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III. An Unsolved Problem in the Application of the Quantum 

 Theory to Chemical Reactions. By W. C. M. Lewis,. 

 Professor of Physical Chemistry, University of Liverpool * . 



IN a series of papers, entitled " Studies in Catalysis," 

 published during the last few years in the Journ. (Jhem. 

 Soc, I have attempted to apply the quantum theory to the 

 problem of chemical kinetics, the fundamental concept being 

 that the thermal radiation in equilibrium with the material 

 system is the source of the energy required to bring about 

 chemical change, and further, that the rate at which a 

 reaction proceeds depends directly upon the density of the 

 radiation of the absorbable type or frequency. By making 

 use of Planck's expression for radiation density, a number 

 of results have been obtained which are in good agreement 

 with experiment. Notably one obtains on this basis an 

 expression for the effect of temperature upon the velocity 

 constant of a reaction which is in o-ood agreement with the 

 empirical equation of Arrhenius, and is also in agreement 

 with the statistical equation of Marcelin and Rice, namely 



^log^T = E/RT 2 V 



where k is the velocity constant, and E is the critical 

 increment reckoned per grammolecuie of the decomposing 

 substance. E represents the additional energy which must 

 be given to a molecule (by the absorption of radiation) in 

 order to render the molecule reactive, for, as finite velocities 

 show, molecules possessing the average amount of internal 

 energy are not chemically reactive. Similar conclusions 

 have been reached independently by Perrin (cf. 'Atoms ' ; also 

 "Matiere et Lumiere," Annates ole Physique [xi.] xi. 1919). 



Further, it has been found possible to calculate the velocity 

 constant of a bimolecular reaction in the gaseous state in 

 absolute measure, on the above assumptions, viz. (1) that 

 radiation of suitable frequency must be absorbed in order to 

 activate the molecule chemically, (2) that one quantum of 

 such radiation is required per molecule (Einstein's law), and 

 (3) that when a collision occurs between two molecules thus 

 activated then and only then does the bimolecular reaction 

 take place (cf. Lewis, Trans. Chem. Soc. cxiii. p. 471, 1918). 

 It is found that numbers thus calculated for the velocity 

 constant are in good agreement with those found by 

 experiment. 



Having attained a certain measure of success in the case 



* Communicated by the Author. 



