The chlorine-oxygen-hydrogen system

Weigert discovered that water is formed if a mixture of CI2, O2, and is irradiated. Sometime later, Norrish and Rideal did experiments at 700 torr total pressure with [H2 ]/ [O2 ] between 0.25 and 4 and found that water formation obeyed the rate law 

The rate was independent of [H2] for mixtures containing 16-90 mole % hydrogen. They concluded that photochemically activated CI2 molecules can transfer their excitation energy to O2 molecules, which then react with H2 to form water. 

Chapman and MacMahon found that small amounts of O2 inhibited the photosynthesis of HCl from H2 and CI2, in agreement with a much earlier observation by Bunsen and Roscoe . The inhibiting effect of O2 was found by Boden-stein and Dux to make the rate of HCl production inversely proportional to [O2]. These facts were confirmed by Chapman who found that HCl production followed the rate law 

Two expressions somewhat different from eqn. (v) were proposed, respectively, by Thon and by Cremer . Thon s was 

He also reported that no reaction occurred unless radiation of sufficient energy to dissociate CI2 is used, concluding that the active initiating species is undoubtedly the Cl atom and not excited CI2. In contrast to the findings of Chapman and Chapman and Underhill , no evidence was found for the retarding effect of excess Hj. 

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