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86 Ralf Steudel 2.1.4 Rates of Formation and Decay of p-Sulfur Several authors have reported that the formation of Sp from S8 in liquid sul- fur is a slow reaction which takes at least 8 h at 120 C and at least 3 h at 150 C [28, 32]. The back reaction Sp!S8 is much faster requiring ca. 30 min at 120 C and ca. 2 h at 97–112 C (in the solid state) [32]. However, the back reaction is not complete: Even after 15 days at 100 C there are still traces of Sp present in the (solid) sample [21]. At 20 C Sp decomposes within two days to a mixture of S8 and Sm in an approximate 1:1 ratio [33]. In solution Sp is much more stable: On heating of S8+Sp in toluene to 100 C no conver- sion was observed within 20 h [20] while heating of such a sample in CS2 to 100 C resulted in a slow decrease of the Sp concentration within several days [21]. In the presence of traces of ammonia the reactions Sp!S8 and S1(or Sm)!S8 are fast and ammonia accelerates also the formation of Sp and S1 in sulfur melts [21, 34–36]. Therefore, investigations on these systems should be carried out in laboratories in which no ammonia (aqueous or gaseous) is stored. Illumination with visible light also accelerates the interconversion re- actions [36]. Assuming a first order reaction, Wiewiorowski et al. [28] calculated the Arrhenius activation energy for the formation of Sp in the temperature range of 130–150 C using the concentrations derived from the freezing point de- pression. An activation energy of 134 kJ mol1 was obtained. However, since these authors—as outlined above—used an outdated value for the enthalpy of melting of b-S8 their cryoscopically determined Sp concentrations are too low. If the concentrations corrected by the present author (see above) are used [18] the activation energy is obtained as 144 kJ mol1 which agrees fairly well with the activation energy of 150 kJ mol1 for the formation of free radicals by homolytic S-S bond dissociation in sulfur melts as deter- mined by ESR spectroscopy [37, 38] and magnetic measurements [39]. It should be pointed out however that in the above calculations the back reac- tion has been neglected despite the fact that it is much faster than the for- ward reaction (Sp formation). 2.1.5 Enthalpy of Formation of p-Sulfur From the corrected concentrations of Sp at various temperatures (see above) and taking the concentration of polymeric sulfur into account the equilibri- um constants Kc=c(Sp)/c(S8) can be calculated for the temperature range 115–150 C. The linear regression log Kc vs 1/T then yields the enthalpy of formation of Sp as +22 kJ mol1 [18].PDF Image | Topics in Current Chemistry
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