Composition and thermochemistry of silver bromide vapor

The mole fractions of AgBr and Ag3Br3 in the saturated vapor at 840 K have been evaluated from the vapor mass spectrum, by comparison with the corresponding spectrum of AgCl vapor, where the monomer/trimer ratio is known accurately from vapor molecular weight measurements. Combination of these results with new measurements of the vapor pressure of molten AgBr by the torsion-effusion method in the range 805936 K yielded the third law enthalpies of vaporization and the standard enthalpies of formation Delta(f)H degrees(298)-(AgBr, g) = 27.8 +/- 0.3 kcal mol(-1) and Delta(f)H degrees(298)(Ag3Br3, g) = -19.0 +/- 1 kcal mol(-1). The dissociation energy, D degrees(0)(AgBr), is found to be 66.4 +/- 0.3 kcal mol(-1), or 2.88 +/- 0.01 eV, some 3.5-5 kcal mol(-1) lower than previous literature values. Approximate thermochemical stabilities of the dimer species Ag2Cl2 and Ag2Br2 have also been evaluated.