Chemical reaction rates, collision theory

An understanding of reaction rates can be explained by adopting a collision model for chemical reactions. The collision theory assumes chemical reactions are a result of molecules colliding, and the rate of the reaction is dictated by several characteristics of these collisions. An important factor that affects the reaction rate is the frequency of collisions. The reaction rate is directly dependent on the number of collisions that take place, but several other important factors also dictate the speed of a chemical reaction. 

An understanding of the nature of chemical reactions requires the details of the elementary-reaction steps in which, the molecules come together, rearrange, and leave as species that differ from the reactants. There are two descriptions that deal with the rates of chemical reactions. The collision theory considers the concept that the reaction of molecules can occur only as a result of collision of the reactant molecules. The transition-state theory focuses on the species that corresponds to the maximum-energy stage in the reaction process. This species is called the activated complex or transition state. The transition state, denoted by the symbol A for reaction (1), is a short-lived species, which is converted to C. The reader is referred to [1-10] for a thorough discussion of the energetics involved in chemical reactions. 

Collision Theory of Chemical Reaction Rates (Widom). 5 353... 

On a microscopic scale, atoms and molecules travel faster and, therefore, have more collisions as the temperature of a system is increased. Since molecular collisions are the driving force for chemical reactions, more collisions give a higher rate of reaction. The kinetic theory of gases suggests an exponential increase in the number of collisions with a rise in temperature. This model fits an extremely large number of chemical reactions and is called an Arrhenius temperature dependency, or Arrhenius law. The general form of this exponential relationship is... 

Before two compounds can react chemically, they must contact each other. Collision theory has been formulated to explain various factors on chemical reaction rates. Part of this theory is the Law... 

The simplest version of the theory of chemical reactions rates is the kinetic collision theory of gas reactions /1/ which has been developed several decades ago by LEWIS (1918), HERZPBLD (1919), POLA-NYI (1920), HINSHELWOOD (1937) a.o./2/. For a simple bimolecular reaction of the type... 

Our approach is very simple, but it has the virtue of providing exact general rate expressions which are closely related to the traditional formulations of both the collision and activated complex theory as given by equations (3A) and (5A), respectively. Thus, it directly yields precise definitions of both the quantum and classical (or semiclassical) corrections to be introduced in these equations, as well as in the properly adiabatic formulations of transition state theory also discussed in this book. We hope, therefore, that the unified treatment presented will contribute to a full elucidation of the relations between the various theories of chemical reaction rates. 

Library of Congress Cataloging in Publication Data. Christov, St. G. Collision theory and statistical theory of chemical reactions. (Lecture notes in chemistry 18) Bibliography p. Includes index. 1. Chemical reaction, Rate of. 2. Collisions (Nuclear physics) I. Title. QD502.K47 541.3 94 80-18112... 

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