3700
JOSEPH B. LEVYAND B. K. WESLEYCOPELAND
The Kinetics of the Tetrafluorohydrazine-Fluorine Reaction’
by Joseph B. Levy and B. K. Wesley Copeland Atlantic Research Corporation, Alexandria, Virginia
(Received February 45,1966)
~~~
~~
The reaction between fluorine and tetrafluorohydrazine has been investigated from 35 to 86’. Nitrogen trifluoride has been shown to be the only product. The kinetics have been measured by a colorimetric technique and have been shown to be homogeneous. It has been found that the kinetic data obey the expression -d(R)/dt = Ic(F2)(N2F4)”’and k = 1.0 x 101l.O*O-a exp(-20,400 400/RT)M-”’ set.-'. It is proposed that k = K1”k2where K is the equilibrium constant for the dissociation of tetrafluorohydrazine and IC2 is the rate constant for the reaction NF2 F2 + NF3 F.
*
+
Introduction In the course of our studies of the hydrogen-fluorine reaction2 we have become interested in finding species that would react with molecular fluorine to yield fluorine atoms a t a known rate. Among the possibilities considered was tetrafluorohydrazine. It seemed reasonable that this would react with fluorine in a manner analogous to that observed for nitrogen dioxide.3 The equilibrium
has been studied4 and the constants have been established. The reactions
+ F2 +NF3 + F
(2)
+ F + AI +NFs + M
(3)
NF2 NF2
would complete the analogy to the nitrogen dioxidefluorine reaction. We have investigated the reaction kinetics for this system and report the results here.
Experimental Part Chemicals. The fluorine used in this work was General Chemical Co. fluorine that was freed of hydrogen fluoride by passage through a potassium fluoride trap. It has been found5 that this treatment yields fluorine containing approximately 0.2 mole % oxygen as the only impurity. The nitrogen was Southern Oxygen Co. prepurified grade specified as 99.998 mole % nitrogen with the remainder oxygen. The tetrafluorohydrazine was Air Products Co. technical grade. Mass spectrographic analysis showed it to be 99.2 mole % pure, containing 0.6 mole % nitric oxide and 0.2 mole The Journal of Physical Chemistry
+
% carbon dioxide as impurities. It was used without further treatment. The nitric oxide and carbon dioxide were taken directly from the cylinders without further treatment. The former was Matheson Co. product rated as 98.5 mole % pure while the latter was Southern Oxygen Co. product with a minimum purity of 99.9 mole %. Apparatus and Procedure. The reaction rate measurements have been made by determining the fluorine concentration optically by means of its absorption a t 2849 8. The reaction vessel was a magnesium block designed to fit into the cell compartment of a Beckman DK spectrophotometer. Two cells path, 1.8 and 0.90 cm. in diameter, and 10 cm. long, machined through the block, constituted the reaction chambers. Sapphire windows were compression-sealed through Teflon gaskets to the block. The block was electrically heated and could be maintained at 1 0 . 5 O manually. Both cell paths were thoroughly passivated before use. The apparatus is described in more detail elsewhere.2 Nitrogen trifiuoride was found to be completely but tetrafluorohydrazine was transparent at 2849 found to have a small absorption, presumably due to a tail of the difluoramino peak a t 2650 As4 A correction
w.,
~
~~
(1) This work was supported by the Air Force O5ce of Scientific Research of the Office of Aerospace Research under Contract No. AF 49(638)-1131. (2) J. B. Levy and B. K. Wesley Copeland, J . Phys. Chem., 69,408 (1965). (3) R.L. Perrine and H. S. Johnston, J. Chem. Phys., 21,2202 (1953). (4) F. A. Johnson and C. B. Colburn, J . Am. Chem. Soc., 83, 3043 (1961). (5) J. B. Levy and B. K. W. Copeland, J. Phys. Chem., 67, 2156 (1963).
KINETICSOF TETRAFLUOROHYDRAZINE-FLUORINE REACTION
was applied for this absorption. Nitrosyl fluoride also showed absorption at this wave length, and, in experiments where nitric oxide was added, a correction was applied for this absorption. Our procedure was to add the reagents consecutively to the cell in the order fluorine, nitrogen, and tetrafluorohydrazine (or tetrafluorohydrazine-additive mixture). Where mixtures were used, they were prepared in a reservoir bulb and mixed by agitating a number of pieces of Teflon tubing by means of a Teflon-covered stirring bar and magnetic stirrer. The reaction began on the introduction of the tetrafluorohydrazine. Only a few seconds were required for this introduction. The fluorine concentration was recorded continuously on the recorder chart. Auxiliary experiments were performed with a magnesium cup fabricated from a cylindrical block of magnesium, 13 cm. in diameter. The inner diameter and depth of the cup were 7 and 5.75 cm., respectively. The cup was closed by a magnesium cover which was bolted to the cup through a Teflon gasket. Entry to the cup was through a 0.635-cm. magnesium tube welded to the top. A Teflon valve was attached to the tube by means of a Swagelok fitting.
Results The Nature of the Reaction. Preliminary experiments were carried out in the 221-cc. magnesium cup described in the Experimental Part. We fist sought to establish that tetrafluorohydrazine did not decompose by itself in the temperature range of our studies. This did not seem probable since equilibrium constants for the dissociation of this species have been measured between 100 and 150" without difficulty.* The possibilities of catalysis by the container walls or reaction with the container still existed however. Two experiments were performed by admitting measured amounts of reagents to the cup at room temperature, immersing the cup in a bath of boiling water for a period of 45 min., cooling it to room temperature, and examining the contents. It was found that: (a) for 50 mm. of tetrafluorohydrazine alone, no pressure change occurred, and the infrared spectrum of the contents at the end of the experiment was that of pure tetrafluorohydrazine; (b) when equal amounts of tetrafluorohydrazine and fluorine were admitted to the cell (&F, = PF2 = 50 mm. at 25"), the pressure at the end of the experiment was unchanged, and the infrared spectrum of the product was that of pure nitrogen trifluoride. Comparison of the intensity of the 1940-cm.--' peak of this product with that of a sample of pure nitrogen trifluoride at almost the same pressure showed the product to be only nitrogen trifluoride.
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The above results were taken as evidence that the stoichiometry of the reaction was that shown by steps 1 and 3. This conclusion was confirmed by examination of the infrared spectrum of the products obtained in kinetic runs where the initial fluorine and tetrafluorohydrazine pressures were equal and where the experiment had been allowed to go to completion. The infrared spectrum of the products was that of nitrogen trifluoride and the amount found corresponded to that calculated from the amount of tetrafluorohydrazine (or fluorine) present. Finally, in an experiment, see Table I, no. 9, in which the initial fluorine pressure was four times the tetrafluorohydrazine pressure, the decrease in fluorine absorbance for complete reaction agreed well with that calculated for the stoichiometry written above. Table I : Rate Constants for the Fluorine-Tetrafluorohydrazine Reaction Table no.
Expt.
Temp.,
no.
OC.
1 2
597 598 643-A 606 607 599 6 19-A 619-B 604 730 643-C 739 603 601 600 600 736
75 75 75 75 75 75 75 75 75 75 75 75 52 61 86 86 35
3 4b 5b 6 7
8' 9 10 11 12 13 14 15 16 17
-Pressures, Fz NzFd
50 50 50 50 50 50 50 50 200 50 50 50 50 50 50 50 50
50 50 50 300 300 500
mm.aDiluent ga8
k, M -l/Z eec. -1
660 (Nz)
0.0170 0.0155 0.0169 410(Nz) 0.0166 410(Nz) 0.0158 210 (Nz) 0.0159 500 210 (Nz) 0.0173 500 210 (Nz) 0.0167 50 510 (Nz) 0.0172 50 659(Nz), 1 (NO) 0.0165 0.0155 50 660(0z) 50 657 (Oz),3 (coz) 0.0173 500 210 (Nz) 0.00204 500 210(Nz) 0.00483 500 210 (Nz) 0.0394 500 .., 0.0394 0.000328 500 210 (Sz)
... ...
Measured a t the temperature of the experiment. 'These runs were made in the 0.9-cm. i.d. cell path. All the others were made in the 1.8-cm. i.d. cell path. I n this run the cell was shut off from the light beam a t all times except for the brief period needed for a reading.
The Kinetics of fhe Reaction. A series of experiments was performed at 75" to determine the order of the reaction in the two reagents, the effects of the impurities on the rate, the effect of total piessure, of surface areato-volume ratio, and of the light of the spectrophotometer beam. The temperature dependence of the rate was determined from experiments at 35, 52, 61, 75, and 86". The Order of the Reaction. In view of the chemical similarity of the tetrafluorohydrazine-fluorine system Volume 69, Number 11 November 1966
3702
JOSEPHB. LEVYAND B. K. WESLEYCOPELAND
to the nitrogen tetroxide-fluorine system, it seemed quite likely that the kinetics of the former would be expressible by -d(F,)/dt = k(Fg)(NtF4)"'. A series of kinetic experiments with varying reagent concentrations was performed at 75", and the results were plotted according to the appropriate integrated form of the above expression. Good straight lines were obtained in all cases, and the rate constants found were in good agreement. The results are shown in Table I, no. 1-12, and confirm the validity of the above expression. In the experiments containing comparatively small amounts of tetrafluorohydrazine-entries where the initial pressure was 50 mm.-calculations were made to see if the extent of dissociation of tetrafluorohydrazine was large enough to require a correction for the fraction dissociated. To do this the following expression for the equilibrium constant was calculated from available thermodynamic data6
K = 3.2 X lo8 exp(-21,500
i 1000/RT) M
The value a t 75" is K = 1.0 X M . For a pressure M), of tetrafluorohydrazine of 50 mm. (2.3 X the fraction of tetrafluorohydrazine undissociated a t 75" is 0.97; for pressures of 25 and 12.5 mm. a t 75" the corresponding fractions are 0.95 and 0.94. Since the tetrafluorohydrazine concentration appears as the square root function in the kinetic expression, the correction for the smallest of the above pressures, corresponding to 75% reaction, is only 3%. We have therefore not corrected our data for dissociation of tetrafluorohydrazine. I n Figure. 1 the data for entry 1 of Table I are plotted for the above rate expression. The data are linear to about 85% reaction. The E$ects of Impurities. The mass spectrographic analysis indicated the presence of 0.6 myle % nitric oxide and 0.2 mole yo carbon dioxide in the tetrafluorohydrazine. The fluorine contained about 0.2 mole yooxygen. Since removal of these small amounts of impurities, although desirable, would present very great diffculties, it was decided to test the belief that they were not affecting the rate by performing experiments where they were added deliberately in amounts substantially greater than were present in the other experiments. The results of such experiments a t 75" are shown in Table I in no. 10-12. In entry 10 the nitric oxide-to-tetrafluorohydraxineratio was 0.02 while in entry 12 the carbon dioxide-to-tetrafluorohydrazine ratio was 0.06. These figures compare to the corresponding figures of 0.006 and 0.002 for tetrafluorohydrazine with nothing added. I n entry 11 oxygen replaced nitrogen as the diluent gas. The results support the belief that these compounds do not affect the reaction. It may be pointed out that, alThe Journal of Ph&d
Chemistry
60
I
50
-
f 4 0
-
I
I
I
8
d
P
I h
6
/
I I
V 0
1200
2400
3600
4800
600
Time, sec.
Figure 1. Test of integrated rate expression for (F$ = (NzF~)O: t x: 75'; (Fz)= 2.21 X 10-8 M ; PFZ = 44.5 mm.
though nitric oxide is known to react with difluoramino radicals to yield nitrosodifluoramine, NFzNO,' this compound would be completely dissociated at the temperatures of our experiments. On the other hand, nitric oxide would react very rapidly with fluorines to yield nitrosyl fluoride so that the effect of adding nitric oxide is really to add nitrosyl fluoride. The E$ects of Total Pressure, Surface Area-to-Volume Ratio, the Light of the Spectrophotometer Beam. A comparison of the results of runs 2 and 3 with those of 1 and 4-9 shows that the rate is independent of total pressure in this pressure region. Runs 2 and 3 were at a total pressure of 100 mm. while the others were a t a total pressure of 1 atm. Runs 12 and 13 illustrate a smaller variation of total pressure, 550 and 760 mm., respectively, a t a higher temperature. The data at the different pressures are in satisfactory agreement. Comparison of runs 4 and 5 with the other runs a t 75" shows that surface effects in t'his reaction are negligible. The cell used in runs 4 and 5 had a surface area-to-volume ratio twice that of the other runs. This is not a large variation, but the agreement of the pertinent data is so good that it seems safe to rule out surface effects. Run 8 differed from the other runs at 75" in that the cell was shut off from the spectrophotometer beam by a shutter at all times except for the few seconds required for a reading. The agreement of the rate obtained in (6) "JANAF Thermochemical Tables," prepared under the auspices of the Joint Army-Navy-Air Force Thermochemical Panel, Air Force Contract No. AF 33 (616)-6149, Dow Chemical Co., Midland, Mich., 1963. (7) C. B. Colburn, Advan. FZuorine Chem., 3, 88 (1962). (8) D.Rapp and H. S. Johnson, J . Chem. Phys., 33, 695 (1960).
KINETICS OF TETRAFLUOROHYDRAZINE-FLUORINE REACTION
this way with the other runs at the same temperature rules out any photochemical effects resulting from the light beam. The Treatment of the Rate Data and the Precision of the Rate Constants. We have extracted the rate constants from our data by drawing a straight line visually through the points, as we have done in Figure 1. When all the experiments at 75" were completed, it was clear that the uncertainty as to the slope of any particular line was much less than the variation in the results of the different experiments. It seemed that the most likely source of this variation was the uncertainty in the temperature. The activation energy found for this reaction (see below) is 20.4 kcal./mole. It is easy to show that the fractional error in the rate constant arising from a given fractional temperature error dT/T ise Eact d T dk/k = - RT T For T = 348"K., dT = 0.5, and R = 20,000, dk/k = 0.04; i.e., the error in k is =k4%. The 12 values of k determined at 75" yield an average value of k = 0.0165 f 0.006 M-'/' see.-', where the uncertainty indicated is the average deviation and is 4% of the value of k. We therefore conclude that the errors in our measurements arise solely from the temperature error, and we report our rate constants as being accurate to h4%. The Temperature Dependence of the Rate. The data of Table I are plotted as the Arrhenius function in Figure 2. In view of the temperature uncertainty discussed above, the points are represented by horizontal lines whose length is a measure of the uncertainty in the 1/T parameters. The average value of the 75" data is plotted instead of putting all 12 data points on the curve. The points are linear enough so that a line can be drawn through them visually. The resultant expression for the rate constant is k = 1.0 X 10" exp(-20,400/lcT) M-"' set.-'. The error in EaOtarising from the h0.5 temperature uncertainty may be shown' to be
dEmt - - dT2 - dT1 Tt - TI Eact If we take T2 and TI as the extreme temperatures and assume dTz and dTl are additive (the worst case), the resultant fractional error in E is 0.02. We therefore report the activation energy as 20,400 =k 400 cal./mole. A similar treatment shows the frequency factor uncertainty to be 1 O 0 e 2 so that the general rate expression is k = 1.0 X 1011.0H.2exp(-20,400 f 400/RT) M-"* sec.-l
3703
3.0 \
. 27.6
28.6
29.6
30.6
32.6
31.6
1041~.
Figure 2. Temperature dependence of the tetrafluorohydrazinefluorine reaction.
Discussion The Mechanism of the Reaction. In addition to the steps 1to 3, we may consider the steps
F
+ NzF4 2F
4 NFa
+ NFz
(4)
+ M * Fz + M
(5)
The kinetics expression found is consist,ent with an equilibrium first step (l), followed by the slower ratedetermining step (2). Support for the belief that step 1 (the dissociation step will be referred to as step 1 and the recombination as step - 1) is fast compared to the over-all reaction rate can be found in the results of Modica and Hornig,l0who studied the rate of dissociation of tetrafluorohydrazine in a shock tube between 350 and 450°K. at pressures of 0.58 to 2.7 atm. In the presence of argon diluent,, the rate of dissociation could be expressed by
k~ = 3.0 X 108T1/'(-)'exp( 18,400 RT
- 18'400) M-l
set.-'
RT
The results for helium, sulfur hexafluoride, and tetrafluorohydrazine as the third body differed from the above by less than a factor of two so that we feel the above expression is approximately valid for nitrogen as the third body. At 348"K., the above expression yields k~ = 5.6 X lo3M-' sec.-l. For a pressure of tetrafluorohydrazine of 50 mm. and a total pressure of 1 atm. the rate of dissociation of tetrafluorohydrazine is 0.45 M set.-'. At the same temperature for 50 mm. each of tetrafluorohydrazine and fluorine our data yield 1.6 X ~
(9) B. K. Morse in "Techniques of Organic Chemistry," Vol. VIII, Part I, S. L. Friess, E. S. Lewis, and A. Weissberger, Ed., Interscience Publishers, Inc., New York, N. Y., 1961, p. 505. (IO) A . P. Modica and D. F. Hornig, Technical Report No. 4, ONR Contract No. Nonr 1858(26),Princeton University, Princeton, N. J., Oct. 31. 1963.
Volume 69, Number 11 November 1966
JOSEPHB. LEVYAND B. K. WESLEYCOPELAND
3704
M sec.-l as the rate of reaction. The rate of dissociation is thus much faster than the reaction rate, and the assumption of step 1 as an equilibrium seems justified. The above discussion has not considered step 5. It seems evident that the concentration of fluorine atoms will be so much smaller than that of difluoramino radicals that step 5 can be neglected in comparison with step 3. Thus, at 348”K., for 50 mm. pressure of tetrafluorohydrazine, the equilibrium pressure of difluoramino radicals is calculated as 2.7 mm. while for the same pressure of fluorine, the equilibrium fluomm.6 rine atom pressure would be about 2.7 X The factor of lo7should suffice to rule out step 5. The reaction mechanism therefore consists of steps 1, 2, and 3 or of steps 1, 2, and 4. From a kinetics point of view the two sequences are indistinguishable. In either case
- d(Fz)/dt
=
2k2K”Z(N~F4)’/2(Fz)
where K is the equilibrium constant in moles per liter for the dissociation of tetrafluorohydrazine, i.e., kl/k-l. We may compare steps 3 and 4. Step 3 is a termolecular reaction, and we may assume that reaction occurs on each collision. At 1 atm. the ratio of termolecular to bimolecular collisions is about 0.001. If we take the nTF2:NZF4 ratio as 2.7:48.6, i.e., 0.056, as computed above for 50 mm. and 348”K., we may write extent of reaction via (3) - k3(NF2)(F)(M) extent of reaction via (4) kq(N2F4)(F) 0.001 X 0.056 exp(E4/kt) where E4is the activation energy for step 4. In order for the above ratio to be 0.01, i.e., for reaction 4 to predominate, E4 would have to be about 1.5 kcal./mole. For the ratio to be 100, i.e., for reaction 3 to predominate, E4would have to be about 4.5 kcal./mole. We have no way of deciding between these possibilities, T h e Kinetics of Step 2. Returning to the kinetics, the rate expression derived from the temperature dependence of the rate may be written 2keK’/’ = 1.0 X 1011.0*o.2 exp(-20,400
* 400/RT) M-”’ sec.-l
The Journal of Physical Chemistry
Inserting the value of K calculated earlier yields k2 = 1.8 X lo7 exp(-9650 i 700/RT)M-l sec.-l
The frequency factor of 1.8 X lo-’ M-l set.-' corwhich is of responds to a steric factor” of about the order expected for a reaction between species of the type involved here. It is of some interest to compare activation energies and exothermicities for this and related reactions since it is widely felt that a simple linear relationship exists between these quantities.12
Table I1 Reaction
NO+Fz+NOF+F NO2 FZ -+ NOlF F NFZ FZ -+ NFa F
+ +
+ +
Exothermicity,* kcal./mole
Activation energy, kcal./mole
24.8 8.1 z t 2 21.1 =k 2
1.58 10.53 9 . 7 =k 0 . 7
Calculated from heats of formation taken from ref. 6.
The three reactions listed in the Table I1 all involve reaction of a free radical with fluorine to form an N-F bond and a fluorine atom. It is clear that no simple relationship between exothermicity and activation energy can fit the data of the Table 11. It is likewise notable that nitrogen dioxide and the difluoramino radical, which are structurally very similar even to the extent that the N-F bonds in the latter like the N-0 bonds in the former exhibit a degree of doublebond character,13show very similar activation energies despite the large difference in exothermicity. This indicates that structural factors may play a more important role than thermocheniical factors in determining reactivity in some cases. (11) 9. W. Benson, “The Foundations of Chemical Kinetics,’’ McGraw-Hill Book Co., Inc., New York, N . Y.,1960,Chapter XII. (12) N . M. Semenov, “Some Problems in Chemical Kinetics and Reactivity,” Princeton University Press, Princeton, N. J., 1968,pp. 29-33. (13) J. J. Kaufman, L. A. Burnelle, and J. R. Hamann, Abstracts, 149th National Meeting of the American Chemical Society, Detroit, Mich., 1966, p. 1J.
