Chemical reaction rates, collision

It is convenient initially to classify elementary reactions either as energy-transfer-limited or chemical reaction-rate-limited processes. In the former class, the observed rate corresponds to the rate of energy transfer to or from a species either by intermolecular collisions or by radiation, or intramolecular-ly due to energy transfer between different degrees of freedom of a species. All thermally activated unimolecular reactions become energy-transfer-limited at high temperatures and low pressures, because the reactant can receive the necessary activation energy only by intennolecular collisions. 

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. 

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... 

Chemical reaction rate depends on the collisions of molecules, per second per unit volume. Since the number of collisions of a species is proportional to its concentration, the chemical reaction rate is proportional to the product of concentrations (mass action law). Thus, for a single homogeneous elementary chemical reaction... 

Hie chemical reaction rate is usually dependent on the molar concentrations of the reactants and not on their mass fractions, because it depends on the chance of collision of molecules. However, here the definition of in terms of mass fractions is preferred, because it can readily be incorporated into mass balances. A definition in terms of moles or molar concentrations might invite the use of mole balances instead of mass balances. Since, contrary to conservation of mass, there is no such thing as conservation of moles (because one molecule might divide into several molecules, or several might condense into one), the use of mole balances is strongly dissuaded. More information concerning the definition of conversion can be found elsewhere [2]. 

Relate rates of chemical reactions to collisions between reacting particles. 

The rate of molecule-molecule collisions given by Zj = /2 N/V)Trd u estimates the upper bound of gas phase chemical reaction rates and calculates the distance a molecule travels between collisions as well as the diffusion coefficient in gases The Boltzmann distribution... 

USE To study chemical reaction rates and mechanisms. The cross section of deuterium for the capture of thermal neutrons is very low which makes it useful, in the form of heavy water> as a neutron moderator in nuclear reactors. Produces a considerable decrease in neutron energy per collision. 

But the stochastic approach is invaluable in several ways. It builds a clear intuitive connection between the imcroscopk and the macroscopic. The microscopic level is characterized by discontinuous, random molecular motion and the probability of collision as the basis for chemical reaction rate. The macroscopic level is characterized by... 

The analogy of molecules acting as if they were hard spheres lead to the Kinetic Model. It permitted the introduction of the Arrhenius pre-exponential factor and of the frequency of collisions between reacting molecules into the study of the chemical reaction rates. 

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... 

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. 

If we now consider molecules A (in concentration colliding with molecules B (in concentration /ig), the number of collisions per second that one A molecule makes with the B molecules is TTp cn. This expression gives the maximum chemical reaction rate, assuming that each collision between A and B molecules results in a reaction. 

Shown in Figure 3.1 is the distribution of the kinetic energies of the molecules in a gas for two temperatures. The number of molecules with kinetic energy > E. is proportional to the shaded area in Figure 3.1. It can be seen that if E. is fairly large, the number of molecules with energy > E. is very sensitive to temperature. Hence, if a certain minimum value of E is required for two colliding molecules to react chemically, it is apparent why the chemical reaction rate should be both smaller and more temperature sensitive than the collision rate. 

Another important factor that determines a chemical reaction rate is the orientation of the colliding molecules. The probability of a particular collision being favorable to a reaction is generally much less than unity. 

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