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.