ELLIOTT GREENBERG AKD WARDN. HUBBARD
222
Fluorine Bomb Calorimetry. XXIII.
The Enthalpy of
Formation of Carbon Tetrafluoride’ by Elliott Greenberg and Ward N. Hubbard Chemical Engineering Division, Argonne National Laboratory, Argonne, Illinois
(Received July 7, 1967)
The energy of combustion of graphite in fluorine was measured with samples of both natural and synthetic graphite. The standard enthalpy of formation, AHf298.15, of CF4(g) was calculated to be -223.04 f 0.18 kea1 mole-’. Combination of this value with other 10H20(1)],dethermochemical data gives -77.02 or -76.75 kcal mole-’ for AHfOzg8[HF. pending on the auxiliary data used.
Introduction The enthalpy of formation of CF4 is a key thermochemical datum. CF4 is an end product of the intensive fluorination of all organic compounds and of inorganic carbides. As such, its importance in fluorine bomb calorimetry is analogous to that of C02in oxygen bomb calorimetry. The importance of AHf”(CF,) led to an early direct determination (- 162kcal mole-’) and more recent indirect measurements, 3-12 with values ranging from -212.7 to -231 kcal mole-’. The selected value of the “JANAF Tables”l3 (-220.5) has an uncertainty of 2.5 kcal mole-1 assigned to it. Domalslii and Armstrong14and the present investigators have nearly simultaneously reinvestigated the direct method of determining AHf O (CF4)by burning graphite in fluorine in a high-precision calorimeter. This investigation differs from theirs in that we avoided the large contribution to the measured energy from the use of Teflon as an auxiliary combustant.
Experimental Section P?“eliminaryObservations and Combustion Technique. It was found that samples of graphite, exposed to fluorine at room temperature, underwent a weight increase in an ill-defined and irreproducible manner. Therefore, some means was necessary to maintain separation of the graphite from fluorine until intentional ignition was desired. It was further found that the amounts of higher fluorocarbons, C2Fa and C3F8, formed upon combustion, decreased with increasing pressure of fluorine in the combustion chamber. At 20 atm pressure the amounts of higher fluorocarbons formed required a calorimetric correction which was only about 0.1% of the total measured heat. We, therefore, built a high-pressure two-compartment bomb,15laboratory designation Ni-7, for this work. Satisfactory combustions were obtained with a Oa4 Of arrangement in which graphite flakes and several milligrams of silicon powThe Journal of Phyaical Chemistry
der, used as an ignitor, were contained in a nickel crucible (1.3 cm in i d . , 2 cm high, 1.5 mm in wall thickness, with a base 3.8 cm in diameter and 1 mm thick). About halfway up from the base, a ring of 1.5-mm diameter holes was drilled through the crucible wall. The height of the crucible (as well as a sintered nickel filter disk in the fluorine inlet port of the combustion compartment) was necessary to prevent scattering of the sample when fluorine was admitted to the combustion compartment. The holes in the crucible helped to improve the circulation of gases around the sample during :combustion, and the relatively large base provided mechanical stability and better thermal contact with the bomb head. (1) (a) This work was performed under the auspices of the U. 9. Atomic Energy Commission. (b) For the previous paper in this series, see E. Rudzitis, H. M. Feder, and W. N. Hubbard, Inorg. Chem., 6, 1716 (1967). (2) H.v. Wartenberg and R. Schutte, 2. Anorg. AEZgem. Chem., 211, 222 (1933). (3) H. v. Wartenberg, ibid., 258, 356 (1949). (4) F.W.Kirkbride and F. G. Davidson, Nature, 174, 79 (1954). (5) R.S. Jessup, R. E. McCoskey, and R. A. Nelson, J. A m . Chem. Soc., 77,244 (1955). (6) (a) D. W. Scott, W. D. Good, and G. Waddington, ibid., 77, 245 (1955); (b) W. D. Good, D. W. Scott, and G. Waddington, J.Phys. Chem., 60,1080 (1956) (7) H. C. Duus, Ind. Eng. Chem., 47, 1445 (1955). (8) C. A. Keugebauer and J. L. Margrave, J . Phys. Chem., 60, 1318 (1956). (9) A. F.Vorob’ev and S. M. Skuratov, Russ. J . Inorg. Chem., 5, 679 (1960). (10) (a) V. F.Baibuz, Proc. Aced. Sci. U S S R , Chem. Sect., 140, 1388 (1961); (b) V. F. Baibua and V. A. Medvedev, T r . , Gos. Inst. Prikl. Khim., 49, 84 (1962) [cf. Chem. Abstr., 60, 3555a (1964)l. (11) E. S. Domalski and G. T. Armstrong, J . Res. Natl. Bur. Std., 69A, 137 (1965). (12) J. D.Cox, H. A. Gundry, and A. J. Head, Trans. Faraday Soc., 61,1594(1965). (13) “JANAF Thermochemical Tables,” The Dow Chemical Co., Midland, Mich., Sept 30,1964. (14) E. 8 . Domalski and G. T. Armstrong, J . Res. Natl. Bur. Std., 71A, 105 (1967). (15) J. L.Settle, E. Greenberg, and W. N. Hubbard, Rev. Sei. Instr., in press. a
ENTHALPY OF'FORMATION OF CARBON TETRAFLUORIDE Calorimetric System. The calorimetric system consisted of the bomb, Ni-7, mentioned before, and the calorimeter, ANL-R2 (a duplicate of ANL-R1, ref 16). For the temperature measurements, we used a quartz crystal therrn~ometerl~ which has as a sensing element a quartz crystal cut in such a manner that its resonating frequency is very nearly a linear function of temperature. The resonating frequency of this thermometer is internally converted to a digitally coded temperature signal which was fed to a digital recorder (HP 562 A), together with the signal from an electronic timer (HP 5512 A), to give simultaneous print-out of time and temperature. Time was measured to 0.1 sec and temperature to 0.0001". An auxiliary timing device initiated temperature measurements at fixed, controllable intervals. This device permitted automatic measurements on a schedule comparable to typical bomb calorimetric measurements.'s The printed data were later transferred to punched cards for computer calculation of the corrected temperature rise. The calorimetric system was calibrated with benzoic acid (National Bureau of Standards Sample 399 whose certified energy of combustion was 26.434 f 0.003 abs kjoules g-1 under prescribed conditions. Fifteen experiments, some preceding and some following the graphite combustions, and with temperature rises comparable to those obtained in the graphite combustions, yielded an average value for E (calor), the energy equivalent of the calorimetric system, of 3262.92 0.46 (std dev) cal deg-'. Materials. Natural graphite, in the form of flakes, was separated from Ticonderoga graphite-bearing marble by acid leaches and flotation in methyl iodide. Graphitization and purification of these flakes was accomplishedl9 by heating to 3000" in nitrogen and above 2500" in chlorine. The sieved fraction between 16 and 30 mesh was retained for calorimetric combustions. Synthetic graphite (Pyrogenics, Inc., Woodside, N. Y .) had bleen prepared by deposition from a carbonbearing vapor at about 2180", and graphitization above 3000" in an argon atmosphere. The graphite flakes were sieved, and the fraction between 16 and 30 mesh was retained for calorimetry. The analyses of the impurities in the samples are given in Table I. Ash contents of
