1318
CONSTANTINE NEUGEBAIJER AND JOHNL. MARGRAVE
Acknowledgment.-The authors wish to acknowledge the mass spectrometer analyses of A. V.
Vol. 60
Jensen and the help of S. R. Smith with the interpretation of mass spectrometer records.
THE HEATS OF FORMATION OF TETRAFLUOROETHYLENE, TETRAFLUOROMETHANE AND I,1-DIFLUOROETHYLENE BY CONSTANTINE A. NEUGEBAUER AND JOHNL. MARGRAVE Contributionf r o m the Department of Chemistry of the University of Wisconsin,Madison, Wisconsin Received April 18, 1966
The standard heats of formation of gaseous tetrafluoroeth lene and tetrafluoromethane have been determined by the hydrogenation and decomposition of tetrafluoroethylene in a t o m b calorimeter. They are -151.7 i 1.1 and -217.1 f 1.2 kcal./mole, respectively. The heat of formation of gaseous 1,l-difluoroethylene was found to be -77.5 i0.8 kcal./mole from combustion experiments,
Introduction The heat of formation of CF, by the direct interaction of the elements was reported by v. Wartenberg' to be -161 kcal. Later, v. Wartenberg2 studied the displacement reaction between CF4 and K to give K F and carbon, and found -231 kcal. for the heat of formation of CF4. Kirkbride and Davidson13using v. Wartenberg's later technique, obtained a value of -218 kcal. for the heat of formation of CF4. Jessup, McCosky and Nelson4 report -220.4 kcal. for the heat of formation of CF4, using the fluorination of methane t o give CF4 and gaseous HF. Later, Scott, Good m d Waddington,s by a method involving the heat of combustion of Teflon polymer in the presence of hydrocarbon oils, obtained -218.3 kcal. for CF4. Finally, Duus6 reports -212.7 kcal. for CF4, using the hydrogenation of CZF4 to give carbon and gaseous HF, and the decomposition reaction of C2F4 to give C and CF4. The final state of the carbon mas assumed to be graphite in both cases. Corrections for the final state of the HF were estimated. Furthermore, no corrections were made for methane produced in the hydrogenation. By the method described above, Duus also obtained the heat of formation of gaseous C2F4 as -151.3 kcal. On the other hand, both v. Wartenberg,7 and Kirkbride and Davidson,* report - 162 kcal. for the heat of formation of CzF4. I n general, the literature values for the heats of formation of both compounds show variations and uncertainties large enough to warrant new determinations. The reactions used to determine the heat of formation of C2F4 and CF4 are similar to those used by Duus. When gaseous C2F4 reacts with hydrogen in a calorimetric bomb that initially contains (1) H. v . Wartenberg and R. Schiitte, Z. anorg. Chem., 811, 222 (1933). (2) H.v . Wartenberg and G. Riteris. ibid., 868, 356 (1949). (3) F. W.Kirkbride and F. G. Davidson, Nature, 174, 79 (1954). (4) R. S. Jessup, R. E. RIeCosky and R. A . Nelson, J . Am. Chem. Soc.. 7 7 , 244 (1955). (5) D. W. Scott, W. D . Good, and G u y Waddington, J . Am. Chem. SOC., ibid., 77, 245 (1955). (6) H. C. Duus, Ind. E n g . Chem., 47, 1445 (1955). (7) H.v . Wartenberg, 2. anory. Chem., 1'78, 320 (1955). (8) F. W. ICirkhride and F. G. Davidson, ref. 3.
water, the net reaction may be represented by the equation CzFa(g) 2Hz(g) = 4HF(aq) 2C(amorphous) (I) From this reaction, the heat of formation of CzFa can be calculated, Tetrafluoroethylene decomposes when ignited according to the equation
+
+
CZFd(g) = CFa(g)
+ C(amorphous)
(11)
This reaction, when coupled with the one above, makes possible the determination of the heat of formation of CF4. The heat of formation of the amorphous carbon formed in the reactions can be determined by separate combustion experiments. The heat of formation of 1,l-difluoroethylene has been determined by measuring the heat of the reaction CHzCFz(g)
+ 202(g) = 2COz(g) f 2HF(aq)
(111)
Experimental Apparatus and Method.-A conventional isothermal calorimeter of the type described by Dickinsong was employed. The temperature of the water jacket was kept constant a t 24.80 f 0.002' with a vibrating mercury regulator in connection with an electronic relay. The calorimeter cup, of 5-1. capacity, was held in place by three Lucite wedges placed along the upper rim of the calorimeter cover and held firm by the lid of the water jacket. The calorimeter stirrer was driven by a small electric motor a t 600 r. .m. A double valve Parr illium bomb having a volume o f 3 6 0 ml. was used in all experiments. The temperature rise in the calorimeter was measured by means of a platinum resistance thermometer, calibrated by the National Bureau of Standards, in connection with a G-2 Mueller resistance bridge, also previously calibrated, and a high sensitivity galvanometer, all supplied by the Leeds and Northrup Co. A change of O.OOO1 ohm (0.001'). in the resistance of the thermometer caused a 3 mm. shift in the reflection from the galvanometer mirror on the scale. The galvanometer was used a's a null-point instrument, the time a t which a predetermined resistance was reached being read from an electric timer. The resistance values were converted t o temperature readings by using the method of Werner and Fraeer.10 The corrected temperature rise (in degrees) was calculated by the Dickinsonll method, assuming Newton's law of cooling to hold. The initial temperature was so chosen that the final temperature was about 0.05' above jacket temperature. For convenience in obtaining the appropriate initial tempera(9) H.C.Dickinson, Bull. Natl. Bur. Standards, 11, 230 (1914). (10) F. D . Werner and A. C. Frszer, Rev. Sci. Instr., 83, 103 (1952). (11) H.C.Diekinaon, ref. 9.
S&.,
1956
HEATS OF
FORMATION O F TETRAFLUOROETHYLENE AND TETRAFLUOROMETHANE
ture, the calorimeter was equipped with a 600-watt heater. The mass of the calorimeter water was determined to 0.05 g. on an analytical balance of 4-kg. capacity and a sensitivity of 1 1 mg./div. at a 4-kg. load. I n calibration experiments t,he mass of the benzoic acid samples was determined to 0.01 mg. on a semi-micro analytical balance. Commercial tank oxygen and hydrogen was used in the combustion and hydrogenation experiments, respectively, and the bomb was flushed 5 times before final filling by admitting the gas t o 6 a t m . and then reducing to atmospheric pressure. Calibration.-Benzoic acid, NBS standard sample 39g., having a n isothermal heat of combustion at 25" of 26.4338 abs. kj. per g. mass under standard bomb conditions,lZ was ustd for determining the energy equivalent of the calorimeter. I n the experiments 2-g. samples were used in order t o achieve about the same temperature rise as is attained in later experiments. The oxygen pressure used in all calibration experiments was 30 atm. Nitric acid, formed by small amounts of nitrogen in the oxygen, was determined by adding 1 ml. of water to the bomb before combustion and t,itrating the washings with dilute sodium hydroxide solution, using methyl orange as indicator. The value for the heat of formatim of nitric acid was taken as 59 kj./mole. The correction from this source never exceeded 12 j . T o facilitate ignition, 10 cm. of iron fuse wire, supplied by the Parr Co., were used. Corrections were made for the quantity of wire burned, taking the heat of combustion to be 6.63 kj ./g. of wire. This correction was of the order of 70 j . Twelve combustions of benzoic acid yielded a value of 4695.00 cal./deg. for the energy equivalent of the calorimeter at 25'. The average deviation from the mean for these experiments was 0.066% and the maximum deviation was 0.14%. Uncertainties and Precision Errors.-The uncertainties in the reported values of the heats of formation were arrived at by taking the square root of the sum of the squares of the uncertainties of the heats of all the reactions which were used t o obtain the heat of formation in question, including the uncertainty in the energy equivalent of the calorimeter. The uncertainty quoted for the heat of a particular reaction is the precision error (twice the st,andard deviation from the mean), as recommended by Rossini,13 and is given l ) , where 4 denotes the by the formula 2 d / Z 4 2 / n ( n deviation from the mean, and n the number of trials. The Hydrogenation of Tetrafluoroethylene .-Compressed tetrafluoroethylene was available in tanks. Most impurities were present to an extent of less than 0.05 mole %, oxygen was present t o the extent of 10 p.p.m., and a polymerization inhibitor t o the extent of approx. 2900 p,p.m. The bomb, with 100.0 ml. of water added, was charged with 7 to 8 a t m . of C*F( a t room temperature. An additional 25 or 36 a t m . of hydrogen were then added. This charge was ignited with a 21 mm. long and 1 mm. thick pencil lead carbon rod, wedged between the two electrodes, and brought t o incandescence by a current of 6-7 amp. at 9-12 volts. The ignition energ was obtained by observing a n ammeter, voltmeter and cironometer simultaneously. The carbon rod can be recovered after the reaction. The HF solution fprmed in the reaction was about 3 molar. The analysis for the substances formed in the reaction was a titration of the HF solution with standard 0.1 N base t o the phenolphthalein end-point. The solution was separated from the carbon by decanting the suspension through a glasswool filter. The residual carbon was washed 3 times with boiling water to liberate any HF ahsorbed in it and the solution then diluted t
