PHOTOCHEMISTRY 23 



The (luaiitum yield ^ expressed in terms of molecules of hydrogen iodide 

 decomposed is 



d[lil]/d( , 

 (f = J = 2. 



^ abs 



This is in excellent agreement with the experimentally determined quan- 

 tum yield of 2.00, the value which has been observed over a wide range of 

 conditions, including pressures down to 0.008 mm Hg. There can be no 

 reasonable doubt of the correctness of this mechanism. 



The thermal formation of hydrogen bromide was perhaps the first reac- 

 tion to which the steady-state approximation method was applied (Chris- 

 tiansen, 1919; Herzfeld, 1919; Polanyi, 1920). At temperatures in the 

 range from 150° to 200°C, a photochemical formation of hydrogen bro- 

 mide can be observed which is relatively free from disturbance by the ther- 

 mal reaction. The quantum yield of this reaction is the following func- 

 tion of concentrations and intensity (Bodenstein and Lutkemeyer, 1924): 



dlHBiydt Av[H2] 



I'^s rH K I [HBr] 



Absorption of blue or near-ultraviolet light leads to optical dissociation of 

 bromine. The primary step of this reaction is therefore 



(1) Br. -\- hu^ 2Br. 



The bromine atoms might be expected to react with either molecular 

 hydrogen or hydrogen bromide: 



(2) Br + H. ^ HBr + H {AH = 16.7 kcal), 

 (2') Br + HBr ^ Br + H {AH ^ 40.6 kcal). 



Both these reactions are endothermic, with the heats of reaction indicated. 

 Since the steric factors for these two reactions are very probably of the 

 same order of magnitude, the rate of the second reaction should be smaller 

 than that of the first bv a factor of about 



g— 40,600/2r 

 g— 16,200/2r 



g— 24,400/2r 



In the temperature range under consideration this is a very small number, 

 and the second reaction may be justifiably dropped from consideration. 

 The hydrogen atoms, formed in step (2), can undergo similar reactions, 



(3) H + Ih:, -^ HBr + Br 

 and 



(4) 11 + HHr^ U,-\- Br, 



