THE JOURNAL OF
PHYSICAL CHEMISTRY Registered in U.5.Patent 0;BEce @ Copyright, 1971, by the American Chemical Society
VOLUME 75, NUMBER 11 MAY 27, 1971
A Shock Tube Study of the C,F,-CF, Equilibrium1 by Gary A. Carlson Sandia Laboratories, Albuquerque, New Mexico 871 16
(Received December 10, 1970)
Publication costs assisted by Sandia Laboratories
The decomposition of CnF4in excess argon has been studied in a shock tube at temperatures from 1240 t o 1600°K. Over this temperature range, a simple C2F4S 2CF2 equilibrium was found. Second- and third-law treatments of the equilibrium data gave a heat of reaction AHr0298= 68.4 =t 0.8 kcal/mol. From this, the heat of formation of CF2 was determined to be AHfo2g8= -44.5 =t 0.4 kcal/mol. Studies of the rate of dissociation of C2F4 supported the simple chemical mechanism proposed.
I. Introduction The decomposition of C2F4 has been studied by Modica and LaGraff (M-L) and by Zmbov, Uy, and Margrave (ZUM). M-L reported kinetics and equilibrium studies of the CzF4-CF2 system from 1165 to 1620"K, shock-heating CsF4 in ArZaand NZzb diluent gases. From a second-law treatment of their equilibrium data, they found AHr0298 (C2F4 --t 2CF2) = 75.5 kcal/mol. ZUM3 studied the CzF4-CFz equilibrium in a Knudsen cell from 1127 to 1244°K mass spectrometri3.0 kcal/mol from cally, reporting AHr"298 = 76.3 both second- and third-law treatments. However, the values of K , obtained by ZUM are in serious disagreement with those of RI-L. I n this paper, an independent shock-tube study of C2F4 decomposition from 1240 to 1600°K is reported. Experimental evidence is given to support a simple C2F4-CFz chemical equilibrium. New values of AHr"298 and AHfo298(CF2) are reported. The somewhat unique method of data treatment employed (the CF2 absorption coefficient used to determine species concentrations was treated as an adjustable parameter) demonstrates potential errors which may result in shock tube chemical equilibrium studies in which reactions approach completion, due to small uncertainties in the determination of species concentrations. Finally, the discrepancies in the earlier equilibrium studies of M-L and ZUM are discussed.
11. Experimental Section Apparatus. The shock tube used in this study consisted of a 150-cm driver section and a 300-cm driven section, each of 7.6-cm i.d., 0.3-cm wall seamless stainless steel 304 tubing. The inside wall of the driven section was honed to a 0.5-p finish. The final 120 cm of the driven section was altered from a circular cross section by pressing 1.9-cm wide flat surfaces on two opposing sides, allowing the use of 1.3-cm diameter flat quartz windows with minimum obstruction to the shocked gas flow. The transition from a circular cross section was accomplished over a length of 10-15 cm. The change in cross-sectional area of the pressed section was calculated to be about 0.1%. Shock-front arrival was monitored by four light-screen schlieren detectors, over a total distance of 75 cm. The signal from the first station activated a 10-RIHz counter with nixie tube readout, which recorded the arrival times at the other three detectors with 0.1-psec resolution. The change in shock velocity over the 75-cm distance was
(1) This work was supported by the United States Atomic Energy Commission. (2) (a) A. P. Modica and J. E. LaGraff, J. Chem. Phys., 43, 3383 (1965); (b) A . P. Modica and J. E. LaGraff, {bid., 45, 4729 (1966). (3) K. F. Zmbov, 0. M. Uy, and J. L. Margrave, J . Amer. Chem. Soc., 90, 5090 (1968).
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1626 within the uncertainty in the measured distances between stations (*0.2%). A 200-W Hg-Xe arc lamp (Hanovia 901 B-1) was used as the light source for the spectroscopic studies. The light was focused and passed through collimating slits on each side of the shock tube before entering a Spex 0.75-m Model 1800 spectrometer. With a 1200 line/mm grating blazed a t 3000 A, and with entrance and exit slit openings of 0.3 and 3 mm, respectively, the resolution of the spectrometer was about 30 8. The signal was detected with an RCA 1P28 photomultiplier tube, amplified, and displayed on a Tektronix 556 dualbeam oscilloscope. Linearity of detector and amplifier response was confirmed using calibrated neutral density filters. For kinetics experiments in which fast signal risetimes were expected, 0.8-mm width collimating slits were used, and a detector frequency response of 2.7 AIHz ( - 3 db) was maintained. For other experiments in which signal risetime was not critical, 1.6-mm width collimating slits and a 500-kHz low-pass filter were used to improve signal-to-noise. Shocks were generated with helium driver gas (hydrogen was used for a few high-temperature experiments), bursting 3- to 7-mil Mylar diaphragms at pressures of 60-105 psia. Crossed concave cutting blades at the inlet t o the driven section ensured full opening of the bursting diaphragms. Both the gas-handling manifold and the shock tube driven section could be evacuated to less than Torr. Evacuation to the Torr range ‘was considered adequate for the shock experiments. The gashandling manifold included two 20-1. stainless steel tanks which were used for storage of the test gas. Test gas pressures from 13 to 60 Torr were measured with a 0-20 Torr absolute pressure gauge (Wallace and Tiernan, Model 160), or with a two-tube mercury manometer (1.3-cm i.d.) read by a cathetometer. Pressure measurements were reproducible within f0.1 Torr. Sample Preparation. Tetrafluorethylene gas was prepared by zinc-debromination of C2F4Br2in a methanol bath.4 Very small amounts of Szand methanol were collected with the CzF4product in a collection trap at liquid nitrogen temperature. These were excluded by a distillation in which only the center third of the original material was saved. N o impurities could be detected in the distilled material by gas chromatographic or mass spectrometric analyses. Matheson ultrahigh purity argon (99.999pI,) was used without further purification. Mixtures of 0.74-0.78 mol % CzF4 in argon were prepared in the storage tanks at 700-800 Torr total pressure and were allowed to equilibrate for a minimum of 24 hr before use t o ensure adequate mixing.
111. Shock Wave Calculations The calculation procedure used in these studies is T h e Journal of Physical Chemistry, Val. 7 6 , X o . 11, 1971
GARYA. CARLSON modeled after that of E b e r ~ t e i n . ~Briefly, the shocked gas parameters are calculated from the shock velocity assuming ideal gas behavior, then modified iteratively to take account of the vibrational and rotational relaxation of the test gas, giving the conditions behind the shock prior to chemical reaction. Chemical reaction is taken into account by numerical integration of a set of coupled first-order different,ial equations which relate changes in temperature, enthalpy, pressure, particle velocity, and density to the degree of dissociation of the test gas. Shocked gas parameters can be obtained for any degree of chemical reaction up to complete dissociation. Changes in the shocked gas parameters with chemical reaction are not particularly sensitive to the assumed heat of reaction for mixtures containing a large excess of inert gas. For example, the temperature drop behind the shock front due to complete dissociation of a mixture of 0.75 mol % C2F4 in argon is about 80”K, assuming a heat of dissociation of C2F4 of about 65 kcal/ mol. Thus, uncertainties in the heat of reaction of a few kilocalories per mole would only change calculated temperatures by a few degrees. We have used the heat of reaction determined in this study as input to the calculations.
IV. Results Experimental studies were performed in three areas. First, the absorption coefficient of CF2 was established at temperatures where complete C2F4dissociation occurred (1800-2900°K). Second, the C2F4-CFZ equilibrium was studied in experiments run at lower temperatures (1240-1800°K). Finally, the rate of CzF4 dissociation was also studied at the lower temperatures. CF, Absorption Coeficient Determination. Ultraviolet light absorption by CF2 was chosen as the diagnostic to be used for determining species concentrations in these studies. The CF2 radical absorbs light from about 2200 to 2900 8,6,7 while C2F4exhibits no absorption in this region. It will be shown in the following section that CF2is the only CzF4decomposition product observed up to about 2400OK. Further, it is found that C2F4 is essentially completely decomposed above 1800°K. Thus, for shocks at temperatures from 1800 t o 24OO0K, one can determine the concentration of CF2 from the initial CzF4concentration and the shocked gas parameters. Assuming Beer’s law, which is reasonable with the rather complete overlapping of the CF2 rotational and vibrational levels at these temperatures, one can then establish an absorption coefficient for CF2, E C F ~ ,over a given wavelength interval and at a given (4) J. J. Daly, Jr., DuPont Organic Chemical Department, supplied the synthesis procedure as well as the CnFaBn reagent. ( 5 ) I. J. Eberstein, “Shock Waves with Chemical Reactions in Shock Tubes,” Report No. YU-PPR-SWCR-66/07, Yale University, New Haven, Conn., 1966. (6) C. W. Mathews, Can. J. Phys., 45, 2355 (1967). (7) A. P. Modica, 1. P h y s . Chem., 72, 4594 (1968).
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SHOCKTUBESTUDY OF THE C2F4-CFzEQUILIBRIUM 2 . O r , ,
,
, , ,
,
,
,
,
,
t
1.0
1800
2000
2200
2400
T
Figurt 1. Variation of 2620 A, bandpass 30 8.
ECF* with
2600
(OK1
temperature.
2800
1
I
*1 3000
Wavelength
temperature. Finding a wavelength int)erval over which ECF, is relatively independent of temperature is more difficult. Each CF2 vibrational level involved in the absorption band has a different temperature-dependent Boltzmann population and, thus, a temperature-dependent absorption coefficient. A temperatureindependent absorption would then necessarily involve a particular combination of vibrational levels for which the sum of the absorption coefficients was temperature independent. I n our study, E C F ~was fotnd toobe constant within experimental error at 2620 A (30-A bandwidth) from 1800 to 2100"K, although some temperature dependence was observe$ at higher temperatures. Figure 1 shows ECF, at 2620 A as a function of temperature. The values obtained a t temperatures above 2400°K were extrapolated back to zero time to allow for the effects of CF2dissociation. The use of E C F ~in the treatment of data will be discussed more fully in the following section. Equilibrium Study. In their shock tube studies, M-L stated that CF2 was the only C2F4 decomposition product of any consequence at temperatures below 2600"K, basing their conclusions on time-of-flight mass spectrometric analysis of the shocked gases. Our studies also support a simple C2F4 $ 2CF2 reaction mechanism. I n all experiments run below 2400"K, the CF2 absorption was constant within experimental error after the initial period of chemical relaxation. Figure 2 shows an experimental trace in which light absorption by CF2 was monitored following shockheating of a typical tetrafluoroethylene-argon mixture. Following the shock front passage a t t = 50 psec, dissociation of C2F4 into CF2 occurred until equilibrium was reached at about t = 100 psec. The constancy of the CFZabsorption after t = 100 psec indicates that no side reactions of either CF, or CzF4occurred later in time. Moreover, kinetics studies which will be discussed later show that no side reactions occurred in these experiments during the initial period of chemical relaxation. Finally, the very good results obtained by analyzing the equilibrium data on the basis of a simple
100 80 3 60 I
' -t
1
40
20
C
0. ~
100
0
200 3w t (psec)
400
Figure 2. CFZabsorption a t 2620 A, equilibrium data: a, light intensity before shock arrival; b, equilibrium intensity; c, baseline. At equilibrium, the temperature was 1495"K, and 81% of the C2F4 had dissociated.
dissociation-recombination mechanism confirm the validity of the assumpt,ion.* The analysis of the equilibrium data is described below. The equilibrium CF, concentration in each experiment was determined from the absorption trace using Beer's law. The equilibrium C2F4 concentration was obtained from the initial C2F4 concentration (taken at the equilibrium temperature and pressure), reduced by half the equilibrium CF2 concentration. Since the trend of ECF* below 1800°K was not clearly established in the high-temperature studies, a temperature-dependent form of E C F ~was used as an adjustable parameter in the Beer's law calculations. E C F ~=
a
+ b(1800 - 5") cc/mol cm
A number of values of a close to 1.60 X lo6,the approximate value of E C F ~at 1800"K, were tested with both positive and negative values of b. For each combination of a and b values tested, equilibrium constants calculated as
Kp
(PCFJ~/PC~F~
were used to evaluate AHr02Q8 by both second- and third-law methods. The second- and third-law results were then examined separately, and "best" values of AHroZ98 were determined. We shall report the results obtained using b = 0, giving ecFz = a, although certain other combinations of a and b, giving both positive and negative slope to ECF,, gave heats of reaction nearly identical with those reported. In the second-law treatment, the van't Hoff equation was used. In Kp = -AW,"T/RT
+ AS,"T/R
A plot of In Kp us. 1/T was made for each value of E C F ~tested, and the plots were visually investigated (8) Although thermodynamic data indicate that F, CF4, CzFz, and
C2Fa should also be important equilibrium species under these condi-
tions, they are apparently not formed due to the short reaction times in the shock tube. The Journal of Physieal Chemistry, Vol. 76, No. 11, 1371
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GARYA. CARLSON 101
t
6.0
I
6.5
I
7. 0
I
7.5
I
8.0
Table I : CzFa Equilibrium Shock Tube Data (PIIs the Initial TesG Gas Pressure, arid olequil the Degree of Dissociation a t Equilibrium)
8. 5
U T x lo4 PK-*l
Figure 3. A second-law plot of equilibrium data for the CpFa-CFz system.
for deviations from linearity. The change in the heat of reaction with temperature is sufficiently small over the experimental temperature range that any noticeable nonlinearity of the plots was an indication that the plotted data were Incorrect. Significantly, although the range of values of C C F ~tested caused changes of several kilocalories per mole in the calculated heats of reaction, nonlinearity was only observed in data taken above 1600°K where dissociation was 95% or greater, so that small changes in E C F ~caused large relative changes in P C 2 F , and, thus, in K,. The implication is that if only data points below 1600°K had been analyzed, there would have been no basis for choosing a "best," straight line data fit. Since the experimental uncertainty was large above 1600"1
