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Knudsen effusion mass spectroscopic studies of the vapor above a gadolinium antimonide-molybdenum phosphide-graphite system led to the determination of the standard enthalpy changes, Δ H298°(kcal), for the following reactions: Sb4(g)=2Sb2(g) (71.3 ± 1.8), Sb4(g) =4Sb(g) (215.0 ± 3.0), Sb2(g) = 2Sb(g) (71.8 ± 1.0), Sb3(g)=3Sb(g)(129 ± 4), P2(g) =2P(g) (117.1 ± 1.1), Sb2(g) + P2(g) = 2SbP(g) (17.6 ± 0.4), SbP(g) = Sb(g)+P(g) (86.0 ± 1.2), and 2SbP3(g)=Sb2(g)+3P2(g) (149 ± 8). These reaction enthalpies have been combined with appropriate literature data to obtain the dissociation or atomization energies D°298 and standard heats of formation Δ H°f,298 of the molecules given. D298°kcal mol−1or kJ mol−1Δ Hf,298°kcal mol−1or kJ mol−1Sb271.8 ± 1.0300.4 ± 4.854.7 ± 2.2288.7 ± 9.2Sb3129.0 ± 4.0539.7 ± 16.760.7 ± 5.8254.0 ± 24.3Sb4215.0 ± 3.0899.6 ± 12.637.9 ± 5.4158.6 ± 22.6SbP85.7 ± 0.9358.6 ± 3.857.3 ± 1.6239.8 ± 6.7SbP3285.8 ± 5.21195.8 ± 21.816.8 ± 6.070.3 ± 25.1

By means of Knudsen effusion mass spectrometric studies the atomization energies, DOo(kcal mol-I), and standard heats of formation, dHof,29lkcal mol-I), respectively, were obtained for the following; Au 2 (52,9±0.5, 122.0± 1.0), Tb 2 (30.5±6.0, 155.2± 7.0). TbAu(69,2±8.0, 1l0.5± 10), HoAu(63.0±8.0, 96.0±8.7), TbAu 2 (139.1±10, 128.0±1l) and HoAuiI27,3±1O, lOO.6±1O). These data were deduced from a critical assessment of the enthalpy changes of the reactions Mig)=2M(g) and M(s ,I) + M(g) = Mig) for gold and terbium. and of the reactions LnAu(g) = Ln(g) + Au(g), LnAu(g) + Au(g) = Ln(g) + Auig), LnAuig) = Ln(g) + 2Au(g) and LnAu 2 (g) + 2Au(g) = Ln(g) + 2Auig)for terbium and holmium. The cited standard heats of formation incorporate appropriate literature data as well as the measured reaction enthalpies. The Pauling model of a polar bond has been used for the interpretation of the measured atomization energies of the Ln-Au molecules and for predicting the dissociation energies of selected intermetallic compo~nds of terbium.The terbium and holmium metals, of 99.9% grade purity were supplied by Research Chemicals, Phoenix, Arizona. . The principal stated lanthanide impurities in terbium were Dy, Gd, and Eu, and in holmium Yb, Tb, and Er.A single-focusing magnetic type deflection mass spectrometer was employed. The mass spectrometer and the experimental procedures used for this work have been described previously. 15 In the pure gold run, the instrumental conditions were standardized at 1 rnA emission current, 18 e V electron

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Mass spectroscopic studies of the effusate from a graphite-lined tantalum Knudsen cell containing a mixture of graphite (excess) -TiP–Rh yielded the enthalpy changes (Δ H°298, kilocalories) for the following reactions: 2C(s) = C2 (199.0 ± 1.6), 3C(s) = C3 (188.2 ± 2.1), P + C(s) = CP (48.6 ± 1.6), P + 2C(s) = C2P(71.9 ± 2.0), P2 +C(s) = CP2 (54.0 ± 2.0), and P2 + 2C(s) = C2P2 (55.8 ± 4.0). From considerations of bond strengths and comparison of Second- and Third-Law thermodynamic data it appeared that the dominant isomeric species of the gaseous carbon-phosphorus compounds were CCP, CPP, and PCCP. With respect to CCP and PCCP, this observation agrees with previous suggestions, but CP2 had been considered to be a mixture of PCP and CPP [Smoes et al., Chem, Phys. Lett. 8, 10 (1971)]. The enthalpy changes of the above reactions and of other complementary homogeneous gas phase reactions permitted the calculation of atomization energies (D°o, kilocalories per mole) and standard heats of formation (Δ H°f,298,kilocalories per mole) respectively, of C2 (142.2 ± 2.5, 199.0 ± 1.6), C3 (322.0 ± 3.5, 188.2 ± 2.1), CP (121.3 ± 2.1, 128.4 ± 1.7), CCP (267.6 ± 4.9, 151.7 ± 4.1), CPP (231.9 ± 4.6, 96.7 ± 2.5), and PCCP (400.0 ± 5.0, 98.5 ± 4.5). Agreement with literature values [Smoes et al., Chem. Phys. Lett. 8,10,1971; Gingerich, Thermochim. Acta 2, 233 (1971); JANAF Thermochemical Tables, Dow Chemical Co., Midland, Mich. (1965)] is generally good.