Termolecular chemical reactions

A mechanism is a description of the actual molecular events that occur during a chemical reaction. Each such event is an elementary reaction. Elementary reactions involve one, two, or occasionally three reactant molecules or atoms. In other words, elementary reactions can be unimolecular, bimolecular, or termolecular. A typical mechanism consists of a sequence of elementary reactions. Although an overall reaction describes the starting materials and final products, it usually is not elementary because it does not represent the individual steps by which the reaction occurs. 

In a termolecular reaction, three chemical species collide simultaneously. Termolecular reactions are rare because they require a collision of three species at the same time and in exactly the right orientation to form products. The odds against such a simultaneous three-body collision are high. Instead, processes involving three species usually occur in two-step sequences. In the first step, two molecules collide and form a collision complex. In a second step, a third molecule collides with the complex before it breaks apart. Most chemical reactions, including all those introduced in this book, can be described at the molecular level as sequences of bimolecular and unimolecular elementary reactions. 

The mechanism is one or more elementary reactions describing how the chemical reaction occurs. These elementary reactions may be unimolecular, bimolecular, or (rarely) termolecular. 

Note that both of the steps in the mechanism are bimolecular reactions, reactions that involve the collision of two chemical species. Unimolecular reactions are reactions in which a single chemical species decomposes or rearranges. Both bimolecular and unimolecular reactions are common, but the collision of three or more chemical species (termolecular) is quite rare. Thus, in developing or assessing a mechanism, it is best to consider only unimolecular or bimolecular elementary steps. 

The molecularity of a single elementary reaction is the number of molecules engaged in the reaction. A simple elementary reaction is referred to as uni-, bi-, or termolecular if one, two, or three chemical species are involved in the chemical reaction, respectively ... 

Because the general principles of chemical kinetics apply to enzyme-catalyzed reactions, a brief discussion of basic chemical kinetics is useful at this point. Chemical reactions may be classified on the basis of the number of molecules that react to form the products. Monomolecular, bimolecular, and termolecular reactions are reactions involving one, two, or three molecules, respectively. 

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

ON 00 NO would be a true transition state in the sense ofEyring theory and represent the only and rate-determining step. It has been shown that this pathway is possible by the expected rate of termolecular encounters, and even the unusual temperature dependence of the gas-phase kinetics can be accounted for (6). However, the idea of a reaction with a negative enthalpy of activation is not convincing, because the alternatives are steady-state formulations with normal chemical physics. The kinetics of many multistep chemical reactions has been successfully explained by applying this model. 

Some elementary chemical reactions follow a third order rate expression at all normally accessible experimental conditions, and according to the definitions of molecularity must be classified as termolecular. The most common example is the combination of two atoms in the presence of a third species. The rate expression is r = A ( J)[A] [M] for combination of Hke atoms. A, in the presence of the collider or heat bath species M. These reactions do not occur by the simultaneous collision of all three species, which is a very rare event, but by two bimolecular steps that take place within lpsec of one another. An energy transfer mechanism of the reaction may be written as follows ... 

The molecularity of a chemical reaction is the number of molecules involved in the transition state of the reaction. The term can only be applied to single-step reactions, also known as elementary reactions. If only a single molecule is involved in the transition state, the reaction is unimolecular. For example, a thermal rearrangement such as a Cope rearrangement is typically unimolecular. If fwo molecules are involved the reaction is bimolecular, with the Sn2 reaction being the prototype. Termolecular processes involve three molecules and are rare, but not unprecedented (we show a few in Chapter 10). 

Chemical reactions are, in general, complex combinations of simple elementary reactions. The molecular number of reactants of the elementary reaction, which are given as the l.h.s. terms of a reaction formula is one, two and three, and these are called unimolecular, bimolecular and termolecular reactions, respectively. The order of reaction is one, two and three corresponding to each case. It is not always true that the reaction of order one is the unimolecular reaction, and this is not always true similarly for other cases of the reaction order. 

There is one reactive intermediate, NO3, which is produced in one step and consumed in the other step. Addition of the steps of this mechanism gives the stoichiometric equation, with cancellation of NO3. Both steps in the mechanism of Eq. (12.1-2) are bimolecular. That is, they involve two reactant particles. Unimolecular steps involve a single particle. Termolecular steps involve three particles. Termolecular processes are relatively slow because of the small probability that three molecules will collide or diffuse together at once, and these processes occur less frequently in mechanics than do bimolecular processes. Elementary processes involving four or more reactant particles probably do not occur in chemical reaction mechanisms. We now make an important assertion concerning the rate law of any elementary process In an elementary process, the order of any substance is equal to the molecularity of that substance. We now justify this assertion for bimolecular elementary processes in the gas phase. 

Chemical reactions include unimolecular, bimolecular and termolecular reactions. Collisions involving four molecules are improbable and even ter-nuclear processes are rare. 

The two steps add up to give the overall reaction. The second step is bimolecular, so it is chemically reasonable. The first step is termolecular, which is possible but rare. In the proposed mechanism, step 1 is the rate-determining reaction. The rate law equation for the first step is written as follows ... 

The third chemical equation, involving nitric oxide, represents a termolecular reaction. Therefore, the overall order of the reaction is expected to exceed that of the second-order reaction generally assumed in the pre-mixed gas burning model. The high exothermicity accompanying the reduction of NO to N2 is responsible for the appearance of the luminous flame in the combustion of a double-base propellant, and hence the flame disappears when insufScient heat is produced in this way, i. e., during fizz burning. 

Trimolecular reactions (also referred to as termolecular) involve elementary reactions where three distinct chemical entities combine to form an activated complex Trimolecular processes are usually third order, but the reverse relationship is not necessarily true. AU truly trior termolecular reactions studied so far have been gas-phase processes. Even so, these reactions are very rare in the gas-phase. They should be very unhkely in solution due, in part, to the relatively slow-rate of diffusion in solutions. See Molecularity Order Transition-State Theory Collision Theory Elementary Reactions... 

The chemical conversion is not without some difficulty because the reagent NO also reacts with the product Cl to form C1NO. In the lower stratosphere, this termolecular reaction is about 10% as fast as the forward reaction. In addition to this removal of Cl is the reaction... 

The first studies of the kinetics of the NO-F2 reaction were reported by Johnston and Herschbach229 at the 1954 American Chemical Society (ACS) meeting. Rapp and Johnston355 examined the reaction by Polanyi s dilute diffusion flame technique. They found the free-radical mechanism, reactions (4)-(7), predominated assuming reaction (4) to be rate determining, they found logfc4 = 8.78 — 1.5/0. From semi-quantitative estimates of the emission intensity, they estimated 6//t7[M] to be 10-5 with [M] = [N2] = 10 4M. Using the method of Herschbach, Johnston, and Rapp,200 they calculated the preexponential factors for the bimolecular and termolecular reactions with activated complexes... 

This is a general fact. For monomolecular (or pseudo-monomolecular) reactions the graphs corresponding to compartments are acyclic. A similar property for the systems having either bi- or termolecular reactions is more complex. It can be formulated as follows. If every edge in the graph of predominant reaction directions for some compartment is ascribed to a positive "rate constant k and chemical kinetic equations are written with... 

There are relatively few systems in either category of termolecular reactions which have been studied in any great detail, and the data for these are presented in Table XII.9. Only three wholly chemical processes are included, and all involve the reaction of NO. The data for the reaction of NO with H2 which has been studied above lOOO K, appear to be third-order, but the mechanism is probably not simple. 

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