essentially the... 

The reaction steps in the mechanism of a homogeneous gas-phase reaction are usually elementary reactions, that is, the stoichiometric equation of the reaction step corresponds to real molecular changes. The molecularity of an elementary reaction is the number of molecular entities involved in the molecular encounter. Thus, an elementary reaction can be unimolecular or bimolecular. Some books on chemical kinetics also discuss termolecular reactions (Raj 2010), but three molecular entities colliding at the same time is highly improbable (Drake 2005). What are often referred to as termolecular reactions actually involve the formation of an energetically excited reaction intermediate in a bimolecular reaction which can then collide with a third molecular entity (e.g. a molecule or radical). 

Each step in a reaction mechanism represents an elementary reaction, one that occurs in a single collision of the reactant molecules. The molecularity of an elementary reaction is essentially the number of reactant molecules involved in the collision. Elementary reactions may be unimolecular (one reactant molecule), bimolecular (two reactant molecules), or termolecular (three reactant molecules). These molecules may be of the same or different types. Each of the elementary steps in the formation of NO2 from NO and O2 is bimolecular, because there are two reactant molecules in each... 

Until now, we have only considered elementary reactions, that is, ones that occur in a single step. These reactions are observed between a certain number (that must be a whole number) of atoms, molecules or ions. The number of species involved in an elementary reaction is designated as the molecularity of an elementary reaction. A chemical reaction is generally described by an equation of the type... 

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