This is a critical evaluation of the thermodynamic properties of the known solid and gaseous compounds of (germanium+sulfur): GeS(cr), GeS(g), GeS2(cr), GeS2(g), and Ge2S2(g). The heat capacity of GeS(cr) at low and moderate temperatures was evaluated from all the information available in the literature, and the properties: C°p,m(T), {H°m(T)-H°m(T')}, S°m(T), and F°m(T )=(D T 0 S°m-D T T' H°m/T ), where T'=298.15 K, were computed to T=930 K, close to the melting temperature, above which decomposition to a (germanium+sulfur) eutectic and uncombined germanium is believed to occur. On the basis of our recent value for DfH°m(GeS, cr, 298.15 K) (J. Chem. Thermodynamics 1994, 26, 727), DfH°m(T ) and DfG°m(T ) were also calculated over the same temperature range. A critical assessment of the enthalpy of sublimation DsubH°m yielded DfH°m(GeS, g, T ). In another part of the present series (J. Chem. Thermodynamics 1995, 27, 99), we determined DfH°m(GeS2, cr, 298.15 K); the corresponding DfH°m(GeS2, cr, T ) is tabulated in the present paper to T=1000 K. Ab initio molecular-orbital calculations showed the cyclic (C2v) arrangement to be the most stable for Ge2S2(g), and the predicted structure and vibrational wavenumbers were used in calculations of its thermodynamic properties as a function of temperature by means of statistical mechanics. A similar treatment of the linear GeS2(g) is described. The assessed DfH°m(GeS, g, T:0) is in harmony with our reinterpreted molar enthalpy of dissociation D°m(GeS) from spectroscopy; and the enthalpies of dissociation of the bonds in GeS2(g) and Ge2S2(g) are also discussed. In summary, the molar enthalpy of dissociation of the (triple) bond in GeS, 535 kJ•mol -1 , is the largest for any Ge-to-S linkage, and the mean molar enthalpy of dissociation of the (double) bonds in GeS2, 404 kJ•mol -1 , is greater by 110 kJ•mol -1 than D°m(Ge2S2) because, presumably, Ge2S2(g) has essentially single Ge-S bonds only. The molar enthalpy of dissociation of the ''primary'' bond D°m(S-GeS) is of comparable magnitude to the mean molar dissociation enthalpy of the Ge-S bonds in Ge2S2(g).
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