20 U\ni\TI<>\ lUOLOGY 



ill homogeneous, coruhMised phases. Steps of this type have very low 

 heats of activation and steric faetors of the ordcf of magnitude of unity; 

 accordingly, practically every collision results in reaction. Examples of 

 this type are the combination of atoms or small radicals, the quenching of 

 the fluorescem-e of dyes by efficient (juenchers, and enzymatic reactions 

 at low concentrations of the substrate. These reactions occur at every 

 encounter of the reactant molecules. The frequency of encounters, 

 unlike that of collisions, is determined by the rate of diffusion, which in 

 turn is dependent on the viscosity of the solution. 



Under the normal conditions in which gas-phase reactions are com- 

 monly studied, recombination of atoms frequently is a wall-catalyzed 

 process, whose rate is fixed by the rate of diffusion of the atoms to the wall 

 of the vessel. The rate of atomic recombination 2 A = A2, occurring 

 both by three-body collisions and by wall catalysis, can be represented by 

 an equation of the following form (Kassel, 1932, pp. 170-180): 



^ = kPcW + ^^ [A]. 



The factors Pc and P« are linear functions of the partial pressures of the 

 components of the gas. The coefficient k' is influenced by the geometry 

 of the vessel, becoming greater as the surface-to-volume ratio of the 

 vessel increases. In a photochemical steady state the homogeneous 

 recombination (whose rate is proportional to the square of the concentra- 

 tion of atoms) is favored by an increase in the intensity of the absorbed 

 light. 



If atoms or radicals are formed in a liquid-phase photochemical reac- 

 tion, the observed quantum yield is likely to be influenced by the rate of 

 stirring of the solution since, in an unstirred solution, the steady-state con- 

 centration of atoms will, in general, be spatially nonuniform. This effect 

 is especially important if a large fraction of the actinic light is absorbed 

 in a thin film near the window through which the light enters. 



MECHANISM OF COMPLEX REACTION 



GENERAL PROBLEM 



Few, if any, chemical reactions are kinetically simple in the sense that 

 they involve only one reaction step which is of simple order and which 

 is identical with the stoichiometric reaction. The observable course of a 

 photochemical reaction is the result of the simultaneous occurrence of a 

 number of reaction steps. A set of reaction steps, which is consistent 

 with the stoichiometry and kinetics of a reaction, constitutes the mecha- 

 nism of the reaction. The kinetic equation for the over-all reaction can be 

 obtained by combining the rate equations for the several steps in such a 

 way that the concentrations of the reaction intermediates are eliminated. 



