Formation energy sulfides

Figure 931 Ellingham diagram showing the variation of the standard free energy of formation of sulfides as a fnnction of temperature. The values refer to the reaction of one mole of S2 in the gaseous state. In order to calculate the curves, data in ref. [1] were used. The boiling...

The high-temperature reaction (9.76) between a metal and gaseous sulfur is called sulfidation. It produces solid or liquid sulfides. The free energy of formation of sulfides is inferior to that of oxides of same stoichiometry (Table 9.35). 

Table 9.35 Standard free energies of formation of sulfides and oxides atlOOO K[l].

Furthermore, the S2p binding energy (162.4 0.1 eV) indicates the formation of sulfides, in agreement with the model proposed by Martin et al. [17]. 

These reactions show sulfur in the role of an oxidizing agent. The properties of compounds such as ZnS suggest they contain the sulfide ion, S-2. The formation of this ion again can be expected on the basis of the fact that the neutral sulfur atom has two electrons less than enough to fill the valence orbitals. Acquisition of two electrons completely fills the low energy valence orbitals and solid ionic compounds can be formed. 

Dissimilatory sulfate reducers such as Desul-fovibrio derive their energy from the anaerobic oxidation of organic compounds such as lactic acid and acetic acid. Sulfate is reduced and large amounts of hydrogen sulfide are generated in this process. The black sediments of aquatic habitats that smell of sulfide are due to the activities of these bacteria. The black coloration is caused by the formation of metal sulfides, primarily iron sulfide. These bacteria are especially important in marine habitats because of the high concentrations of sulfate that exists there. 

Hallberg, R. O. (1972). Sedimentary sulfide mineral formation - an energy circuit system approach. Mineral. Deposit. 7,189-201. 

The copper remaining in lead after the above operation is removed by matte formation. Finely divided sulfur is added to molten lead at temperatures slightly above its melting point, and the melt is stirred continuously. Copper sulfide forms and floats on the surface, leaving the bullion substantially free of copper (less than 0.005%). The standard free energies of formation of cuprous sulfide and lead sulfide are about the same the observed separation must, therefore, be due to kinetic factors or to the influence of certain minor impurities that are present in the lead. 

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