FLUORINE-COMBUSTION CALORIMETRIC DETERMINATIONS OF THE STANDARD MOLARENTHALPY CHANGES FOR THE FORMATION OF SISE2(CR), SISE1.94(CR), AND SISE1.94(VIT), AND FOR THE TRANSITION - SISE1.94(VIT)=SISE1.94(CR) AT THE TEMPERATURE T=298.15-5 - IMPLICATIONS OF THE RESULTS FOR THE ENTHALPIES OF DISSOCIATION D(M)DEGREES(SE-SISE) AND D(M)DEGREES(SISE) - THERMODYNAMIC PROPERTIES OF SISE(G)

Fluorine-bomb calorimetry was used to determine the standard massic en ergies of combustion DELTA(c)u degrees of the silicon selenides SiSe2( cr), SiSe1.94(cr), and SiSe1.94(vit), according to the combustion reac tion: SiSe(v)(s)+(2+3v)F2(g) = SiF4(g)+vSeF6(g). The following additio nal results were derived for the standard molar energy of combustion D ELTA(c) U(m)degrees; the standard molar enthalpy of combustion DELTA(c )H(m)degrees; and the standard molar enthalpy of formation DELTA(f)H(m )degrees, all at the temperature T = 298.15 K and for the standard pre ssure p degrees = 101.325 kPa: SiSe2(cr) SiSe1.94(vit) SiSe1.94(cr) DE LTA(c)u degrees/(J.g-1) -19682+/-16 -19873+/-13 -19837+/-22 DELTA(c)U( m)degrees/(kJ.mol-1) -3661.05+/-2.98 -3602.38+/-2.36 -3595.85+/-4.35 D ELTA(c)H(m)degrees/(kJ.mol-1) -3673.4+/-3.0 -3614.5+/-2.4 -3608.0+/-4. 3 DELTA(f)H(m)degrees/(kJ.mol-1) -178.4+/-3.1 -170.2+/-2.4 -176.7+/-4. 5 Standard thermodynamic properties of the gas SiSe(g) were calculated by means of the conventional formulae of statistical mechanics. On th e basis of the thermochemical quantities given above, the standard mol ar enthalpy of the transition: SiSe1.94(vit) = SiSe1.94(cr) is DELTA(t rs) H(m)degrees = -(6.5 +/- 2.2) kJ.mol-1 at T = 298.15 K. (All uncert ainties in this abstract correspond to twice the standard deviation of the mean.) Implications of the thermochemical quantities for the enth alpies of dissociation D(m)degrees(SiSe) and D(m)degrees (Se-SiSe) are discussed, and the latter are compared with D(m)degrees(SiX) and D(m) degrees(X-SiX), where X denotes O, S, or Te.