Photochemical equilibrium of hydrogen bromide exposed to light in the extreme ultraviolet: also submitted for the Gunning Victoria Jubilee Prize in Chemistry

Abstract

The photochemistry of the formation and decomposition of hydrogen bromide has now been fairly fully investigated. Thus we have the work of Bodenstein and Lütkemeyer on the mechanism of the formation of this substance under the action of light in the visible spectrum, together with the same authort ' researches on the same action taking place thermally in the dark. Also there exists the work of Warburg, who showed that the decomposition or hydrogen bromide by ultraviolet light followed Einstein's law or photo chemical equivalence. Along with these researches on the separate reactions of formation and decomposition of the hydrogen bromide, we have the work of Coehn and Stuckardt investigating the final condition of equilibrium reached when either hydrogen bromide or its constituent elements are exposed to the action of light of different wave-lengths.The research just described is an extension of that of Coehn and Stuckardt. The facts learned from it may be summarised thus:-(1) Hydrogen and bromine combine to a small extent when exposed to the light from the aluminium spark, which radiates strongly at λ=185μμ. At equilibrium under these conditions, the partial pressure of the hydrogen bromide is about 1.4 to 1.8 per cent. that of the bromine.(2) The absorption coefficient of bromine increases slowly with decreasing wave-length for wave-lengths shorter than 254μμ, but the gas is still very transparent even at 185μμ.(3) Theoretical considerations, especially the analogy with Coehn and Stuckardt's results obtained with hydrogen iodide, leads one to anticipate that a small amount of formation of hydrogen bromide from its constituent elements must take place under the influence of radiation of wave-length l85μμ. Also a consideration of the values of the absorption coefficients of bromine and hydrogen bromide for light of this wave-length suggests that at equilibrium the amount of hydrogen bromide formed would be about that actually found by experiment.