394 TRANSACTIONS OP SECTION B. 



3. Discussion on Homogeneous Catalysis. 



Opened by Prof. W. C. McC. Lewis {University of Liverpool). 



[Ordered by the General Committee to be printed in extenso.'] 



Prof. Lewis.— In a reaction between two different molecules A and B the usual 

 kinetic method of representing the rate at which this can take place as being proportional 

 to the product of the concentrations of A and B rests, as is well known, simply on the 

 probability of the A and B molecules coming into contact with one another. It has 

 long been recognised, however, that the reactivity depends on other factors as well. 

 Thus, if it were simply a question of probability of collision, one might expect the 

 velocity to be independent of the specific nature of the reaction, being simply deter- 

 mined by the temperature, the mean free path, and the speed of the molecules. 

 Owing, however, to the highly specific nature of the reaction-rate as well as the 

 remarkable effect brought about by a change in temperature, a considerable number 

 of hypotheses generally based upon the formation of intermediate substances have 

 been suggested to account for the mechanism of a given process. There can be no 

 doubt that several of these suggestions are of fundamental importance as regards the 

 molecular mechanism involved. They mainly restrict themselves, however, to 

 material molecular changes and do not attempt to attack the problem of the cause of 

 such changes. In other words, whilst postulating a molecular mechanism which 

 takes account more or less of experimental facts, they give us little or no information 

 respecting the nature, magnitude, and source of the energy exchange, which must 

 accompany or rather precede molecular change. In the same connection we have the 

 whole question of the relation of concentration to active mass which has been studied 

 more particularly in America and which has shown fairly conclusively that concen- 

 tration per se is not an accurate measure of activity except in the limit. 



Of the few attempts made to deal with the subject of the rate of a reaction from 

 the standpoint of energy exchange as a necessary accompaniment of molecular 

 change perhaps the simplest and most direct is that of Marcelin as modified by Rice. 

 Marcelin's idea is that a molecule in the mean or average state as regards internal 

 energy characteristic of the system under the given conditions is not capable of 

 reacting in a chemical sense, and that it only becomes reactive when its internal 

 energy rises to or beyond a certain critical value. The energy difference between the 

 mean state and the critical may be conveniently termed the ' critical increment.' 

 Marcelin makes no assumptions as to the mechanism whereby this change in energy 

 content may take place. His view has, therefore, the advantage of generality in not 

 being restricted to any particular type of molecule, atom, or ion. On the basis of the 

 idea of the critical increment it has been shown by Marcelin, and more completely by 

 Eice from the standpoint of statistical mechanics, that the effect of temperature on 

 the velocity constant is given by the expression 



d log 7i; _ E 

 ~dT ET'' 



where E is an energy term approximately identical with the critical increment 

 reckoned per gram molecule. This equation is obviously of the Arrhenius type, 

 which has been shown by experiment to be ajDplicable to a large number of reactions 

 of very dissimilar chemical kind. In view of the fact that the Marcelin-Rice equation 

 is in good agreement with experiment, one is prompted to inquire somewhat further 

 into the physical mechanism which underlies the concept of critical energy and 

 critical increment. Two distinct questions arise : first, what is the nature of the 

 energy which is communicated to the molecule, thereby raising its energy content 

 from the mean to the critical state ? and, secondly, what internal changes occur in 

 the molecule during this absorption of energy ? The second question is obviously 

 the more complicated of the two, involving, as it does, a knowledge of the structure 

 of the molecule. It is not proposed to attempt to deal with this question. The first, 

 however, regarding the form of the energy which is communicated to the molecule 

 appears to be somewhat simpler. The writer would venture to suggest that the 

 energy thus supplied exists in the form of infra-red thermal radiation which is 

 necessarily present throughout the system in virtue of its temperature. 



The idea that infra-red radiation may cause chemical reaction is not new. It 

 seems to have been first suggested by Trautz in 1900, mainly from analogy with 



