THERMOCHEMISTRY OF (GERMANIUM PLUS SULFUR) .4. CRITICAL-EVALUATION OFTHE THERMODYNAMIC PROPERTIES OF SOLID AND GASEOUS GERMANIUM(II) SULFIDE GES AND GERMANIUM(IV) DISULFIDE GES2, AND DIGERMANIUM DISULFIDE GE2S2(G) - ENTHALPIES OF DISSOCIATION OF BONDS IN GES(G), GES2(G), AND GE2S2(G)

This is a critical evaluation of the thermodynamic properties of the k nown 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 l ow and moderate temperatures was evaluated from all the information av ailable in the literature, and the properties: C-p,m(o)(T), {H-m(o)(T) -H-m(o)(T')}, S-m(o)(T), and Phi(m)(o)(T) = (Delta(o)(T)S(m)(o) - Delt a(T)(T)H(m)(o)/T), where T' = 298.15 K, were computed to T = 930 K, cl ose to the melting temperature, above which decomposition to a (german ium + sulfur) eutectic and uncombined germanium is believed to occur. On the basis of our recent Value for Delta(r)(H)(m)(o)(GeS, cr, 298.15 K) (J. Chem. Thermodynamics 1994, 26, 727), Delta(r)H(m)(o)(T) and De lta(f)G(m)(o)(T) were also calculated over the same temperature range. A critical assessment of the enthalpy of sublimation Delta(sub)H(m)(o ) yielded Delta(r)H(m)(o)(GeS, g, T). In another part of the present s eries (J. Chem. Thermodynamics 1995, 27, 99), we determined Delta(r)H( m)(o)(GeS2, cr, 298.15 K); the corresponding Delta(r)H(m)(o)(GeS2, cr, T) is tabulated in the present paper to T = 1000 K. Ab initio molecul ar-orbital calculations showed the cyclic (C-2v) 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 sim ilar treatment of the linear GeS2(g) is described. The assessed Delta( r)H(m)(o)(GeS, g, T --> O) is in harmony with our reinterpreted molar enthalpy of dissociation D-m(o)(GeS) from spectroscopy; and the enthal pies of dissociation of the bonds in GeS2(g) and Ge2S2(g) are also dis cussed. 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(o)(G e2S2)] because, presumably, Ge2S2(g) has essentially single Ge-S bonds only. The molar enthalpy of dissociation of the ''primary'' bond D-m( o)(S-GeS) is of comparable magnitude to the mean molar dissociation en thalpy of the Ge-S bonds in Ge2S2(g).