Technical Papers and Notes - Institute of Metals Division - Determination of the Standard Free Energies of Formation of Zinc Sulfide and Magnesium Sulfide

FREE energies of formation of zinc sulfide and magnesium sulfide had previously been estimated to an accuracy of ± 5.0 kcal per g mol. In the present work, these values were determined experimentally. As the determinations of the free energies of formation of zinc sulfide and magnesium sulfide presented different problems, separate experimental procedures were employed for each. ZINC SULFIDE It is difficult to measure the free energy of formation of either zinc sulfide or magnesium sulfide by measuring their decomposition pressures directly, as they are very stable compounds. An attempt was made by Jellinek and zakowski3 to study the equilibrium. zns(s) + H2(g) = Zn(g) + H2S(g) [I] They passed hydrogen over zinc sulfide and analyzed the gases leaving the reaction zone for H2S. They were unable to determine the true partial pressures of the H2S by this method, as some of the H2S on being cooled from the reaction temperature, reacted with the zinc in the gas to produce ZnS. Consequently, they were unable to establish any true equilibrium data for reaction [I]. More recently,' the equilibria ZnS(sphaledte) + Fe (s) — FeS(s) + Zn(g) [2] and ZnS(sphalerite) + 2Cu(s) — Cu2S(S) + Zn (g) [3] were studied. As the standard free energy of formation of Cu2S is known to * 3.0 kcal, and of FeS to ± 1.0 kcal, it should be possible upon performing equilibrium studies on reactions [2] and [3] to deduce the standard free energy of formation of ZnS (sphalerite). However, as both of these equilibria involve 2 solid sulfides, and as it is possible for these sulfides to dissolve in one another, it is necessary first to ascertain the activities of the various sulfides in these solid phases.