NOTES
1572
Ti to form TiO. This appears to be a rather arbitrary assumption. We propose that the valencebond approximation employed here, which approximates only the vibrational contribution, yields a more reliable estimate of the entropy of sublimation. Ti0.-In our study of the vaporization of Ti0 the measured vapor pressures are within a factor of 2 of those reported by Groves, et al. However, there appears to be an error in their calculated entropy of sublimation. The third law entropy of TiO(g) which we have used, which is the same as that recently tabulated by the National Bureau of Standards,I2is 1.13 e.u. higher at 2000°K. than the value reported by Skinner, Johnston and Becketti2 and subsequently employed by Groves, Hoch and Johnston.2 Hence, their AH of sublimation for Ti0 is very likely too low by 2.3 kcal./mole. Ti20a.-Groves, et al., report that their vaporization and X-ray data indicate that Tiz03vaporizes stoichiometrically within lo%, with the Ti0 to TiOz ratio at 0.9 to 1.1. Our results a t comparable temperatures are Ti:TiO:TiOz = 600: 1460: 17 (12) National Bureau of Standards, Selected Values of Chemical
Thermodynamic Properties, Series 111. (13) G. B. Skinner, H. L. Johnston and C. Beckett, “Titanium and Its Compounda,” Herrick L. Johnston Enterpriaes, Columbus, Ohio, 1954.
Vol. 61
This discrepancy, particularly in the TiO/TiOz ratio, could be explained if the condensate in the experiments of Groves, et al., had absorbed oxygen before it was weighed. An alternative explanation is the possibility that the samples used by Groves, et al., were of a composition different from ours. Conclusions The only gaseous species observed in equilibrium with various solid concentrations in the titaniumoxygen system were Ti, Ti0 and TiOz. The heats of sublimation obtained for the latter molecules (TiO2) AHozes(TiO)= 139 A 5 kcal./mole and AHozss = 146 5 kcal./mole are in approximate agreement with the results of Groves, Hoch and Johnston,2 although some cancellation of errors between experimental observations and computations appear to have occurred. Our results indicate that the mechanism of vaporization of TizOa(s)reported by Groves, et al., is probably in error. The sublimation energy of Ti0 obtained in this study combined with the heat of formation of TiO(s),9 the heat of vaporization of Tiloand the dissociation energy of OZ1’yields 6.8 e.v. as the dissociation energy of TiO. The corresponding atomization energy of Ti02 is 13.5 e.v. Within the precision of the above results (ca. 4%), the addition of a second oxygen to titanium liberates as much energy as the first.
*
NOTES HEATS OF FORMATION OF ALUMINA, MOLYBDENUM TRIOXIDE AND MOLYBDENUM DIOXIDE BYALLAD. MAE Contribution from the Minerals Thermodwaamics Experiment Station h e i o n I I , Bureau of Minae, United State8 Department of the Intarior: Waashington, D . (3. Recrived June 6, 1967
Recent measurements’-* of the heat of formation of alumina (corundum) at 298°K. have given values ranging from -399.0 to -402 kcal./mole. It appeared desirable, therefore, to determine this quanti6y again, to enable the selection of a “best” value. Also, occasion arose to determine the heats of formation of molybdenum trioxide and molybdenum dioxide. The results confirm the recent work of Staskiewicz, Tucker and Snyder,4 who obtained results that differed appreciably from the values selected by Brewer6from older literature. Materials.-A small aluminum bar of 99.998%. purity was furnished for this work by T. H. Hazlett, Division of Mineral Technology, University of California. Fine lathe turnings, cut with a Carboloy tool, were used for the measurements.
(1) P. E. Snyder and H. Selts, J . A m . Cham. Soc., 67, 683 (1945). (2) C. E. Holley, Jr., and E. J. Huber, Jr., ibid., 73, 5577 (1951). (3) A. Schneider and G. Gattow, 2. anorg. aZZgem. Chem., 377, 41 (1954). . . (4) B. A. Staskiewios, J. R. Tucker and P. E. Snyder, J . A m . Chem. SOC.,77, 2987 (1955). (5) L. Brewer, Chem. Rsvs., Sa, No. 1 (February, 1953).
The molybdenum metal was Fansteel Metallurgical Corp., type 352, 200-mesh powder. It was heated in a stream of pure hydrogen for 2 hr. at 900’ before use in the measurements. Molybdenum dioxide was prepared from high purity molybdenum trioxide by prolonged treatment with hydrogen at 400’. The oxygen content, determined by hydrogen reduction at goo”, was 25.01%, which is the theoretical amount for pure molybdenum dioxide. The X-ray diffraction pattern agreed with the ASTM catalog. Methods.-The energy-of-combustion measurements were conducted with previously described apparatus.6 All weights were corrected to vacuum and all heat values are in terms of the defined calorie ( 1 cal. = 4.1840 abs. joules). National Bureau of Standards benzoic acid, sample No. 39g, wa8 used for calibration. The mean calibration values were 32,480.6 (f0.02%) cal./ohm for the aluminum combustions and 32,495.3 (f0.02’%)cal./ohm for the molybdenum metal and molybdenum dioxide combustions. All combustions were made under 30 atm. oxygen pressure. The samples were ignited by an electrically heated platinum spiral and a small filter paper fuse. The substances showed no oxidation under bomb conditions before ignition. The bomb gases after combustion contained only negligible amounts of oxides of nitrogen. Alundum disks were used to hold the aluminum samples during comb‘ustion. A small piece of freshly cleaned magnesium, for which roper correction was made, was used a8 a kindler. More tgan 98% of the combustion product stayed on the disk; the remainder appeared as a deposit on the bomb walls. The percentage completion of combustion was determined by weighing the total combustion roduct. Completions varied from 99.14 to 99.66%. %-Ray diffractions of the combustion products from the disks agreed with the pattern for corundum reported by (6)
G. L. Humphrey, J . Am. Chcm. SOC..78,
1587 (1951).
1573
NOTES
Nov., 1957
TABLE I ENERQY OF COMBUSTION AT 30' g.
Total energy evolved, cal.
0.40359
3030.77
2.00032
3373.68
Maaa of substance,
Energy from EIt, fuee,
Cor. for incomplete aombustion, cal.
Total energy cor., cal.
-cal./g. AUB,
A1 (av. of 7 runs) 58.28 15.16
2987.65
7402.7 f 2 . 8
Mo (av. of 7 runs) 9.84 328.67
3692.51
1846.0 f 0 . 5
2324.26
290.5 AO.1
NlOo,kindler, cal.
Mooz (av. of 8 runs) 8.00034
1714.18
11.22
Rusaell?; there was no evidence of the presence of any other modification of alumina. The wall deposits, however, did contain other modifications. No correction was attempted for this as the amount of wall deposit was so small. The molybdenum metal and molybdenum dioxide samples were held on molybdenum trioxide.plates during combustion. About 6?%. of the metal combustion product and over 99% of the dioxide combustion product remained on the plates. Wall deposits on the bomb accounted for the rest. Completion of combustion of the metal samples ranged from 84.23 to 93.43'%, as determined by weight gain of the total combustion product upon prolonged low-temperature ignition in air. Completions of combustion of the dioxide samples ranged from 72.07 to 75.03%. This was determined by weighing the total combustion product.
Results The average values of the energy of combustion measurements are in Table I. The assigned uncertainties were calculated by the method of Rossini and Deming.8 The energy of combustion of aluminum corre= -399.45 kcal./mole. Correcsponds to A.E,OB.~~ tion to unit fugacity of oxygen (-138 cal.), to a constant pressure process (-904 cal.) and to 298.15"K. (16 cai.) leads to AH298.16 = -400.48 f 0.25 kcal. as the heat of formation of corundum from the elements. This agrees excellently with the value of Holley and Huber2 (-400.29 f 0.31 kcal./ mole), and the mean value, AH298.16 = -400.4 0.3 kcal., is recommended. The mean result for the energy of combustion of molybdenum corresponds to AE303.15 = - 177.12 kcal./mole. Correction to unit fugacity of oxygen (-132 cal.), to a constant pressure process (-904 cal.) and to 298.15"K. (-7 cal.) leads to m 2 9 8 . 1 6 = -178.16 f 0.11 kcal./mole as the heat of formation of molybdenum trioxide from the elements. This completely confirms the result of Staskiewicz, Tucker and Snyder4 (-178.01 i 0.10 kcal.), and the mean value, AH298.16 = -178.1 i 0.1 kcal., is recommended. Likewise, the mean value for the combustion of molybdenum dioxide corresponds to AE303.16 = -37.17 kcal./mole. Again, correcting to unit fugacity of oxygen (-45 cal.), to a constant pressure process (- 301 cal.) and to 298.15"K. (-2 cal.) gives AH298.16 = -37.52 f 0.03 kcal. as the heat of combustion of molybdenum dioxide to trioxide. Combining with the heat of formation of the trioxide, there is obtained AH298.15 = - 140.64 0.13 ked./ mole as the heat of formation of the dioxide from the elements. This value also agrees well with the
*
(7) A. S. Rusaell, Aluminum Company of America Technical Paper No. 10, 1953. (8) F. D. Roseini and W. E. Deming, J . Waah. Acad. SOe., 80, 416 (1939).
621.30
result of Staskiewicz, Tucker and Snyder4 (- 140.88 f 0.13 kcal.), The mean value, AH298.16 = - 140.8 0.2 kcal., is recommended.
*
THE HEAT CAPACITY O F ALUMINUM OXIDE IN THE RANGE 300 TO 70OoI
