T H E
J O U R N A L
OF
P H Y S I C A L CHEM.ISTRY Registered in U.S . Patent Ofice @ Copyright, 1973, by the American Chemical Society
VOLUME 77, NUMBER 12 JUNE 7, 19738
Spectrophotometric Determination of the Rate of Dissociation of Nitrogen Trifluoride behind Shock Wavesla K. 0. MacFaddenlb and E. Tschuikow-Roux* Department of Chernisfry, University of Calgary, Calgary, Alberta T2N 1N4, Canada (Received November 14, 1972)
The rate of dissociation of nitrogen trifluoride in excess argon has been determined by measuring the rate of formation of NF2 in the temperature range 1050-1390°K using a shock tube-spectrophotometric technique. In the range of 2.7-6.0 atm total pressure the reaction was found to be first order in both NF3 andi Ar concentrations. The reaction may be represented by NF3 + M NF2 F M ( k ~K ;- M ) and for small conversions, other reactions are shown to be unimportant. The temperature dependence of the bimolecular rate constants can be described by k~ (M-1 sec-I) = 1010.10 * 0.02 exp[-(30.1 f 2.3 kca'l mol-l)/RTj where it is noted that the apparent activation energy is significantly lower than the bond dissociation energy, D(NF2-F).
*
Introduction The species NF2, NF3, and N2F4 are prototypes of the classical XY2, XY3, and x2Y4 molecules. The structure determination and the physical properties of these molecules have been the subject of numerous investigations and reviews.2.3 In contrast, kinetic data on the thermal decomposition of these compounds are relatively scarce. The only decomposition studies reported hitherto are those on tetrafluorohydrazine4-6 and the NF2 radical.7-9 The kinetic parameters for the decomposition of NF3, HNF2, and ClNF2 have apparently not been reported. In this communication we report on the kinetics of the shock-induced thermal decomposition of NF3 in the presence of inert gas at temperatures in the range 10501390°K and total pressures 2.7-6.0 atm. Experimental Section A detailed description of the experimental apparatus has been reported elsewhere.6 An aluminum shock tube (80 mm i.d.), designed for cold driver operation, was used to heat dilute mixtures of NF3 in argon. In these experiments, the driver section was 305 cm and the expansion channel 417 cm long. Helium as driver gas and aluminum diaphragms were used to obtain the desired shock strengths. The incident shock velocity was measured by three pressure transducers of 1-psec risetime (Kistler, Model 603A/623F) located 40, 60, and 80 cm from the
+ +
endplate. The signals from the transducers were amplified and fed to two universal counters (Hewlett-Packard, Model 5325 A) which provided a direct readout of the transit times of the shock wave. The reaction kinetics were monitored by following the NF2 radical concentration in absorption at 260 nm. The optical observation station, placed perpendicular to the shock tube axis, was located in the vicinity of the last, pressure transducer, 36 cm from the endplate. It consisted of a xenon arc lamp (Hanovia, Model 538 C-l), a pair of' quartz lenses to spatially define the light beam, a light chopper to produce a reference signal for 100% absorption. followed by a 0.3-m plane grating monochromator (McPherson, Model 218) and a high-gain photomultiplier (EMI-9635 QB). The output of the photomultiplier, after passing through a variable load resistor, was displayed on the face of an oscilloscope and recorded photographically. (1) (a) Work supported by the Defence Research Board of Canada, under DRB Grant No. 9530-107.(b) Postdoctorate Fellow, 19711973. (2) (a) C. J. Hoffman and R. G. Neville, Chem. Rev., 62, 1 (1962);(b) A. V. Pankratov, Usp. Khim., 32,336 (1963). (3) C. 6.Coiburn, Endeavour, 24, 138 (1965). (4) L. M. Brown and B. de B. Darwent, J. Chem. Phys., 42, 2158 (1965). (5) A. P. Modica and D. F. Hornig, J. Chern. Phys., 49, 629 (1968). (6) E. Tschuikow-Roux, K. 0. MacFadden, K. H. Jung, and D. A. Armstrong, J. Phys. Chem., 77, 734 (1973). (7) A. P. Modicaand D. F. Hornig, J. Chem. Phys., 43, 2739 (1965). (8) R. W. Diesen, J. Chern. Phys., 41, 3256 (1964). (9) R. W. Diesen, J. Chem. Phys., 45, 759 (1966).
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K. 0.MacFadden and E. Tschuikow-Roux
1476
Nitrogen trifluoride of 99.8% purity was obtained from the Air Products Co. and used without further purification. Reaction mixtures of 1 and 2% NF3 in research grade argon were prepared in stainless steel tanks and allowed to mix prior to use. The temperature and density conditions behind the shock wave were calculated from the Rankine-Hugoniot equations, modified to account for the enthalpy of dissociatiom6 These calculations were made using data from the JANAF Tables.10 Absorption Coefficient of NF2. Above 500”K, the absorption coefficient of the NF2 radical has been shown to be slightly temperature d e ~ e n d e n tIn . ~ the present study, the absorption coefficient was redetermined by shock heating a 1% mixture of N2F4 in argon to temperatures between 800 and 1400°K. In this temperature range the N2F4 is completely dissociated and the NF2 concentration can be calculated from the known equiiibrium constant,5,6 since the NF2 should be stable with respect to decomposition under conditions of the experiment.7 This was confirmed by observing that the NF2 concentration reached its equilibrium value very rapidly and no further change in concentration occurred during the time of these experiments (150 psec). The absorption coefficient was evaluated from the Beer-Lambert law in the form E = 1n [HI(H - h,,)l/LlNFzI (I) where H is the deflection of the oscilloscope trace for 100% absorption produced by the light chopper; heq is the deflection due to NF2 absorption at equilibrium; and L is the optical path length. The NF2 concentration is given by [NFz] = 2a’pzl[N2F4]01, where p21 is the density ratio across the shock front, [N2F4]01 is the initial NzF4 concentration ahead of the shock, and a’ i s the fraction of NzF4 1).The absorption coefficient was found dissociated (a’ to change very little with temperature, and the average value over this temperature range, t = 603 M-1 cm-l, was used for all calculations. Kinetics. In contrast to the situation in “3, the first N-F bond in NF3 is broken more easily than the second and third. The strength of the first N-F bond is 57 kcal mol-111 while the average strength of the latter two bonds is 71 kcal rno1-1.11J2 The most likely mode of decomposition is therefore uia N-F bond scission, and the reaction is reversible
+
NF,
hM
M
NF,
+
F
+
M
TABLE I: Arrhenius Parameters Reaction
A,
M-’ sec-’
E, kcal mol-’
Ref
a
1013
35.6 52.0 15.3
106 10’0
-0 35.0
8.8 X 1O’O 6.0 X 10”
3
4 -5
3.6 4.3 2.0
6
7
x x x
9 6
b 13
a Computed from the reverse reaction (ref 13) and the equilibrium constant using data in ref 10. * k Gwas assumed to be equal to 0.1k5‘ where k j ‘ is the rate constant for NF2 radical recombination in the limit of high pressures. The latter was deduced from the reverse dissociation rate conStan:, k - 5 - (ref 6) and the equiiibrium constant using data in ref 10.
the N2F4 is converted to NF2 while very little of the NF3 decomposes. The NF2 concentration was observed to reach a maximum in less than 50 psec and then remain constant over the observation period (1 msec). A 0.5% mixture of N2F4 in argon was then heated to the same temperatures yielding identical results. Based on this evidence it was concluded that reaction 2 is not significant under the conditions of the NF3 decomposition experiments. In order to determine the rate parameters for the NF3 dissociation reaction it was next assumed (to be justified later) that over the temperature-time regime of this study reactions 3-7 were negligible in comparison to the NF3 decomposition rate. Since for reaction 1 the equilibrium constant is much less than unity over the entire temperature range, K, = k M / k - M
