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Speciation and Thermodynamics of Sulfur Vapor 119 Table 1 Reaction enthalpies DH (kJ mol–1) for the equilibrium reactions cyclo-S8Ð8/n Sn in the gas-phase at selected temperatures according to various experimental investigations Temperature S8Ð4 S2 S8Ð8/3 S3 S8Ð2 S4 S8Ð8/5 S5 S8Ð4/3 S6 S8Ð8/7 S7 Reference 400 K 393 251 172 96 35 27 [28] 400 K 414 255 231 96 35 28 [30] 435–669 K 298 405€9 420 257€11 275 169€13 190 85€13 73 35€2 34 28€2 28 [31] [24] K 298 K (2nd law) 404 253 150 90 48 28 [32] 298 K (3rd law) 387 231 137 77 34 20 [32] cycloS8 Ð 8=nSn ðn1⁄427Þ ð1Þ The presence of even larger molecules such as S9 and S10 in equilibrium sulfur vapor has been suspected [20] but the mass spectroscopic evidence is weak. Sulfur atoms can be expected in sulfur vapor only at extremely high temperatures (>2000 K) owing to their high enthalpy of formation of 277 kJ mol1 [29]. Many authors have tried to derive the enthalpies of the gas-phase reac- tions (Eq. 1) as well as the entropies S of the various Sn molecules for n=2– 8 from the temperature dependence of the mass and Raman spectra in com- bination with the total pressure of saturated and unsaturated sulfur vapor. These results are summarized in Table 1 and will be commented on in detail in the following sections. Most authors assumed that only one species is present for each molecular size; in other words, possible isomers were ne- glected. It has later been found out that this is an oversimplification (see be- low). 2.2 Mass Spectrometry The first reliable spectroscopic analysis of saturated sulfur vapor was pub- lished by Berkowitz and Marquart [28] who used a combination of a Knud- sen effusion cell with a mass spectrometer and generated the sulfur vapor by evaporating either elemental sulfur (low temperature region) or certain met- al sulfides such as HgS which decompose at high temperatures to sulfur and metal vapor. These authors observed ions for all molecules from S2 to S8 and even weak signals for S9+ and S10+. From the temperature dependence of the ion intensities the reaction enthalpies for the various equilibria (1) were de- rived (see Table 1). Berkowitz and Marquart carefully analyzed their data to minimize the influence of fragmentation processes in the ion source of the spectrometer. They also calculated the total pressure of sulfur vapor from their data and compared the results with the vapor pressure measurements by Braune et al. [26]. The agreement is quite satisfactory but it probablyPDF Image | Topics in Current Chemistry
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