PHOTOCHEMISTRY 269 



results are not easily interpretable. This energy of activation is of great 

 importance in the theory of chemical kinetics. It represents the energy 

 which must be introduced into a gram molecule of the reactants in order 

 to give them sufficient activation so that the reaction may take place. 

 Combining the effect of temperature and concentration, the general 

 formula (16) is obtained: 



k = se-^^^'^ {16) 



This is a very useful equation which applies to most reactions and 

 expresses the specific reaction rate A; as a function of the temperature 

 and the energy of activation. The constant s has the physical signifi- 

 cance of collision frequency when the reaction is bimolecular. When 

 the reaction is unimolecular, the significance is not so clear, but it may 

 be connected with the oscillation of atoms within the molecule and this 

 in turn is related to the infra-red absorption spectrum. In most unimolec- 

 ular reactions s has a value in the neighborhood of 10"^^, and in many 

 reactions E/RT has a value in the neighborhood of 40. In gas-phase 

 bimolecular reactions the frequency of collision s can be calculated from 

 standard formulas of the kinetic theory of gases involving the molecular 

 diameter. The value of the energy of activation E is usually about 

 25,000 cal. for unimolecular reactions which proceed with a measurable 

 velocity at room temperature. Reactions having an energy of activa- 

 tion in the neighborhood of 50,000 cal. usually require a temperature 

 in the neighborhood of 400°C. in order to give a measurable reaction 

 rate. It must be emphasized that the range of measurable reactions 

 is somewhat limited. These statements are, of course, only rough 

 approximations. 



PHOTOCHEMICAL KINETICS 



Photochemical kinetics constitute a complicated and difficult field. 

 Not only are photochemical reactions subject to all the variations which 

 affect thermal reactions, but they involve also complications from the 

 absorption of radiation. In the study of photochemical reactions it is 

 desired to determine the primary photochemical process caused by the 

 absorption of the radiation. As explained before, this primary process is 

 very frequently obscured by secondary reactions. The photochemical 

 reaction may institute a series of successive reactions constituting a chain. 

 The primary reaction may be of the same nature as a reaction which is 

 proceeding in the dark but at a slower rate. Again, the photochemical 

 reaction may be reversed by a thermal process when the light is removed 

 or the velocity of the primary process may vary with the intensity of 

 the light absorbed. This in turn may change with the absorption and the 

 extent of the chemical reaction. AMien the absorption of light is com- 

 plete, the study of the photochemical kinetics is somewhat simplified. 

 At least the change in the amount of absorption is not a variable in the 



