Reaction Kinetics

Homogeneous Gas Reactions

Keith J. Laidler, Robert Robinson, H. M. N. H. Irving, L. A. K. Staveley

• Publisher: Pergamon
• Year: 2013
• Print ISBN: 9781483197388
• eBook ISBN: 9781483222417
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Reactions Kinetics: Volume I: Homogeneous Gas Reactions presents a general introduction to the subject of kinetics, including the basic laws of kinetics and the theoretical treatment of reaction rates. This four-chapter book deals mainly with homogeneous reactions in the gas phase. Chapter 1 presents the kinetic laws based on experimental results in terms of their simple concepts, with a special consideration of the way in which rates depend on concentration, while Chapter 2 deals with the interpretation of rates in terms of more fundamental theories. Chapter 3 covers the overall reactions that are believed to be elementary, such as the reaction between hydrogen and iodine, the reverse decomposition of hydrogen iodide, the corresponding reactions involving deuterium instead of hydrogen, and the dimerizations of butadiene and cyclopentadiene, as well as a few elementary termolecular reactions, all involving nitric oxide. This chapter also includes a general account of some of the elementary reactions that occur as steps in more complex mechanisms. Chapter 4 examines the reaction rates of numerous complex gas reactions. Undergraduate physical chemistry and chemical kinetics students, as well as advanced students in other fields, such as biology and physics, will find this book invaluable.

CHAPTER 1

Basic Kinetic Laws

Publisher Summary

This chapter presents basic kinetic laws and experimental results in terms of simple concepts. The subject of chemical kinetics is concerned with the rates of chemical reactions and with the factors upon which the rates depend. The most important of these factors are concentration, temperature, and hydrostatic pressure. By making systematic studies of the effects of these factors on rates, it is possible to draw conclusions about the detailed mechanisms by which chemical reactions proceed. The ultimate objective of a kinetic study is to arrive at a reaction mechanism. The studies of a nonkinetic nature, such as stereochemical studies, provide valuable information regarding mechanism. The rate of a chemical reaction, which may also be referred to as its velocity or speed, may be expressed in various ways. In a kinetic investigation, one measures, in some direct or indirect fashion, concentrations at various times. The method of integration is more widely used in the interpretation of kinetic data than is the differential method.

The subject of chemical kinetics is concerned with the rates of chemical reactions, and with the factors upon which the rates depend. The most important of these factors are concentration, temperature and hydrostatic pressure. By making systematic studies of the effects of these factors on rates it is possible to draw conclusions about the detailed mechanisms by which chemical reactions proceed. It is probably true to say that the ultimate objective of a kinetic study is to arrive at a reaction mechanism. Studies of a non-kinetic nature, such as stereochemical studies, may also provide valuable information as to mechanism, and must always be taken into account in a kinetic investigation.

In any branch of science it is convenient to distinguish between the phenomenological, or empirical, laws that are obeyed, and the theories that are formulated in order to provide an explanation for these laws. The present chapter is mainly concerned with the kinetic laws, and with the analysis of experimental results in terms of simple concepts; it is almost entirely devoted to a consideration of the way in which rates depend on concentration. Chapter 2, on the other hand, deals with the interpretation of rates in terms of more fundamental theories.

RATE OF REACTION

The rate of a chemical reaction, which may also be referred to as its velocity or speed, may be expressed in various ways. In some investigations it is convenient to measure the concentration x of a product of reaction at various times, and curve a in Fig. 1 shows schematically how such a concentration may vary with the time. The slope dx/dt of such a curve at any time then provides a measure of the rate at that time. If the units of concentration are moles per litre the units of the rate are clearly moles litre⁻¹ sec⁻¹.

FIG. 1 Schematic curves showing the concentration of a product, and the concentration of a reactant, as functions of time.

Alternatively, one may measure the concentration of a reactant, and curve b of Fig. 1 shows how such a concentration may vary with time. The slopes, dc/dt, are now all negative; it is convenient to drop the negative sign and define the rate as −dc/dt.

It is important to note that the rate of a chemical reaction may have a different numerical value according to the way it is defined and measured. Consider, for example, the reaction

Since every time one molecule of nitrogen reacts two molecules of ammonia are formed it is evident that the rate of formation of ammonia, υ_NH2, is twice the rate of disappearance of nitrogen, υ_N2:

(1)

Similarly the rate of disappearance of hydrogen, υ_H2, is three times the rate of disappearance of nitrogen,

(2)

Order of Reaction

In some reactions the rates are proportional to the concentrations of reactants raised to...

Table of contents

• Cover image
• Title page
• Table of Contents
• Inside Front Cover
• Copyright
• Preface
• A Note to the Student
• Chapter 1: Basic Kinetic Laws
• Chapter 2: Molecular Kinetics
• Chapter 3: Elementary Gas Reactions
• Chapter 4: Complex Gas Reactions
• Correlation, and the Method of Least Squares
• Numerical Values
• Bibliography
• Name Index
• Subject Index