A spectrophotometric study of the equilibrium formation of the gaseous species bismuth (I) tetrachloroaluminate and tellurium (II) chloride

A spectrophotometirc study of the vapor species present at equilibrium in high temperature, metal-metal salt systems was performed in an effort to characterize the nature of the species present and evaluate the thermodynamic functions which describe their interaction. A Cary 14 recording spectrophotometer was modified to permit the investigation of the high temperature systems. In addition to the characteristic BICL$sub 3$ and BiCl bands, the absorption spectra of Bi-BiCl$sub 3$-AlCl$sub 3$ systems exhibited a band at 723 m /lower mu/ which corresponded to BiAlCl$sub 4$. A quantitive study of the system over the temperature range 650 to 900 degrees yielded a value of -28.1 +- 0.5 Kcal for /cap delta/H$sup o$$sub 1050$ for the reaction BiCl(g) + AlCl $sub 3$ (g) = BiAlCL$sub 4$(g). Using this enthalpy and an estimated entropy, a value of 16.8 Kcal was computed for /cap delta/F$sup o$$sub 1050%. Systematic deviations in the data at low temperature (<650 degrees) were ascribed to the formation of compounds in the melt and the resultant decrease in the activities of the components in the system. A series of previously unreported bands in the near infra-red spectrum (~ 1500 m/lower mu/) was assigned to BCl(g) although the exact nature of the transitions was unknown. Two broad bands (510 and 670 m/lower mu/) observed in the spectra of the Te-TeCl4 system were found to correspond to TeCl2(g). Quantitative studies of the data in the temperature range 275 to 350 $sup o$ yielded a value of 23.5 +- 2 Kcal for /cap delta/H $sup degree$$sub585$ for the reaction Te(s) + TeCl $sub 4$ (g) = 2 TeCl$sub 2$ (g). The value 38.7 +- 4 eu computed for /cap delta/S$sup o$ using this entalpy and K$sub p585$ (0.49 atm) corresponds quite well with a value of 44.7 eu estimated from data available in the literature. This equilibrium accounts for the low temperature transport of tellurium which has been observed in the Te-TeCl$sub 4$ systems. As a result of the extensive compound formation in Te-TeCl$sub 4$-AlCl$sub 3$ melts, quantitative studies of the vapor species above such systems are not possible. (author)