Termolecular reaction, defined

Termolecular Reactions. If one attempts to extend the collision theory from the treatment of bimolecular gas phase reactions to termolecular processes, the problem of how to define a termolecular collision immediately arises. If such a collision is defined as the simultaneous contact of the spherical surfaces of all three molecules, one must recognize that two hard spheres will be in contact for only a very short time and that the probability that a third molecule would strike the other two during this period is vanishingly small. 

We can define a termolecular reaction as one that requires the participation of three individual particles in a single kinetic process. Thus we might think of the reaction of NO with CI2 as proceeding through the simultaneous interaction of two NO molecules with CI2 to form the transient reaction complex (NO)2Cl2, which then decomposes in a single step into two molecules of NOCl. Or in more general terms, for the reaction of A + B + C —> products we may write... 

It is useful to classify elementary reactions according to their molecularity, which is defined as the sum of the exponents appearing in a rate equation for a single elementary reaction. The term unimolecular reaction is used to describe an elementary reaction involving one chemical species. A bimolecular reaction involves the interaction of two chemical species. The interaction of three chemical species, or a termolecular reaction, is quite rare and will not be considered for further discussion. Molecularity is often confused with the order of a reaction, which refers to the sum of the exponents appearing in an experimental rate equation. 

An alternative method of treating termolecular reactions which avoids the formal difficulty of defining the conditions for ternary collisions is that of estimating the partition functions for the transition state. The procedure has already been illustrated (p. 382). The formal superiority of the theory is, however, counterbalanced by the arbitrariness of the molecular constants assigned to the transition complex. 

The number of molecules that participate as reactants in an elementary reaction defines the molecularity of the reaction. If a single molecule is involved, the reaction is unimolecular. The rearrangement of meth)d isonitrile is a unimolecular process. Elementary reactions involving the collision of two reactant molecules are bimolecular. The reaction between NO and O3 is bimolecular. Elementary reactions involving the simultaneous collision of three molecules are termolecular. Termolecular reactions are far less probable than unimolecular or bimolecular processes and are rarely encountered. The chance that four or more molecules will collide simultaneously with any regularity is even more remote consequently, such collisions are never proposed as part of a reaction mechanism. 

The rate of a termolecular reaction is controlled by the three-body collision frequency Zabc- This frequency is defined as the number of two-body collisions Zx.YZ for each particle X (X = A, B, C) with an unstable complex YZ formed from other particles (YZ = BC, AC, AB) [443]. 

A reaction mechanism is the sequence of elementary reactions, or elementary steps, that defines the pathway from reactants to products. Elementary reactions are classified as unimolecular, bimolecular, or termolecular, depending on whether one, two, or three reactant molecules are... 

Define what is meant by unimolecular and bimolecular steps. Why are termolecular steps infrequently seen in chemical reactions ... 

Each of these two reactions is called an elementary step, a reaction whose rate law can be written from its molecularity. Molecularity is defined as the number of species that must collide to produce the reaction indicated by that step. A reaction involving one molecule is called a unimolecular step. Reactions involving the collision of two and three species are termed bimolecular and termolecular, respectively. Ter-molecular steps are quite rare, because the probability of three molecules colliding simultaneously is very small. Examples of these three types of elementary steps and the corresponding rate laws are shown in Table 12.7. Note from Table 12.7 that the rate law for an elementary step follows directly from the molecularity of that step. Eor example, for a bimolecular step the rate law is always second order, either of the form k[l Y for a step with a single reactant or of the form A [A][B] for a step involving two reactants. 

The distinctive features of Warneck s photoionization technique are the pressure range covered (up to 0.2 Torr) and the direct measurement of ion residence times. The capability of working at high pressures makes possible the study of reactions with low rates, even termolecular association reactions. The residence time may be varied considerably and well-defined ion temperatures and drift velocities established at the higher pressures. The direct measurement of residence time eliminates certain errors which can occur in the calculation of this quantity—e.g., the electric field may be affected to an unknown extent by surface charges, space charge, contact potentials, and electric field penetration. The rate constant is directly determined from measured values of the ion residence time and of the initial and final concentrations of reactants or of products or of both. 

The molecularity of a reaction is defined as the number of reactant molecules appearing in the stoichiometric equation. Thus, reaction (2.1) is bimolecular, while eq. (2.5) is termolecular. Often, the rate of a reaction is experimentally found to be proportional to the concentrations of the reactants (and, less frequently, the products or some other, catalyst, species, Q) to some power ... 

Section 14.6 A reaction mechanism details the individual steps that occur in the course of a reaction. Each of these steps, called elementary steps, has a well-defined rate law that depends on the number of molecules (the molecularity) of the step. Elementary steps are defined as eitirer unimolecular, bimolecular, or termolecular, depending on whether one, two, or three reactant molecules are involved, respectively. Termolecular elementary steps are very rare. Unimolecular, bimolecular, and termolecular steps follow rate laws that are first order overall, second order overall, and tiiird order overall, respectively. An elementary step may produce an intermediate, a product tiiat is consumed in a later elementary step and therefore does not appear in tiie overall stoichiometry of the reaction. 

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