1326
The Heat of Reaction of Nitrogen Trifluoride and Hexafluoroethane by G. C. Sinke Thermal Restarch Laboratory, Dow Chemical Go., Midland, Michigan (Receiced October 15, 1965)
Published work on the thermochemistry of hexafluoroethane is limited to one investigation' in which CzF6 reacted with elemental potassium to form potassium fluoride and amorphous carbon. A recent paper2 called attention to discrepancies in the energy relationships involved in the dissociation of C2F6 to CF3 radicals. The heat of reaction of NF3 and CzFs has been measured in an attempt to resolve the discrepancies. The method developed should be useful for other volatile fluorocarbons. Experimental Section
Materials. Commercial technical grade NF3 was purified by low-temperature distillation. A combination of infrared spectroscopy, mass spectroscopy, and gas chromatography indicated the product was at least 99.9 mole % pure. Commercial CzF6 was found to contain a trace of air which was removed by condensing the CzFein liquid nitrogen and pumping. No other impurities were detected. Commercial "ultrapure" hydrogen was stated by the supplier to be 99.995% pure. No impurities were detected by infrared and mass spectroscopy. Procedure. A conventional Parr platinum-lined combustion bomb of 348-m1 volume was employed. The bomb cylinder and head were dried at 110" for 1 hr, assembled, connected to a vacuum line, and evacuated while still hot. After cooling to room temperature, the bomb was surrounded by a constanttemperature water bath and pumped until a pressure of about 1 p was achieved. The bomb was then charged with 800.0 mm of hydrogen as measured by a Wallace & Tiernan precision dial manometer which could be read to 0.1 mm. Hexafluoroethane and nitrogen trifluoride were contained in small stainless steel cylinders which were weighed on a 200-g capacity analytical oalance. The cylinders were connected to the vacuum line and the bomb was successively charged with C ~ to F a~total pressure of about 1270 mm and T\TF~ to a pressure Of about 2300 mm* The bomb was closed except when charging and sample remaining in the connecting lines was recondensed in the cylinders with liquid nitrogen. ~h~ cylinders were reweighed to obtain the exact amounts of sample charged. T h e Journal o/ Physical Chemistry
NOTES
The bomb was placed in a conventional combustion calorimeter and the charge was fired by discharging a condenser through a fine platinum fuse wire. The condenser voltage before and after firing was a measure of the ignition energy. The calorimeter was calibrated with National Bureau of Standards benzoic acid 39h. An average of 0.55 g of acid was used and the bomb charged with 1 ml of water and 30.3 atm of oxygen. Under these conditions the standard sample was calculated to have a heat of combustion of 6317.5 thermochemical cal/g of mass. The calorimeter equivalent was 3407.10 cal/deg with a standard deviation of 0.42 cal/deg for five experiments. After the calorimetry was completed, the gaseous reaction products were passed over 85% KOH pellets and the scrubbed gases were examined by infrared and mass spectroscopy. Only traces (less than 0.05%) of the original NF3 and CzFBwere found, indicating complete reaction of hydrogen to HF, C2F6to CF4, and excess NF3 to Nzand Fz. Because the thermochemistry of H F vapor is poorly defined, comparison experiments were run in which NFs and Hz mixtures were exploded to give the same final amount of H F in the bomb. A washer of Teflon fluorocarbon used on the insulated ignition post was slightly attacked by fluorine. I n the comparison experiments, the extent of this attack was determined by mass spectrometer analysis of the CFg in the reaction products; in the CzFe experiments an average value was obtained by weighing the washer before and after the series of five runs. Results The results for five comparison runs are given in Table I. The calorimeter equivalent adjusted for changes in the bomb contents was 3403.54 cal/deg. The correction for fluorine attack on Teflon was taken as 2.474 ~ a l / m g . ~All runs were adjusted to a standard quantity of 0.9600 g of S F 3 using an enthalpy of dissociation under constant volume conditions of 41 1.4 ~ a l / g . Results ~ for the hexafluoroethane experiments are given in Table 11. The adjusted calorimeter equivalent was 3403.79 cal/deg and all runs were again adjusted to a standard of 0.9600 g of KF3 above that required for reaction with CzFO. The assumption was made that the final state of the comparison experi(1) F. W. Kirkbride and F. G. Davidson, Nature, 174, 79 (1954). (2) E. Tschuikow-Roux, J . P h y s . Chem., 69, 1075 (1965). (3) E. S. Domalski and G. T. Armstrong, J . Res. ,Vatl. Bur. Std., 69A, 137 (1965). (4) D. D. Wagman, W. H. Evans, I. Halow, V. B. Parker, S. M. Bailey, and R. H. Schumm, National Bureau of Standards Technical Note 270-1, U. 8. Government Printing Office, Washington, D. C., Oct. 1, 1965.
NOTES
1327
Table I: Results of Comparison Experiments Washer correction, cal
Adjustment to 0.96 g of NFs, cal
temp rise,
0
Ignition energY , cal
0.9621 0.9698 0.9382 0.9746 0.9757
0.24 0.21 0.22 0.22 0.18
24.74 29.69 23.30 16.33 16.33
0.86 4.03 -8.97 6.01 6.46
0.49399 0.49488 0.49740 0.49143 0.49103
Mass of NFa,
Corrected Calories per expt.
O C
1657.19 1658.47 1660.43 1662.06 1661.19 Av 1659.87
Table I1 : Results of Hexafluoroethsne Experiments Ignition energy. cal
Adjustment to 0.96 g of NFa, oal
At,
-AEr/M,
g
Washer correction, cal
O C
CaVg
1.3769 1.3523 1.3448 1.3407 1.3511
21.53 21.53 21.53 21.53 21.53
0.21 0.20 0.21 0.20 0.20
6.42 -3.70 -5.72 -5.39 -4.57
0.75203 0.75604 0.75578 0.74940 0.75365
of CzFs,
Mass of NF3,
g
1.1701 1,1702 1.1625 1.1481 1.1727
Mass
ments was thermally equivalent to the final state in the C2F6 experiments. The calorimeter starting temperature was selected so that the final temperature in each series was 25.00'. I n the C2Fe series, however, CF4 and additional N2 were present as well as H F vapor. The comparison experiments therefore did not exactly duplicate the final state in the CzFe experiments. Further work on this aspect of the method is planned. The effect of the additional gases on the thermal state of the H F vapor is probably small. Employing atomic weights of 12.0111 and 18.9984 for carbon and fluorine, respectively, there is derived for the reaction CZF6(g)
+ 2/3NF3(g)
+2CF4(g)
+ '/aNz(g)
AEr2ss = -104.3 kcal/mole Calculating to constant pressure conditions
756.0 758.9 761.5 752.4 749.6 Av 755.7
*
102 2 kcal as the most reliable value for the CFS-H bond dissociation energy, which gives for the enthalpy of formation of CF3 radical -114.6 f 2.5 kcal/ mole. The present work yields an enthalpy of formation of CzF6 of -318.2 i 2 kcal/mole. From these data is calculated a CF3-CF3 bond dissociation energy of 89 =k 4.5 kcal. Tschuikow-Roux6 measured 93 4 kcal by direct observation of the dissociation of C2F6 in a shock tube, in agreement within experimental error. Other work listed by Tschuikow-Roux2 leading to much higher or lower values now appears to be in error.
*
Acknowledgment. This work was supported by the United States Air Force under Contract No. AF04(611)-7554(4). ( 5 ) E. Tschuikow-Roux, J. Chem. Phys., 43, 2251 (1965).
AHrzos = -103.9 kcal/mole
The over-all uncertainty is estimated as i1kcal/mole.
Conclusions
Schmidt Number Correction for
Although accurate heats of formation for fluorocarbons cannot be computed because of the uncertainty in the true value for aqueous HF, a consistent set based on the most recent selections at the National Bureau of Standards4 allows some comparisons to be made. The selected enthalpies of formation (all in kcal/mole) are -29.8 for NF3(g), -221 for CF*(g), and - 164.5 for CHFa(g). Tschuikow-Row2 selected
the Rotating Disk by John Newman Department of Chemical Engineering, University of California, Berkeley, California (Received October 18, 1966)
According to Levich,1$2the rate of mass transfer to a rotating disk is given by Votume 70,Number 4
April 1966
