22 RADIATION HIOLOGY 



thermic (A// = 34 kcal). Tlu* .socoikI, l)oiiit>; an cxijlhc'iiuic reaction 

 hetwocn an atom and a diatomic molecule, .sliould have a .small heal of 

 activation and a steric factor not dirt'eriiig {greatly from unity. The two 

 possible alternative reactions that a hydrogen atom can undergo, 



M + H + I-^ III + M 



and 



M + 11 + II -^ U, + M, 



can occur only by three-body collisions with any molecule, M, or by 

 diffusion to the wall. The relativ^e importance of these atomic combina- 

 tions is further reduced by the fact that the steady-state concentrations of 

 the atoms must be much smaller than the concentration of the reactant, 

 hydrogen iodide. Since the hydrogen atom concentration is kept very 

 small by its efficient reaction with hydrogen iodide, the only reaction 

 which iodine atoms can enter into, appreciably, is association to form 

 molecular iodine. Accordinglj^, the mechanism for the reaction may be 

 written as follows: 



(1) HI + /;i^-^ H + T* (primary step), 



(2) H + HI ^ Ho + I I 



(3) M + 21 -^ lo + M ) 



(secondary steps). 



Expressing the "intensit.y " lahs of the absorbed light in the photochemical 

 units of einsteins per liter per second and the rates of the chemical reac- 

 tions in moles per liter per second, the rate equations for the three steps of 

 the mechanism may be written 



Ih = labs, 



Vo = k.j[Hl][Ul 



ih = k,Pc[l]-. 



The rate of decomposition of hydrogen iodide is the sum of the rates of 

 steps (1) and (2), 



- '-^ =^ V, + Vo = La,. + A-.[HIlfin. 

 Introducing the steady-state assumption 



then 



dt -^' 



lab. = A-2[HI][H] 

 may be written. Therefore 



dt ~ *'"''• 



