PHOTOCHEMISTRY 271 



such conditions the reaction rate increases for a time whereas in other 

 chemical reactions the rate decreases. No generaUzations can be made 

 concerning this induction period. It is comparatively rare and in each 

 case the catalyst is specific. Examples will be given later. Sometimes 

 the catalyst is produced by the photochemical reaction, sometimes it is 

 produced by a slow thermal reaction, and in still other cases an inhibitor 

 is present and this inhibitor is destroyed by the photochemical reaction. 

 After destruction of an inhibitor the rate of reaction increases. 



After-effects. — Not only is a time lag observed at the beginning of a 

 reaction in some cases, but reactions are also known in which there is a 

 time lag when the light is turned off. When the reaction proceeds in 

 the dark after illumination, the phenomenon is known as an after-effect. 



INFLUENCE OF VARIOUS FACTORS ON PHOTOCHEMICAL REACTIONS 



Temperature. — Temperature has a very marked effect on ordinary 

 chemical reactions as explained in a preceding section. It is of such a 

 nature that most chemical reactions which proceed with measurable 

 velocity at room temperature double or triple their rate for a rise of 10°C. 

 in temperature. Photochemical reactions, on the othet hand, are usually 

 rather insensitive to temperature changes. It is to be expected that a 

 purely primary photo process would be largely independent of the tem- 

 perature. The reason that ordinary chemical reactions have large tem- 

 perature coefficients lies in the fact that activation is produced by 

 molecules of very high energy content and the number of these highly 

 activated molecules increases greatly with the temperature. In photo- 

 chemical reactions the excitation comes from absorption of light from an 

 outside source and the intensity of radiation in the system is quite 

 independent of the temperature of the absorbing system. However, 

 the primary photo process is very frequently followed by other processes 

 which do depend on the temperature of the reacting system. For exam- 

 ple, in a chain reaction the chemical reaction which follows the primary 

 photochemical reaction is subject to the same factors which influence 

 any ordinary chemical reaction. Thus it is found that many photochemi- 

 cal chain reactions do have large temperature coefficients approaching 

 those of ordinary thermal reactions. Again, the photochemical reaction 

 may be influenced by collisions; and collisions, in turn, are influenced 

 by a change in temperature. Sometimes the absorption of light by the 

 reacting system is changed by a change in temperature and this absorp- 

 tion may affect the photochemical reaction. Such cases are not very 

 common. Experimentally it is found that a large number of photo- 

 chemical reactions change in velocity about 10 per cent for a 10° rise in 

 temperature. At one time it was thought that most photochemical 

 reactions could be grouped into three classes with temperature coefficients 

 of 1.00, 1.2, and 1.4, but such a classification is now believed to be without 

 significance (13). In general, one may say that if the temperature coeffi- 



