The Photochemical Reaction Limit

It foUows from Eq. (27.2) that the photochemical reaction rate becomes zero, i.e. the reaction terminates when all the hydrogen or bromine are consumed, i.e. after one of the reactants is completely converted to the product HBr. This reaction limit corresponds to the equilibrium Hg + Brg 2 HBr shifted to the HBr side owing to exothermic character of the reaction H2 + Brg 2 HBr. 

This coincidence of the reaction limit with its equilibrium is not always observed. Usually, the reaction limit does not coincide with equilibrium when the action of light on the forward and reverse reactions is different. 

Taking this into account, the process HBr + hv - H -j- Br must be included into the above HBr photosynthesis mechanism, allowing also for the process Br + HBr Brg + H. Denoting the amounts of light absorbed by bromine and hydrogen bromide as AI and AI2, respectively, the stationary reaction rate is 

Equating the reaction rate to zero, we find for a stoichiometric mixture [Hg] = [Brg] and [Hale, = [BiaJ  

This expression defines the ratio of the reactant (Brg) to the product (HBr) concentrations when the reaction stops, i.e. the extent of conversion in a photochemical reaction. 

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