t 671 ] 



LXXVII. Note on the Theory of the Velocity of Chemical 

 Reaction. By F. A.' Lindbmann *. 



SOME years ago Professor W. C. McC. Lewis put forward 

 mi interesting theory in which he endeavoured to 

 prove that the velocity of a chemical reaction was deter- 

 mined by the energy density of radiation of a certain 

 frequency. This theory, it' true, would be of such funda- 

 mental significance in all future work on the mechanism of 

 chemical combination, that it requires most careful con- 

 sideration ; and it may therefore be worth while to call 

 attention to a difficulty which appears to be fatal to the 

 whole theory. 



According to Arrhenius's well-known empirical relation, 

 the temperature coefficient of the reaction velocity may be 

 written A/T 2 , A being a constant and T the absolute tempera- 



~dv 



ture. If v is the reaction velocity therefore, l/u^, =A/T 2 



AdT ® 



or dlogv— r|12 , whence i< = Be~ A/T . It is clear, therefore, 



that any theory on which v is proportional to e~ A,T will 

 agree with the observed facts. Marcelin and Rice assume 

 this to be the case because only molecules whose kinetie 



Alt 



energy is greater than -rr react. The number of these 



is proportional to e~ A;T , so that Arrhenius's relation is 

 satisfied. Prof. Lewis assumes that the reaction velocity 

 is proportional to the radiation density 







Sirhv z 



1 





.J. ~ 



u v 



= _ 7" 



hv 

 e k?_ 



5 

 -1 



te 



n 



8t^v 3 - 

 — s—e 



c z 



hv 



' AT = 



Be- 



hich may be written 



ft-fc™3 _ hv 



■AT 



if hv is large compared to &T, so that again the Arrhenius 

 relation is bound to be found. 



The only check obviously would be to establish some 

 relation between the value of v derived from the observed 

 value of A by Prof. Lewis and the optical properties of the 



* Communicated by the Author. 



