3168
JOSEPH B. LEVYAND B. K. W. COPELAND
The Kinetics of the Mydrogen-Fluorine Reaction.
111.
The Photochemical Reaction1
by Joseph B. Levy and B. K. W. Copeland Kinetics and Combustion Group, Atlantic Research Corporation, Alexandria, Virginia (Received February 6 , 1968)
Mixtures of hydrogen, fluorine, oxygen, and nitrogen can be prepared which are stable in the dark a t room temperature. On irradiation of such mixtures with a 3130-A light, hydrogen and fluorine react a t a measurable rate. The effects of light intensity, reagent pressures, oxygen pressure, and total pressure on reaction rates a t 15" have been measured. The results show that oxygen inhibits the reaction through the step H O2 M + H02 M and allow an evaluation of the energy of activation for the step H Fz+ H F F as 1.5 & 0.3 kcal mol-'. Earlier results on the thermal oxygen-inhibited reaction are reinterpreted in the light of the present results.
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Introduction We have been studying the kinetics of the reaction of hydrogen and fluorine a t reactant pressures of about 20-80 torr and have recently reported on the oxygeninhibited reaction in the temperature range 122-162O.2 We report here results of our study of the photochemical reaction at 15" in the same general reactant-pressure region. Experimental Part Chemicals. The fluorine used in this work was fluorine (General Chemical Co.) that was freed of hydrogen fluoride by passage through a potassium fluoride trap. The nitrogen was prepurified grade (Southern Oxygen Co.). Mass spectrographic analyses$showed it to contain 0.1% of oxygen as the only impurity. The hydrogen was dry electrolytic grade (Southern Oxygen Co.) containing no more than 0.2% impurities, consisting of nitrogen and oxygen. The helium was Southern Oxygen Co. Grade A, specified as being 99.99% pure, The hexafluoroethane was Freon 116 (Matheson Co.) specified as being 99.6% minimum purity. The Reaction Cells. The reaction cells were fabricated as in our earlier work2p4except that the cell bodies were of aluminum rather than magnesium. Two cells were used, each had a 4.41-cm i.d.; one was 10 cm long and the other was 20 cm long, Temperature control was achieved by circulating water from a thermostatically controlled bath through copper tubing wrapped around the cell body. The runs reported here were performed at 15 f 0.5". The Irradiation Apparatus. In the early part of this work, the light from a mercury arc lamp was passed through a solution, containing nickel sulfate and cobaltous sulfate, which cut off all light below about 3000 A.6 This arrangement was later replaced by a system utilizing a monochromator in place of the filter solution. The Journal of Phvsical Chemistry
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All the data reported here refer to this later arrangement, unless otherwise specified. I n the later arrangement, the light from a mercury arc lamp was focused on the entrance slit of a 33-86-40 Bausch and Lomb grating monochromator by means of two quartz lenses. The monochromator was set at 3130 A. The emergent beam was collimated by a quartz lens and the collimated beam passed through the cell, filling it. A quartz lens downstream of the cell served to focus the light onto an RCA 935 phototube. The output of this tube was continuously monitored by a microammeter. The monochromator, collimating lens, reaction cell, focusing lens, and phototube were all mounted on an optical bench. The initial experiments were performed with a Hanovia 100-W mercury arc lamp; the subsequent experiments which comprise the bulk of the results utilized a Westinghouse 1000-W mercury short-arc high-pressure lamp. Procedure for Making a Run. The general procedure for making a run was to evacuate the cell and to admit into it oxygen, fluorine, nitrogen, and hydrogen in the order named. This was accomplished by using a manifold constructed from 0.25411. heavy-wall Teflon tubing, which used Teflon tees and valves which were attached by Teflon fittings. At one end of the manifold was a glass mercury manometer with the exposed side of the mercury covered by a layer of fluorolube oil. A tilting McLeod gage was attached to one of the manifold valves. (1) This work was supported by the Air Force Office of Scientific Research of the Office of Aerospace Research under Contract No. AF 49(638)-1131. (2) J. B. Levy and B. K. W. Copeland, J . Phys. Chem., 69, 408 (1965). (3) We are indebted to Dr. Robert Nugent of this laboratory for thia analysis. (4) J. B. Levy and B. K. W. Copeland, J . Phys. Chem., 69, 3700 (1965). (5) M. Kasha, J. Opt. SOC.Amer., 38,929 (1948)
THEKINETICSOF
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3169
HYDROGEN-FLUORINE REACTION
Oxygen pressures that were of the order of a few torr were measured using the McLeod gage, which had a 0-10-torr range. Higher pressures were read on the manometer. ‘The other gases were added by building the pressure in. the manifold up to an appropriate value with the cell valve closed and then admitting the gas to the cell, while building the pressure up to the desired final value. The manifold was evacuated and flushed with the next gas to be added before each addition. The fluorine pressure was measured optically a t 2850 A in a Beckman DU spectrophotometer whose cell compartment had been modified to allow insertion of the reaction cell. By adding oxygen first of all, it was possible to measure low pressures of it accurately with the McLeod gage. By adding fluorine before hydrogen it was possible to see if any dark reaction occurred by checking the 2850-A absorption. It was necessary t o add nitrogen before hydrogen to prevent reaction in the dark (see the Results). The order of addition was set by the above considerations. In making ab run, the cell was brought to the desired temperature, filled with the gases as described above, and was allowed to sit at least 15 min in the darkened laboratory. The voltage was adjusted on the lightsource power supply to bring the reading on the phototube to a predetermined value, This seemed to keep the light intensity reproducible from day to day. The absorption at 2850 A was then checked to see if any reaction had occurred. For all the runs below, no reaction occurred in this period. The cell was then put in place on the optical bench with a shutter blocking the light beam. ‘The shutter was removed and the timer simultaneously was started. After the desired irradiation period, $he shutter was replaced and the absorption a t 2850 d was measured in the Beckman spectrophotometer. The readings in the spectrophotometer were constant, which showed that the reaction ceased when the irracliation ceased. The above procedure was repeated as desired. The Beer’s llaw curve for fluorine was constructed by measuring the absorption at pressures ranging from 0 to 200 torr. A pressure of 20 torr in the 20-cm cell resulted in a value of loglo/l = 0.160. The 10-cm cell gave values in. agreement with those of the 20-cm cell. The constant was checked each day at a few points and was found to be reproducible. I n the experiments reported here, the fluorine pressures never exceeded 100 torr. The Actinometric Measurements. Light-beam intensities were measured by means of uranyl oxalate actinometry.6 A Pyrex cell, with quart5 faces cemented on, placed downstream of the reaction cell, was filled with the actinometer solution. Phototube readings showed that the actinometer solution absorbed >99% of the light.
Results The Dark Reaction. We have found in earlier worka that, for certain compositions of fluorine, hydrogen, helium, and oxygen, stable mixtures were obtained a t room temperature. Thus mixtures prepared from partial pressures of fluorine, hydrogen, and oxygen a t 50 torr each and a partial pressure of helium of 610 torr (to make the total pressure 1 atm) were stable at 25’. For lesser amounts of oxygen with the total pressure maintained a t 760 torr with helium, the reaction occurred a t a rate that increased as the proportion of oxygen decreased. We have examined the question of preparing such mixtures in somewhat greater detail prior to conducting the photochemical experiments. The experiments were performed in a darkened room, but the cell was exposed to the light beam of the spectrophotometer. I n these experiments the cell was filled with fluorine to a partial pressure of 50 torr, an inert gas was added to some pressure, and hydrogen was admitted. Where no reaction occurred as the hydrogen was admitted, its partial pressure was built up to 50 torr and the fluorine absorption was monitored to see if reaction occurred. The results are shown in Table I. Table I: The Dark Reactions of Hydrogen and Fluorine a t 15” (I
Run
Pressure of inert gas, torr
Inert gas
154-A
None
...
154-B
Helium
500
155 154-C
Helium Nitrogen
1544 154-E 154-F
Nitrogen Nitrogen Hexafluoroethane
400 660 100
154-G 154-H 162
Hexafluoroethane Hexafluoroethane Oxygen
200 400 50
a
1470 300
Observations
Fluorine consumed as hydrogen admitted Fluorine consumed as hydrogen admitted Blow reaction Fluorine consumed as hydrogen admitted No reaction No reaction Explosive reaction as hydrogen admitted Very slow reaction No reaction No reaction
= 50 torr; a 10-om reaction cell was used.
The above results are essentially qualitative in nature, but they establish that the presence of inert gas acts to stabilize hydrogen-fluorine mixtures and that the order of decreasing effectiveness is hexafluoroethane > nitrogen >> helium. Although hexafluoroethane is more effective on a volume basis than nitrogen, it is less available and, in addition, yielded carbonaceous (6) C. R. Masson, V. Boekelheide, and W. A. Noyes, Jr., “Technique of Organic Chemistry,” Vol. 11, 2nd ed, A. Weissburger, Ed., Interscience Publishers, New York, N. Y., 1966, pp 289-299.
Volume 79,Number 9 Xeptember 1968
JOSEPH B. LEVYAND B. K. W. COPELAND
3170 products in the case where an explosion occurred. We, therefore, chose nitrogen as our inert gas. It may be pointed out that since oxygen inhibits the reaction,2 it should be possible to prepare mixtures richer in fluorine and hydrogen and leaner in the inert gas than indicated by Table I if oxygen is present. The Photochemical Reaction. The kinetics of the photochemical reaction have been measured in detail a t 15". The effects of reagent concentration, light intensity, and total pressure have been determined. The results are presented below. The Nature of the Reaction Products. We have demonstrated in our studies of the thermal oxygeninhibited reaction of hydrogen and fluorine that hydrogen fluoride was the only reaction product. We have not performed additional analytical experiments but assume that this is so for the photochemical reaction a t 15". This assumption is supported by the absence of the rate acceleration that would be expected if oxygen were consumed in any way as the reaction progressed. The Effect of the Hydrogen Pressure. The dependence of the rate on hydrogen pressure was one of the first aspects of this reaction that we investigated. This was done by keeping all other parameters constant and varying the hydrogen pressure. The results of a set of experiments are shown in Table 11. Data for two duplicate experiments with a pressure of 100 torr of hydrogen and for experiments in which the hydrogen pressure was increased by factors of 2 and 4 are shown. There is some scatter in the data, but it is clear that the rate is independent of the hydrogen pressure. The above experiments were performed with the 100-W lamp. Data obtained with the 1000-W lamp, see below, also show that the rate of reaction is independent of the hydrogen pressure. The Effect of Oxygen Pressure. The general inTable I1 : The Effect of Hydrogen Pressure on the Reaction Rate" Time, min
-
Pnz, torr
loo--------.
200
Per cent reaotion
0
0.5 1.0 1.5
2.5 3.0 4.0 5.0 8.0 10.0
0
5.7 10.9 14.4 21.9 24.2 29.0 32.9 42.8 46.8
0
7.0 12.2 15.6 24.0
28.6 33.7 37.5 47.8 50.1
0 7.3 13.0 16.5 23.5 26.5 31.5 35.8 48.6 51.6
-400
--r
0 5.7 10.7 14.7 20.5 24.3 29.8 34.5 42.8 49.2
0 7.2 12.9 17.0 19.0 25.8 31.2 35.5 46.3 51.0
" The temperature was 15'; the cell length was 10 cm; the light source was a 100-W Hanovia lamp; the light was filtered through a nickel sulfate-copper sulfate filter solution; the total pressure was made up to 760 torr with nitrogen. The pressures of fluorine and oxygen were 100 and 5.2 If 0.1 torr, respectively. The Journal of Physical Cherniatry
0.1
4
0
8
20
12 16 TIME (mlnules)
24
Figure 1. The dependence of rate on oxygen pressure.
On = 5.0 TORR. B ~ L A N C ENITROGEN TO n TEMPERATURE = 15'C
0.2
0
2
I
I
4
6
1
8
1
10
o TORR. 1
l
12 14 I6 TIME (minutqs)
1
18
1
1
~
20 22 24
1
26
Figure 2. The dependence of rate on light intensity.
hibiting effect of oxygen on the reaction is shown in Figure 1. The curves show, qualitatively, that the inhibiting effect is most pronounced for small amounts of added oxygen-the drop in rate in going from 2.2 torr of 0 2 to 5.0 torr is much more pronounced than that in going from 20 to 100 torr. The Effect of Light Intensity. Experiments were performed with the 1000-W lamp and the 20-cm cell in which wire-gauze screens were introduced between the monochromator and the cell to reduce the light intensity to correspond to appropriate values as indicated by the phototube. The results are shown in Figure 2 and the dependence of rate on light intensity is evident. A fourfold range of light intensities is represented. Tangents were taken to the curves at equal fluorine concentrations in order to investigate the quantitative dependence of rate on light intensity, If the Beer's law expression is written Io/I = 10r(Fz), where E is a constant for a given cell, then I , = Io(l and at equal fluorine concentrations I , is directly proportional to IO. For a system where Beer's law is obeyed, I , is thus directly proportional to Io. The dependence of the rate on Io,hence on I,, is shown in Table 111. The data are compared in Table I11 by calculating the value of n in the equation VI/Vz = (IdIz)",where VI and V2 refer to the instantaneous rates at equal fluorine pressures. Values of n were calculated using the data for the highest intensity in combination with
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HYDROGEN-FLUORINE REACTION
Table I11 : The Dependence of Rate on Light Intensity' Io = 9.45 PFa,
torr
93.2 84.8 79.0 67.8 56.5
>:
IO =
1015 3.90
x
2 , :26 1.41 1 ,107 1.02 0.43
1.80
iola
quanta quanta 8eo-1 see -1 dPFa/dt, torr/min
0.88 0.59 0.48 0.34 0.14
20 cm CELL F i 5 Hi = 20 TORR.
IO
x
5.0 TORR. BALANCE NITROGEN TO TOTAL PRESSURE TEMPERATURE * 15'
O2
IOlfl
n
quanta see-1 dPsz/dt, torr/min
n
1.1 1.0 0.9 1.3 1.3
0.34 0.24 0.20 0.13 0.085
1.1 111 1.0 1.3 1.1
-
0.8
160 TORR,
2 = 5.0 a The initial pressures were Fz = Hz = 100 torr, 0 torr, and Nz = 555 torr at a temperature of 15'. The cell length was 20 cm and the light source was a 1000-W lamp.
the corresponding values for each of the lower two densities. We conclude from the above that the rate depends on the first power of the absorbed light. The E f e c t o j Total Pressure. A series of four experiments was performed at 15" with standard pressures of fluorine and hydrogen equal to 20.0 torr, an oxygen pressure of 5.0 torr, and various pressures of nitrogen to yield total pressures varying from 95 to 760 torr. The results are shown in Figure 3. The data have been plotted as ( F z ) / ( F J o . It is clear that the reaction rate increases as the total pressure is decreased. Experiments at Other Temperatures. We have attempted to perform experiments a t temperatures above 15" and below. It was clear that the reaction would not have a largle temperature coefficient, so that it seemed necessary to have temperature intervals of significant magnitude if the rate change were to be significant. Accordingly, experiments were attempted at 50". These were abandoned because reaction was observed in many cases in the dark for mixtures similar to those used in the experiments at 15". Experiments were then performed at -42". Here a new phenomenon was observed. On irradiation of reaction mixtures, the optical density at 2850 A decreased, but on standing in the dark the optical density increased again. For example, for a run with initial pressures of 20 torr each of hydrogen and fluorine, 5.3 torr of oxygen, and 715 torr of nitrogen, a 6-min irradiation period yielded a fluorine concentration corresponding to 26% reaction; the absorption increased as the cell sat in the dark and leveled off at a reading corresponding to 14.5% reaction after 3 tnin. This phenomenon was observed repeatedly. It can only be explained on the basis of the photochemical formation of a species that decomposes in the dark to give back fluorine and which absorbs the 2850-A light less strongly than fluorine does. We cannot identify this species a t this time, but, because of its intrusion we have not pursued the kinetics a t this temperature. We emphasize that no evidence for a species of this sort in the 15" experiments was ever found.
380 TORR. 190 TORR.
0.1
o
a
4
6
a
io
12
14
16
TIME (minutes)
Figure 3. The dependence of rate on total pressure.
It is worth reporting the results of some collateral experiments a t -42" in which fluorine and okygen alone were irradiated. In these experiments a species more intensely absorbing at 2850 A than fluorine was formed, and this species decayed in the dark to give back fluorine. This species was shown to be dioxygen difluoride by measuring its extinction coefficients. These measurements were made in the following way. A mixture of 20 torr each of fluorine and oxygen was irradiated for 1 hr at -42", and the cell contents were pumped through a Teflon-tubing trap cooled in liquid nitrogen. A reddish orange solid which melted to a reddish orange liquid was observed in the trap. This is characteristic of FzOz. This material was readmitted to the cell, was allowed to decompose, and the absorption at 2850 A was measured. The reading corresponded to 1.5 torr of fluorine. From the initial absorption due to fluorine, the increase in absorption on irradiation, and the final value, an approximate extinction coefficient of 8.5 (defined as r in I = Io X with p in atmospheres and d in centimeters) at 2850 was calculated. Similar readings weroe made over a range of wavelengths from 2250 to 3500 A. The resulting curve of absorption vs. wavelength agreed well with that reported by Brodersen, Frisch, and Schumacher for FzOza7These results led us to conclude that dioxygen difluoride is formed by the irradiation of oxygen-fluorine mixtures at -42". We emphasize here that irradiation of such mixtures at 15" results in no observable changes. The Dependence of the Rate on the Fluorine Concentration. The Mechanism of the Reaction. Discussion of
d
(7) P. H. Brodersen, P. Frisch, and H. J. Schumaoher, 2. Physik. Chem. (Leipzig), B37, 26 (1937).
Volume 78, Number Q September 1968
JOSEPHB. LEVYAND B. K. W. COPELAND
3172 the dependence of the rate on the fluorine concentration is complicated by the fact that the fluorine concentration can enter the rate equation in two ways, i.e., as a result of the dependence of I, on fluorine and as a result of fluorine being involved directly in the kinetics as a reacting molecule. It is most convenient to discuss the effect on the rate of the fluorine concentration, or, what is more pertinent, of the fluorine to oxygen ratio, in terms of a reaction mechanism. The evidence which has been presented above placed certain restrictions on any mechanism, and the steps which seem most plausible suggest themselves rather readily. For these reasons we present a mechanism for the photochemical reaction a t this point and consider thereafter how the predictions of the mechanism concur with our results. The mechanism is
+ light -% 2F F + H2kl, H F + H H + F2 -% H F + F H + + M -% HO2 + M HO2 + F 2 k l ” F + +F F + HO2 -% H F + F2
0 2
(1) (2) (3)
(4)
For the case of the inequality 8, the rate expression found is
(9) This is consistent with our results on the lack of dependence on (Hz), on the direct dependence on I,, and on the inverse dependence on (Oz). We, therefore, proceed to examine the expression quantitatively and to postulate inequality 8 for the photochemical reaction at 15”. Quantitative Treatment of the Data. In order to examine the validity of expression 9 from a quantitative viewpoint, it is necessary to consider the dependence of I , on fluorine concentration. We write the Beer’s law expression for fluorine in the 20-cm cell in our irradiation apparatus as
Io log - = P(F2)
I
where the units of concentration are moles per liter and the units of P, the proportionality constant, are liters per mole. It then follows that
I, = Io(1 - lo-@(Fz) 1
(10) Incorporation of this expression for I, into expression 9 0 2 (6) yields a differential rate equation containing the fluorine concentration as the only variable, but one that Step 5 has been postulated earlier2 to explain our cannot be integrated to give a useful expression. results with the thermal reaction a t 132-162”. Step 6 We have determined the relationship between I, and has been chosen in preference to other termination (F2) by measuring I o , the intensity of the light beam steps that might be written for HO2 because, as will be passed by the empty cell, and values of I at fluorine shown below, it is required for the results at the higher pressures in the cell of 20 and 50 torr. From these temperatures. This is not compelling evidence for this values we calculate the Beer’s law constant, and from step at 15”; however, since any step that destroys HO2 Beer’s law we calculate I, as a function of (Fz) from 0 to without continuing the chain or consuming oxygen gives 50 torr. Three determinations of Ioyielded an average the same result, we write step 6 for the sake of simvalue of 1.06 X 1017 quanta sec-l, with an average plicity. From the above mechanism the reaction rate deviation of *0.07 X 1017 quanta sec-l. This value is has been multiplied by 1.05 to allow for reflection losses,8 so that we set lo = (1.11 0.07) X 1017quanta sec-’, einstein la-’ min-’. The equivalent to 3.69 X Beer’s law constant calculated from the intensities at k6(Fz)(H02) ks(F)(HO2) 1 20 and 50 torr was (3 = (88 8) 1. mol-l. The plot of I,, calculated using the Beer’s law conIt is possible to obtain a useful rate expression from stant, us. (Fz) was found to be only slightly curved and the above mechanism only for the two limiting cases was very closely approximated by a straight line for (see the Appendix) (Fz) 5 1.8 X 10-3 mol. I.-’ (30 torr). For this region ) of concentration, therefore, the relation I, = ~ ( F zcan ks(F2) (HOz) >> ka(F)(HOz) (7) be introduced, where y is the empirical slope obtained and from the linear plot. Since the introduction of this expression for I, leads to considerable simplification in h ( F ) 0 3 0 2 ) >> ks(Fz) (HOz) (8) treatment of the data, we have made use of it in our For the case of the inequality 7, the rate expression for quantitative treatment of runs where fluorine pressures (Fz) >> (02) shows a dependence on (F~)(Hz)~”I~”’,did not exceed 30 torr. and this is contrary to our results with regard to the dependence on (Hz) and on I,. For this reason we re(8) J. G . Calvert and J. N. Pitts, Jr., “Photochemistry,” John Wiley and Sons, Ino.,New York, N. Y., 1966, p 794. ject inequality 7. 0 2
+
The Journal of Phvsical Chemistry
(5)
*
THEKINETICB OF
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HYDROGEN-FLUORINE REACTION
3173
The reaction rate then becomes
which may be integrated9 to obtain
+
2
4
10
6
TIME (mlnutes)
where a = (F2)0/(F2), and r = (F2)0/(02). The exponential may be replaced by the first two terms of the Taylor expansion, i.e., by 1 - y t , to give, withrearrangement
It should be pointed out here that the value obtained actinometrically for y was 0.00465; the third term in , a contribution of the expansion, ( ~ t ) ~ / 2represents about 0.1% for t = 10 min and 0.4% for t = 20 min. Since a time of 10 min in most cases represented a considerable reaction, the neglecting of terms higher than the second in the expansion was deemed justifiable and data for the first 10-20 min of reaction were plotted. The results for three representative experiments, plotted for expression 12, are shown in Figure 4. The data are obviously linear and the agreement of the values of k&(M) derived from the slopes is satisfactory. We have treated our runs where the initial fluorine pressure was
