Collision Theory A Model for the Reaction Process

Gasoline and oxygen coexist quietly until a spark from a spark plug propels them into violent reaction. Why Why are ozone molecules in the stratosphere destroyed more rapidly in the presence of chlorine atoms released from CFG molecules In order to understand these situations, we need to take a look at a model called collision theory, which is useful for visualizing the process of chemical change. 

We will demonstrate the basic assumptions of collision theory by using it to describe the reaction of an oxygen atom and an ozone molecule to form two oxygen molecules. 

Objective 2 Step 1 Reactants collide The process begins with a violent collision between an O 

Objective 3 atom and an O3 molecule, which shakes them up and provides them with enough 

An oxygen atom collides with an ozone molecule. 

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Section 16.1 Collision Theory A Model for the Reaction Process... 

As the fundamental concepts of chemical kinetics developed, there was a strong interest in studying chemical reactions in the gas phase. At low pressures the reacting molecules in a gaseous solution are far from one another, and the theoretical description of equilibrium thermodynamic properties was well developed. Thus, the kinetic theory of gases and collision processes was applied first to construct a model for chemical reaction kinetics. This was followed by transition state theory and a more detailed understanding of elementary reactions on the basis of quantum mechanics. Eventually, these concepts were applied to reactions in liquid solutions with consideration of the role of the non-reacting medium, that is, the solvent. 

A fully microscopic treatment of this problem is a very difficult task. It is usually the motion of some internal coordinate of a complex molecule that is important for the description of the isomerization reaction (cf. Sections III and IV). A microscopic theory at the same level as that for the bimolecular processes described in the previous sections would entail a full description (or model) of the internal structure of the molecule and its interactions with the surrounding solvent. The collision dynamics for such a process are necessarily complex, but a theory at this detailed level is not out of the question for some models of small molecule isomerization reactions. However, it is probably premature to embark on such a program, since the implications of the kinetic theory for the reactions for which it is more easily formulated have not yet been fully explored. 

The simple collision theory for bimolecular gas phase reactions is usually introduced to students in the early stages of their courses in chemical kinetics. They learn that the discrepancy between the rate constants calculated by use of this model and the experimentally determined values may be interpreted in terms of a steric factor, which is defined to be the ratio of the experimental to the calculated rate constants Despite its inherent limitations, the collision theory introduces the idea that molecular orientation (molecular shape) may play a role in chemical reactivity. We now have experimental evidence that molecular orientation plays a crucial role in many collision processes ranging from photoionization to thermal energy chemical reactions. Usually, processes involve a statistical distribution of orientations, and information about orientation requirements must be inferred from indirect experiments. Over the last 25 years, two methods have been developed for orienting molecules prior to collision (1) orientation by state selection in inhomogeneous electric fields, which will be discussed in this chapter, and (2) bmte force orientation of polar molecules in extremely strong electric fields. Several chemical reactions have been studied with one of the reagents oriented prior to collision. ... 

A transition-state theory (Safron et al, 1972) has been developed within the context of scattering theory, to provide suitable models for crossed molecular beam processes. As in the case of RRKM theory, it is based on the premise that the probability of complex decomposition is a product of a probability of break-up and a probability Of departure from the collision region. But it adds restrictions peculiar to bimolecular reactions, such as a limit on the maximum angular momentum that allows formation of the complex from reactants. Let p(E t) indicate the probability density for finding a product pair with kinetic energy E. This may be written as... 

The simplified-kinetic-theory treatment of reaction rates must be regarded as relatively crude for several reasons. Numerical calculations are usually made in terms of either elastic hard spheres or hard spheres with superposed central attractions or repulsions, although such models of molecular interaction are better known for their mathematical tractability than for their realism. No account is taken of the internal motions of the reactants. The fact that every combination of initial and final states must be characterized by a different reaction cross section is not considered. In fact, the simplified-kinetic-theory treatment is based entirely on classical mechanics. Finally, although reaction cross sections are complicated averages of many inelastic cross sections associated with all possible processes by which reactants in a wide variety of initial states are converted to products in a wide variety of final states, the simplified kinetic theory approximates such cross sections by elastic cross sections appropriate to various transport properties, by cross sections deduced from crystal spacings or thermodynamic properties, or by order-of-magnitude estimates based on scientific experience and intuition. It is apparent, therefore, that the usual collision theory of reaction rates must be considered at best an order-of-magnitude approximation at worst it is an oversimplification that may be in error in principle as well as in detail. 

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