INTRODUCTIOX TO T H E SYMPOSIUM ON ANOMALIES I N REACTION KINETICS’ MILTOX BURTOS Department of Chemistry, University of Notre Dame, Nolre Dame, Indiana Received August 10, 1960
“Anomaly” is one of the misused words of science. Some phenomenon inconsistent with past experience is called anomalous anti later explained, but the word “anomalous” survives. We note, for example, the thoroughly understood ‘Lanomalousdispersion.” Our concern is exclusively with current anomalies. In reaction kinetics, as in other branches of science, two classes of anomalies exist: the known, obtrusive and the subtle, undetected. In the first class are those in which two (or more) sets of apparently reliable data lead to two (or more) seemingly irreconcilable sets of conclusions. Born of intransigence their children are polemics, diatribes, and doctoral dissertations. In the second class are principally the theoretical inconsistencies. Developed as logical consequences of what seem to be the same basic premises are mutually inadmissible conclusions, usually not recognized as such because they may relate to different phenomena or dzerent aspects of the same phenomenon. I had hoped a t this point to give illustrations, from these two classes, of past anomalies which are now no longer in that category. In branches of chemistry other than kinetics it is easy to find such illustrations. For example, we have the ancient, no longer anomalous, inversion of pairs of elements in the atomic table; in kinetics there was the confusion of order and molecularity, of concentration and effective concentration. Broadly, such anomalies have been solved. The more detailed ones remain. This situation is particularly true in the field of mechanisms. I recall a statement by Professor 0. K. Rice that the best that can be said for a mechanism is that neither it nor the ones yet unconceived are necessarily wrong. We see this fact illustrated in such a simple case as mercuryphotosensitized hydrogenation. The first explanation was on the basis merely of energy relationships, the second considered also the Franck-Condon principle, the third takes into account the total potential energy surfaces. In surface catalysis we have become so accustomed to anomalies that we hardly regard them as such. Each class of reactions may have its own special theory, even when various reactions of one simple substance on one catalyst may be involved. In kinetics, and particularly in reaction mechanisms, me proceed from one anomaly to another. As one is clarified, another appears, and there is apparently no final solution. It was the intention of the organizers of this Symposium that it should have a twofold function: namely, to smooth some ruffled waters where storms of controversy have served merely to obliterate underlying truth, and to create
* Presented before the Symposium on Anomalies in Reaction Kinetics which was held under the auspices of the Division of Physical and Inorganic Chemistry and the hfinneapolis Section of the American Chemical Society at the University of Minnesota, June 1921, 1950. 763
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disturbance, if possible, where there has been complacency regarding a seemingly generally satisfactory situation. Thus, we were essentially to eliminate some overworked fields of research and to provoke some new ones. How well we succeed in this purpose mill depend on the authors of the various papers and the participants in the discussion. We propose, if possible, to reconcile some old antagonists and to estrange some old friends. In the first session we have papers which we hope ‘will be troublesome for those who have a major interest in the theory of the activated complex. Unfortunately, Professor Prigogine himself could not be here, but we can expect that his views will be well presented by Professor Hulburt and well debated by the other participants. This work is placed early in the program because of our reasonable expectation that it may have considerable influence on the development of the later discussions. Professor Kilpatrick has assembled for the second session some provocative papers on medium effects. I n order to eliminate, so far as possible, any arguments about the facts, he had proposed to circulate these papers to the participants well in advance of this meeting. I hope that the speakers have had time to examine each other’s efforts. If so, arguments should be on interpretation. The presence on the program both of those who accept and of those who reject the applicability of the Debye-Huckel theory and the Br0nsted theory of salt effects to the kinetics of solutions may serve to settle some controversial points. Although neither Professor Wynne-Jones nor his collaborators can be here, we are fortunate that Dr. Liebhafsky is here to present his work. Professor Olson has recently presented evidence that concentration and character of particular ions are the significant factors rather than ionic strength. Perhaps, in the course of the discussion he may have opportunity to comment on other papers from his point of view. For the two sessions on free radicals Professor 0. K. Rice has succeeded in gathering a number of people who have very definite opinions on such matters as the steric factor and the energy of activation of free-radical reactions. Perhaps the divergent views of Dr. Steacie and Dr. Sewarc will be reconciled; if not, they will certainly be exposed. It is pleasant also to have on the program a paper by Professors F. 0. Rice and K. F. Herefeld, who proposed the theory which started all the discussion. One phase of dispute has revolved around the question of the actual contribution of free-radical chains in some of the most important reactions studied and of the nature of the chain-continuing and chain-terminating steps. The papers of these two sessions will clarify many questions-and perhaps raise others. On the last day of the meeting we have the papers on some surface reactions. All the reactions to be discussed are presumably simple, and many of them are taught as well understood. Perhaps Dr. Farkas had in mind the idea in their selection that the discussion regarding reactions of hydrogen on metallic and catalytic surfaces might solve some open questions. More probably, however, he wished to emphasize that even the simplest aspects of such reactions require not only more detailed study but also more precise interpretation than has been their lot in the past.
