A STUDY OF THE UV-VISIBLE ABSORPTION-SPECTRA OF BR-2 AND BRCL

A mathematical representation of the UV-visible absorption spectrum of Br-2 vapour, which is based on a four-band semi-logarithmic gaussian distribution function, was derived in an assessment of the literature spectra [GRAPHICS] where tanh=tanh(hcx325.3213 cm(-1)/2kT) and lambda is the wavelength in vacuum (200 less than or equal to lambda less than or equal to 650 nm). The UV-visible absorption spectrum of BrCl (200 less than or equal to lambda less than or equal to 600 nm) and the equilibrium coefficient for the reaction (Br-2)(g) + (Cl-2)(g) <-- K-BrCl --> 2(BrCl)(g) were obtained by analysis of the absorption spectra of gaseous mixtures of Cl-2 and Br-2, which were investigated at 298 K with a spectral resolution of 0.2 nm. The absorption cross-sections of BrCl are in good agreement with those reported by other workers and can be adequately described by a three-band semi-logarithmic gaussian distribution function [GRAPHICS] where tanh=tanh(hcx443.1 cm(-1)/2kT) and lambda is the wavelength in vacuum (200 less than or equal to lambda less than or equal to 600 nm). In contrast with the spectrum of Br-2 vapour, no evidence was found for a fourth band in the spectrum of BrCl in the wavelength range studied. The value obtained for the equilibrium coefficient K-BrCl(298 K)=[BrCl](2)/([Br-2][Cl-2])=10.1+/-1.1 is the largest reported so far. This yields a standard enthalpy for the formation of BrCl of Delta H(BrCl)degrees(298 K)=14307 J mol(-1).