3.091_Notes_8 - LN8 3.091 Introduction to Solid State Chemistry Lecture Notes No 8 THEORY OF REACTION RATES Sources for Further Reading 1 2 3 4 5

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LN–8 1 3.091 – Introduction to Solid State Chemistry Lecture Notes No. 8 THEORY OF REACTION RATES * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * Sources for Further Reading: 1. Laidler, K.J., Principles of Chemistry , Harcourt, Brace & World, New York, 1966. 2. Moore, W.J., Physical Chemistry , Prentice-Hall, 1962. 3. Moeller, T., Inorganic Chemistry , John Wiley, 1982. 4. Campbell, J.A., Why Do Chemical Reactions Occur? , Prentice-Hall. 5. Ebbing, D.D., General Chemistry , Houghton-Mifflin, 1984. * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * 1. INTRODUCTION We can readily understand that chemical reactions normally are preceded by collisions of atoms, ions, or molecules. However, the rate of such collisions in solids, liquids and gases is so great that all reactions would be very rapid were it only necessary for collisions to occur. “Chemical” reactions will not proceed more rapidly than molecular collisions allow, but many reactions proceed much more slowly. It is thus apparent that not every molecular collision leads to reaction. We have also seen from earlier examples that the driving force for physical as well as chemical reactions is the (free) energy change - which must be negative for reactions to occur spontaneously. However, while providing a “go/no-go” answer, this criterion alone cannot give us any information concerning the rate at which reactions occur, nor can it tell us which factors influence the reaction velocity. Rates at which reactions occur vary considerably. For example, the nuclear reaction: U 238 92 ³ Th 234 90 ) a is 50% completed ( t 1 ń 2 ) after 5 x 10 9 years. t 1 ń 2 + 5x10 9 years
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LN–8 2 On the other hand, the chemical reaction: SO ** 4 ) H ) ³ HSO * 4 is 50% completed after about 10 –4 s. t 1 ń 2 + 3x10 * 4 seconds Reaction kinetics ( rate theory ) deals to a large extent with the factors which influence the reaction velocity. Take corrosion (rusting of iron), for example. We all know that it requires air and water to provoke rusting and we also know, much to our sorrow, that rusting proceeds much more rapidly near the ocean where salt is present. We also know that the rate of rusting depends strongly on the composition of iron (pure Fe and steel corrode much less rapidly than cast iron, for example). It is primarily kinetic studies which lead to the elucidation of chemical reactions and, in the case of corrosion, to the development of more corrosion resistant materials. The principal experimental approach to the study of the reaction process involves the measurement of the rate at which a reaction proceeds and the determination of the dependence of this reaction rate on the concentrations of the reacting species and on the temperature. These factors are grouped together in the term reaction kinetics and the results for a given reaction are formulated in a rate equation which is of the general form: Rate = k(T) x function of concentration of reactants The quantity k(T) is called the rate constant and is a function only of the temperature if the term involving the reactant concentrations correctly expresses the rate dependence on concentration. Thus the experimental information on the reaction process is
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This note was uploaded on 11/02/2009 for the course CHEMISTRY 3.091 taught by Professor Donsadoway during the Fall '04 term at MIT.

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3.091_Notes_8 - LN8 3.091 Introduction to Solid State Chemistry Lecture Notes No 8 THEORY OF REACTION RATES Sources for Further Reading 1 2 3 4 5

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