VII 

 PHOTOCHEMISTRY 



Farrington Daniels 

 Department of Chemistry , University of Wisconsin, Madison 



PART 1. THEORY 



Theory of chemical reaction. Activation of molecules. Types of radiation. Types 

 of spectra. Laws involved in photochemical reactions. Quantum theory. Photo- 

 chemical phenomena. Chemical kinetics. Photochemical kinetics. Influence of various 

 factors on photochemical reactions. Experimental technique. General procedure. 

 References. Part 1. 



Photochemical reactions are chemical reactions which are produced 

 directly, or indirectly, by the absorption of radiation. Qualitative 

 photochemistry has been studied for a very long time, but only within the 

 past decade or so has the quantitative study of photochemical reactions 

 been reduced to an exact basis. The theory of photochemistry also has 

 received important impetus within the last few years through advances 

 in the interpretation of spectra and in the new developments of chemical 

 kinetics. In order to understand photochemical reactions a brief survey 

 must be given first of our present theories of chemical reactivity. 



THEORY OF CHEMICAL REACTION 



It is now generally accepted that chemical reaction must be preceded 

 by an activation process. The mere existence of slow chemical reactions 

 supports this view. If activation were not necessary, it would be 

 expected that all molecules would be in similar states of reactivity and 

 that the reaction would be nearly instantaneous. It is true that many 

 instantaneous reactions are known, but these occur at very high tem- 

 peratures where there is a high activation or they occur between ions, 

 particularly in aqueous solutions, and these ions may be regarded as 

 atoms in a state of activation. 



The heat of a chemical reaction is not a measure of the energy required 

 to activate the molecules. The heat of a chemical reaction is equal to 

 the difference between the energy required to activate the forward reac- 

 tion and the energy evolved when the products are formed from the 

 activated reactants. This relationship is shown in Fig. 1. Activation 

 in ordinary chemical reactions is assumed to be caused by collisions 

 between molecules, and is thus due to heat. In a given number of 

 molecules at a definite temperature some of the molecules will be moving 

 rapidly and some slowly and the distribution of the velocities among the 



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