PHYSICAL CHEMISTRY 191 



The frequency of the theoretical absorption line is exactly five 

 times as great as the most intense absorption line of NgOg. 



Blue light, 400 iMfx to 460 /*//-, accelerates the decomposition 

 of N2O5, but only in the presence of N2O4, which acts as an 

 autocatalyst. Since the adsorption lines of N2O4 and NgOg 

 overlap in the infra-red, the authors suggest that N2O4 adsorbs 

 blue light, and emits light in the infra-red region, which causes 

 the photochemical decomposition of N2O5. Baly and Barker 

 {Trans. Chem. Soc, 1921, 119, 662), however, consider that the 

 absorption by the pentoxide of energy, radiated by the peroxide, 

 cannot take place on the quantum theory, unless the frequencies 

 possessed by the two are exactly equal. 



The photochemical decomposition of nitric acid (Reynolds 

 and Taylor, Trans. Chem. Soc, 191 2, 101, 131), which only 

 occurs in the gaseous phase, is inhibited by the presence of 

 concentrated sulphuric acid. The autocatalyst, nitrogen 

 peroxide, is in this case removed from the system as soon as 

 it is formed. 



Tolman {J.A.C.S., 1921, 43, 269) points out that Dushman's 

 equation for the velocity constant of a monomolecular reaction, 



Qa "Rt 



u; ^ ^h 



or (2) log ^= 10-0203 + log Qa ^ 



4-57! 



(where Q^ denotes the heat of activation, N and R possess the 

 usual significance, and h is the quantum constant) is the only 

 equation which fits the results for the rate of decomposition of 

 nitrogen pentoxide. This equation has been derived from 



which represents successfully the dependence of reaction velocity 

 on temperature. Dushman, in agreement with Lewis and 

 Perrin, assumes that Q=N/w. 



He also assumes that s, which possesses the dimensions of a 

 frequency, is the same as v already introduced. This equation 

 (Dushman, J.A.C.S., 1921, 43, 397) gives values for Qa in 

 substantial agreement with the experimental results. It has 

 been applied, not only to the decomposition of N2O5 and PH3, 

 but also to those chemical reactions where one of the opposing 

 reactions is monomolecular. Thus for reactions of the type 



A, '^ 2 A 

 and A.B^AH-B 



the velocity of the monomolecular reaction is calculated from 



