Chemical Kinetics – Retrospect and Prospects

1 Introduction

From time to time my now 16-year-old son, repeating a time-honoured but half-forgotten ritual, will ask me what I do for a living and as I absentmindedly chant in reply the litany - '... chemistry ... physical chemistry ... chemical kinetics ... speed of chemical reactions ...,' I can see the expression of curiosity changing slowly and familiarly to one of resigned bafflement. The situation is quickly recovered with another set of words, '... rocket engines ... fires ... explosions ... atomic bombs ... digestion ...,' but neither of us has yet had the courage to explore the gulf between these languages.

Chemical kinetics as a formal science can be today reckoned to be about a hundred years old, but, despite its pervasive involvement with nearly every branch of science and technology, it has never stirred many sparks in the public imagination. Probably, it is too far behind the front lines. This is, I believe, an unfortunate situation since the demands of our growing, complex, industrial–technological society will place an increasing burden of responsibility on chemical kinetics to provide answers to problems which seem to grow exponentially in their molecular complexity. In the present Report I would like to review some of the changes and involvements which have occurred in chemical kinetics, and then, hopefully, to look into the crystal ball and try to make some educated guesses about what the future is likely to hold.

Any special field of knowledge starts in the observer mode with an assembly of described experiences. These may become categorized if common variables can be discerned, and the final stage sees the emergence of quantitative relations which completely describe the observations and permit their expression in mathematical form. The variables in a kinetic system are basically chemical composition (reactants, products, catalysts), pressure, and temperature, ignoring for the moment physical state and external fields. Chemical kinetics emerged as a quantitative science with the statement of the law of mass action by Guldberg and Waage and attained some form of adolescence with Arrhenius' expression for the temperature dependence of the rate of chemical reactions. If one delves into the history of this early period one cannot help but be struck by the incredibly small data base which served to inspire both of these fundamental generalizations.

In the next 50 years (roughly 1870 — 1920) there followed a period of very slow growth. Very few scientists were attracted to study kinetic phenomena. On the other hand the same could probably be said of most fields of chemistry during this period. Chemistry had not yet made a great social impact. As a hobby, however, chemistry was still relatively inexpensive and any enthusiast with a stopwatch and the patience to perform many repetitive chemical analyses could engage in the action in kinetics. Adequate tools had not yet been developed to explore many of the most interesting phenomena such as combustion and explosion. However, the real flowering of physical chemistry during this period gave an early inspiration to the application of physico-chemical methods of analysis to the study of kinetic processes. Optical and electrical properties were used to follow the course of reactions in solutions while gas reactions could frequently be followed simply by observing pressure changes.

World War I gave a large impetus to the acceleration of scientific research and witnessed the extensive involvement of both industry and government in the support of science. The period between World War I and World War II also witnessed the development of the conceptual basis for chemical kinetics. The methods of describing molecular behaviour, using the tools of statistical mechanics, were outlined in useful detail for gas reactions by the important contributions of Lindemann, Rice, Ramsperger, Kassel, Evans, Polanyi, and Eyring, leading to the most recent expressions for unimolecular reactions in the RRKM (Rice–Ramsperger–Kassel–Marcus) formulation of Transition State Theory. Transition State Theory itself has blossomed into a primary underpinning of all 'equilibrium' kinetic theory for both gases and liquids.

World War II gave a further enormous impetus to the development of both science and technology. The status of science in various countries is measured today in terms of the fraction of the total national effort (G NP) expended on science (more properly, technology). The very concept of such a measure would have been the occasion of great humour in scientific circles prior to 1930. Perhaps an even greater impact introduced by World War II was the development of electronic tools for measurement. The unbelievable rate of growth and sophistication of electronic devices, particularly solid-state devices, since 1940 is probably the single biggest common feature in the research activity of the past three decades. In a very profound sense we, meaning all scientists, may be said to be in the 'Electronic Age'.

What this has done primarily has been to give us the ability to explore the details of chemical interactions on a molecular level and to answer questions which would have been considered moot or meaningless just a short time ago. It has not changed our conceptual understanding of chemical kinetics (which was basically complete with the Dirac Equation) but it has made it possible to use quantum theory to explore the rich and complex phenomena of many-body interactions both in space and in time.

One eloquent testimony to this rich development in chemical kinetics has been the growth of specialities. We have today specialists in gas kinetics, solution kinetics, and catalysis, and in these areas we have sub-specialities of ion–molecule reactions, atom–electron reactions, free-radical reactions, and unimolecular reactions. There are kineticists today who have devoted almost their entire professional lives to the kinetics of energy-transfer processes. In the areas of solution kinetics, scientists who study ionic reactions or 'redox' reactions rarely talk to kineticists who deal with enzyme kinetics or 'concerted' reactions. Finally, there are 'mission-oriented' kineticists whose field of application may cut diagonally across many of these specialities. Lasers, space travel, rocket engines, automobile exhausts, photochemical smog, electrical discharges, and most recently the spectroscopy of interstellar dust have all generated...