The photosynthesis of hydrogen chloride. I.—A new experimental method; the inhibiting effect of hydrogen chloride

Abstract

In spite of the many valuable investigations which have been carried out in recent years on the photochemical reaction between hydrogen and chlorine, no general agreement can be said to exist regarding the complete mechanism of the process. Examination of the various semi-empirical formulæ, which have been proposed to represent the rate of reaction, shows that the relative importance of all the experimental factors cannot have been fully realized; and while the divergence in the results of different workers has frequently been ascribed to the effects of small amounts of oxygen or other impurities in the gases used, or to various chain-breaking processes associated with the surfaces of the reaction vessels, yet a complete scheme has by no means been evolved to co-ordinate the results of all the experimental investigations. There are indeed major discrepancies in the published data of different workers which must be satisfactorily cleared up before such an attempt is made. For example, there is the question as to what extent the rate of the formation depends on the amount of light absorbed. Thus the reaction rate in oxygen-free systems has been given by Kornfeld and Müller as directly proportional to the light intensity over a sixty-fold variation, a conclusion supported also by Marshall for gas mixtures of low total pressures (less than 10 mm. Hg) and by Bodenstein and Unger for higher pressures; on the other hand, the results of Chapman and Gibbs indicate that the reaction rate is proportional to a power of the absorbed light which is definitely less than unity, the conclusion being that the limiting value of 0⋅5 would be reached in gas mixtures entirely free from oxygen. The role of hydrogen has also been the subject of considerable discussion. The data of Chapman and Underhill indicate a slight but distinct inhibiting effect at high hydrogen concentrations when a small but definite amount of oxygen was also present. Such an inhibiting effect was confirmed by M. C. C. Chapman, but is in disagreement with the findings of Bodenstein and Dux, the discrepancy being ascribed by Thon to the unsuspected presence of oxygen in the hydrogen used. A similar state of uncertainty exists about the reaction kinetics in mixtures containing larger percentages of oxygen, various formulæ having been proposed from time to time by different workers. In addition no scheme of reaction which does not admit of the calculation of the absolute quantum efficiencies under all conditions can be said to be complete. The difficulties attendant on previous methods of investigation have, however, prevented any fruitful attempt being made in this direction.