3 139
HEATOF FORMATION OF POLY (CARBON MONOFLUORIDE)
The Heat of Formation of Poly(carbon monofluoride) by J. L. Wood, R. B. Badachhape, R. J. Lagow, and J. L. Margrave Department of Chemistry, Rice Universitg, Houston, Texas 77001 (Received March 85, 1960)
EYoly(carbon monofluoride) (CF), has been burned in a fluorine bomb calorimeter. The heat of combustion of this material is -176.26 -I: 0.15 kea1 mol-'. Its heat of formation based on the currently accepted values for AHr" CF4 (g) is -46.7 -I: 1.0 kea1 mol-'. By combining this value with the observed heat of formation of CF(g), one can show that the carbon-fluorine bond energy in poly(carbon monofluoride)is 115 kcal mol-', about the same as in other fluorocarbons.
-
Introduction I n 1934 Ruff, et al.,I prepared a gray compound of composition CF0.92by the action of fluorine on coarse graphite at 420-460". I n a further investigation Rudorff and Rudorff prepared a series of materials of composition CFo.68 to CFo.995 with colors varying from black in the fluorine-poor products to white in the case of CFo.gg5.The chemistry of these poly(carbon monofluorides) has been reviewed by R u d o ~ Pand by Hennig. 3b Interest in this compound developed in this laboratory when Kuriakose and Margrave4studied the kinetics of fluorination of graphite between 315 and 900". These workers pointed out the need for thermodynamic data on the heat, of reaction of fluorine with graphite to form poly(carbon monofluoride). By studying the reactions of CF2C1-CFC12and CF,-CCI, with graphite a t 500-1O0O0K, Porter and Smiths calculated the heat of the reaction Fz(g)
+ graphite e 2F (graphite)
(1)
to be - 100 f 6 kcal/mol Fz. Their method, however, left an uncertainty as to whether this heat was for a reaction involving only active surface sites on the graphite or one involving internal sites also and required the use of estimated heats of formation for the chlorofluoroethanes. I n this work, a direct calorimetric determination of the heat of reaction of a poly(carbon monofluoride) of known stoichiometry with fluorine is reported.
Experimental Section Preparation and Characterization of Poly (carbon monofluoride). Graphite powder (-325 mesh) supplied by Union Carbide Gorp., Carbon Products Division (SP-2 grade, Lot No. G96) was used in all preparations. The total concentration of impurities in this graphite was less than 1 ppm. The fluorine used was supplied by the Matheson Co. (98% purity, typical) and was passed through a sodium fluoride trap prior to use. The temperatures reported were measured with a Pt-10% Pt-Rh thermocouple in conjunction with a Model 2745
Rubicon potentiometer. The procedure used was a modification of that used by Rudorff. z I n typical preparation approximately 1-1.5 g of graphite powder was uniformly spread out in a prefluorinated nickel boat 7.5 in. long and 0.5 in. wide. The reaction chamber consisted of a l-in. i.d. nickel tube in a Hevi-Duty Electrical Co. furnace. Prior to passing fluorine through the furnace, helium was slowly passed over the graphite at a rate of 5 cc/min for 1 hr to remove moisture and oxygen from the reaction zone. During this time the furnace was slowly heated up to 200". At the end of this time a slow fluorine flow was initiated and the furnace was brought up to a After several hours a t temperature of -600". these conditions the fluorine supply was cut off, and helium was passed over the sample until the furnace cooled down to room temperature. The white product was removed and stored in glass bottles. Poly(carbon monofluoride) is not shock-sensitive nor does it hydrolyze even on standing in a glass bottle for several months. The yield of poly(carbon monofluoride) could not be calculated on the basis of a gain in weight of the sample; actually, a net loss in weight was observed due to loss of some of the sample in the fluorine gas flow arid to the formation of volatile fluorocarbons. The infrared spectra of all the samples were identical. A strong absorption at 1215 cm-' was observed, which corresponded to the report of Rudorff and Brodersen,6 and is probably the (CF) stretch (4C-F group). I n addition, absorptions of medium intensity were observed at 1345 and 1070 cm-l which are probably the asymmetric and symmetric stretching vibrations of (1) 0. Ruff,0. Bretschneider, and F. Elert, 2. Anorg. Allg. Chem., 217, l(1934). (2) W-.Rudorff and G. Rudorff, ibid., 253, 281 (1947). (3) (a) W;,Rudorff, "Advances in Inorganic Chemistry and Radiochemistry, Vol. I, H. J. Emeleus and A. G. Sharpe, Ed., Academic Press, Inc., New York, N. Y . , 1959, p 230; (b) G. R. Hennig, "Progress in Inorganic Chemistry," Vol. I, F. A. Cotton, Ed., Interscience Publishers, New York, N. Y., 1959, p 125. (4) A. K. Kuriakose and J. L. .Margrave, J. Phys. Chem., 69, 2772 (1965). (5) R. F. Porter and D. H. Smith, ibid., 66, 1562 (1962). (6) W. Rudorff and K. Brodersen, 2. Naturforsch, 126, 595 (1957).
Volume 79, Number 0 September 1060
3140 peripheral CF2 groups. A far-infrared absorption a t 332 cm-l was also observed. Two of the six preparations of poly(carbon monofluoride) were chosen a t random and analyzed for carbon and fluorine by Schwarzkopf AIicroanalytical Laboratories, Woodside, N. Y . Trouble was encountered with the fluorine analyses and carbon content was finally accepted as the only good check on the material’s composition. The results of the two analyses were 36.01% C, 63.57% F, and 35.98% C, 61.11% F. Based on the average per cent carbon, the empirical formula of the poly(carbon monofluoride) was CFi .iz+o.o3. The density of this material was determined using the pycnometer method described by Reilly and Rae.’ The liquid used to wet the sample was J. T. Baker Co. reagent grade xylene (boiling range 137.4-139.4”) with a measured density of 0.8609 g/ml. The results of two density measurements on the poly (carbon monofluoride) were 2.46 and 2.51 g/ml for an average value of 2.485 g/ml. Ruff reported a density of 2.39 g/ml for a similar material while Palin and Wadsworth8 reported a density of 2.78 g/ml for a gray-colored product whose €ormula was -CF1.04. An X-ray powder pattern obtained with copper radiation yielded an ill-defined pattern with only four intense but diffuse lines. From theseolines interplanar spacings of 5.80, 2.89, 2.23, and 1.29 A were calculated. In addition, yery weak lines yielded interplanar spacings of 3.29 A. Palin and Wadsworth reported interplanar spacings of 6.0, 2.23, and 1.30 8 for their material.
Calorimetric Measurements A rotating bomb calorimeter built to Argonne h’ational Laboratories design CT-3986 was used in all combustion experiments. The jacket temperature of the calorimeter mas maintained at approximately 26.000 f 0.002” with a Bayley Instrument Co. Model 123 temperature controller. A Parr Instrument Co. Model 1003 monel bomb was used with all internal fittings constructed of nickel. The ignition system was a condenser discharge design similar to that described by Coughlin, et al.9 A 1.8-cal pulse of electrical energy supplied by the condenser discharge heated a short 40gauge nickel fuse wire to glowing. Temperature measurements were made with a Dymec Model 2801A quartz thermometer with a resolution of 0.0001” and a 10-sec time interval between measurements. A quartz crystal probe, serial no. 603-30-AX6, was used for all combustion experiments. Temperature measurements u.ere recorded manually with the time scale being constant and determined by the thermometer itself. The corrected temperature rise for each combustion experiment was calculated using a computer program especially written for the output from the quartz thermometer using standard proceThe Journal of Physical Chemistry
J. WOOD,R. BADACHHAPE, R. LAGOW, AND J. MARGRAVE dures1° to make corrections for heat losses to the jacket and for the heat of stirring. A series of calibration experiments was run over a temperature interval of approximately 1.0” using NBS sample 39h benzoic acid. The heat of combustion of this benzoic acid (taking into account the actual bomb conditions, which were a bomb volume of 360 ml, an average sample weight of 0.56 g, an initial oxygen pressure of 30 atm, and 1 ml of water in the bomb) was -6318.22 f 0.72 cal/g (mass in vacuo). The result of this series of experiments was I: (calor) = 3580.93 A 0.98 cal/”C. As a secondary check of the apparatus the heat combustion of reagent grade succinic acid was determined as -3017.4 f 0.8 cal/g (average of two determinations) which is in reasonable agreement with the value of -3019.8 f 0.4 cal/g from the Bureau of Mines.” The combustion experiments on the poly(carbon monofluoride) were carried out using samples formed into 0.25-in. diameter pellets with a manual pellet press. The samples were weighed into a 10-g prefluorinated nickel cup. Ignition of the samples was accomplished using a short strand of 0.2-mil thick Teflon weighing approximately 10 mg. One end of this fuse was tied to a nickel fuse wire, and the other end was placed under the pellet. The heat of combustion of this Teflon sample in fluorine was determined in this laboratory to be -2467.7 i= 0.7 cal/g (four experiments). This value is in close agreement with the value of -2474.0 =k 0.1 cal/g reported by Domalski and ArmstrongI2 for a different Teflon sample. For all combustion experiments a fluorine pressure of 60 psi was used. Standard techniqueP were used for filling the bomb and handling fluorine under pressure. Combustion of a poly(carbon monofluoride) in fluorine proceeded very smoothly once a suitable ignition method was developed. Analyses of the bomb gases on two experiments using both infrared and mass spectral methods showed only CF4 as the combustion product. No detectable amounts of other gases (including any possible impurities in the fluorine) were detected. The limit of detectability of higher fluorocarbons on the mass spectral analysis was approximately 0.1%. The infrared analyses were performed using a gas cell equipped with CaFz windows. After each combustion (7) J. Reilly and W. N. Rae, “Physico-Chemical Methods,” Methuen and Co., London, 1954, p 557. (8) D. E. Palin and K. D. Wadsworth, Nature, 162, 925 (1948). (9) J. L. Lacina, A. J. Elzer, and J. P. Coughlin, 18th Calorimetry Conference, Bartlesville, Okla., Oct 1963. (10) W. N. Hubbard, D. W. Scott, and G. Waddington, “Experimental Thermochemistry,” F. D. Rossini, Ed., Interscience Publishers, Inc., New York, N. Y., 1965, p 75. (11) W. D. Good, et al., J . Phys. Chem., 63, 1133 (1959). (12) E. s. Domalslri and G. T.Armstrona - J. Res. Nat. Bur. stand.,
699 137 (1965). (13) W. N. Hubbard, “Experimental Thermochemistry,” Val. 11, H. A. Skinner, Ed., Interscience Publishers, Inc., New York, N. Y., 1962, 95.
3141
HEATOF FORMATION OF POLY (CARBON MONOFLUORIDE)
Table I : Combustion D a t a for Poly( carbon monofluoride)
1. m(CFs), g" 2. m(residue), g 3. Atol 4. &(calor)(- AL), calb
"c
5.
AEoontents,
6. 7. 8. 9.
AEipn, ea1 AE,,,, cal AEruss, cal AE~OIM,cal g-''
ca'l
I
I1
0.74834 0.00081 1.11270 - 3984.50 - 1.86 1.80 - 0.13 27.17 - 5288.40
0.68119 0.00088 1,01628 - 3639.12 1.70 1.80 - 0.11 36.57 - 5288.63
a small amount, of white residue weighing approximately 0.8 mg was found deposited evenly over the bottom and sides of the nickel crucible. A mass spectrum of this material heated to 200" revealed it was not volatile. An X-ray analysis showed it to be noncrystalline. Based on its distribution about the nickel crucible, this residue was assumed to be unburned sample or some solid fluorocarbon resembling poly(carbon monofluoride). A sample weight correction was therefore made, based on the weight of this residue. This procedure was judged reasonable since calculations of the heat of combustion based on a correction due to the residue being unburned Teflon showed that the difference between the two heat of combustion values was smaller than the uncertainty in the combustion experiments themselves. The results of five combustion experiments on poly(carbon monofluoride) are given in Table I. The enthalpy of combustion for reaction 2 is AH,' = - 176.26 rt 0.15 kea1 mol-'
+ 1.44Fz(g)
--t
CF4(g)
(2)
based on the currently accepted ~ a l u e ' for ~ ~the ' ~ heat of formation of carbon tetrafluoride, AHr' [CF4(g)] = -223 rt 1 kcal mol-', the heat of formation of CF1.12 is -46.7 f 1.0 kcal mol-'.
Discussion The heat of formation of poly(carbon monofluoride) calculated from the above combustion data is in fair agreement with the heat of reaction 1 reported by Porter and Smith when their value is based on the number of fluorine atoms attached to the graphite. The exact structure of poly(carbon monofluoride) is still in question, although several attempts have been made to specify the exact positions of the fluorine atoms in the CF, lattice.2~8~1e,17 The infrared C-F stretching frequency of 1215 cm-16 is strong evidence in favor of Rudorff's original structure2 of an aliphatic-type lattice involving sp3 hybridization of the carbon atoms, with each bonded covalently to a fluorine atom and three other carbons.
300
V
0.68210 0.00084 1.01600 - 3638.22 1.69 1.80 - 0.12 27.86 - 5293.02
0.67884
0.68646 0.00100 1.02062 - 3654.77 - 1.70 1.80 - 0.12 28.26 - 5282.94
0,00087 1.00811
- 3609.97 - 1.68 1.80 0.12 23.96 - 5282.56
-
An explanation of these symbols is given in ref 13.
a Mass in vacuo corrected for residue. dev of the mean = rt4.4 cal g-'.
CFi.n(s)
IV
I11
Av AE,O/M = -5287.1 cal g+;
std
4
//
100
0
,' 0
/'
c a i o r i m e t r i c data i r c m varlous sources
MOnOflUOIIOC
I
2
3
4
n
Figure 1. Heats of formation of saturated carbon-fluorine compounds of general formula CF,; AHfO = - (44n f 3nZ) kcal mol-'; 0, experimental data.
Of special interest is the manner in which the heats of formation of "saturated" carbon fluorides (CF,; n = 0, 1, 2, 3, 4) vary with the number of fluorine atoms attached to the carbon atom. By using the heats of formation of Teflon, perfluoro-%-heptane*' (AHf" = 821.2 kea1 mol-') and perfluoroethane'* (AHfO = -323.2 (AHfO= -223 kcal mol-') and of carbontetrafl~oride'~)~~ f 1 kcal mol-'), one finds (see Figure 1)that there is an approximately constant increment per fluorine atom added to the carbon as one goes from graphite to carbon tetrafluoride. Actually, one can fit the currently available data with the equation AHr"(CF,) = - (44n 3712) f 3 kcal mol-l. The heat of formation of the other known carbon subfluoride, tetracarbon monofluoride, C4F(s),19 can be estimated to be - 11 f 3 kcal mol-' by using Figure 1.
+
(14) E. Greenberg and W. N. Hubbard, J . Phys. Chem., 72, 222 (1968). (15) J. L. Wood, R. J. Lagow and J. L. Margrave, J. Chem. Eng. Data, 12, 2 (1967). (16) Struct. Rept., 11, 212 (1947). (17) A. R. Ubbelohde and F. A. Lewis, "Graphite and Its Crystal Compounds," Oxford University Press, London, 1960, p 119. (18) G. C. Sinke, J . Phys. Chem., 70, 1326 (1966). (19) W. Rudorff and G. Rudorff, Chem. Ber., 80, 417 (1947).
Volume 79, Number 9
September 1969
J. WOOD,R. BADACHHAPE, R. LAGOW, AND J. MARGRAVE
T
CF(g, valence state) = CF(s)
i STATE )
-122 kcol mole-' c
3/z E I C - C )
1
-
CF2(g, ground state) = - (CzF&(Teflon) 2n
Figure 2. Energy level diagram for carbon monofluorides (VS = valence state).
The exact nature of the bonding in this compound is unknown, and work is currently under way in this laboratory to learn more about its thermal properties. Likewise, the heat of formation of C3Fs is estimated as -414 It 9 kcal mol-' and the heat of formation of C4F10 as -516 h 15 kcal mol-'. The heat of formation of poly(carbon monofluoride) reported here indicates that the C-F bond is indeed covalent and has an energy of approximately 115 kcal mol-'. Figure 2 shows an energy level diagram for carbon monofluoride. The JANAF tablesz0 gave AHfO [CF(g)] as 74.4 kcal mol-l, but ModicaZ1has recently found AHfo[CF(g)] = 49 f 9 kcal mol-'. By combining this latter value with the heat of formation of the poly(carbon monofluoride), one calculates for reaction 3 CF(g, ground state) = CF(s)
AH, = -97 kcal mol -' while, for reaction 4
The Journal of Physical Chemistry
(4) one has AHr = - 122 kcal mol-' which is in good agreement with the exothermicity predicted for formation of three new carbon-carbon bonds (- 3/2 X 83 = - 124.5 kcal mol-'. With Modica's value for AHiO [CFZ(g)] = -39.7 i 3.0 kcal and AHfO (Teflon) = -200.6 hl.1 kcal mol-1 23 one calculates for the reaction
(3)
(5)
AH, = -60.3 kcal mol-' which is some 20 kcal mol-l less exothermic than one predicts from average bond energies. This difference is again presumed to be the energy necessary to promote the electrons in CFZ(g) to the valence-state configuration. Acknowledgment. Research in fluorine chemistry at Rice University is supported by the U. S. Atomic Energy Commission and the U. S. Army Research Office, D u r ham, North Carolina. Calorimetric studies are supported by the donors of the Petroleum Research Fund, administered by the American Chemical Society, and by the Robert A. Welch Foundation. The authors also wish to acknowledge helpful conversations with Dr. C. F. Cook. (20) "JANAF Thermochemical Tables," D. R. Stull, Ed., Clearinghouse for Federal Scientific and Technical Information, Springfield, Va., Aug 1965,No. PB-168-370. (21) A. P. Modica, J . Chem. Phys., 44, 1585 (1966). (22) A. P. Modioa and J. E. LaGraff, ibid., 43, 3383 (1965); 44, 3375 (1966). (23) D.W. Scott, W. D. Good, and G. Waddington, J . Amer. Chem. Soc., 77, 224 (1955).
