THETHERMAL DISSOCIATION OF OXYGEN DIFLUORIDE
2307
The Thermal Dissociation of Oxygen Difluoride. I. Incident Shock Waves by Jay A. Blauer and Wayne C. Solomon Air Force Rocket Propulsion Laboratory, Edwards, California 05623 (Received J u l y 16, 2067)
The thermal dissociation of OF2 behind incident shock waves was investigated with an Ar diluent in the temperature range 860-1300°K. The course of the dissociation was followed by means of uv absorption spectroscopy utilizing a light beam centered at 220 mp. The dissociation exhibits two phases: a slow initiation phase followed by an accelerating rate. Initial slope measurements gave a second-order rate constant of IC = 1014.2e-318500/RT cm3/molsec. The results are interpreted as being indicative of a chain mechanisni.
Introduction The thermal dissociation of OF2 has been the subject of at least two independent studies in the past.lI2 The over-all second-order rate constant for the reaction M
+ OFz+
F
+ OF + M
below its high pressure limit was given as
k =
1017.7e-39*Q00/RT
cm3 mol-’ sec-I
by Koblitz, et al.’ Dauerman2 also reports an activation energy of 40 kcal for the reaction. Although both of the cited studies consider the reaction as a simple cleavage of an OF bond, there is much current speculation that the actual reaction proRecently a ~~ ceeds by means of a chain m e c h a n i ~ m . ~ thorough investigation of this reaction was undertaken a t this laboratory in conjunction with studies of the OF2-Hz and OF2-CHd systems. Sufficient data have now been collected to make a valid comparison with the previously published results. The investigation proceeded by two paths. The high-temperature (ca. 860-130OoK), high-pressure (ca. 10-20 atm) region was investigated behind incident shock waves. The lowtemperature (ca. 500-65ODK), low-pressure (ca. 0-10 atm) region was investigated by means of a static reactor coupled to a mass spectrometric sampling system.
Experimental Section The shock tube, designed a t Avco Corp., Wilmington, Mass., is of stainless steel and has an inside diameter of 3.75 cm. The downstream section has a length of 7.5 m and its entire inside surface is finished t o a grade-8 smoothness. The driver, having an over-all length of 1.7 m, was separated from the downstream aection by means of a scribed-steel or aluminum diaphragm. The downstream section was, in turn, separated from a 220-1. dump tank by means of a thin sheet of Mylar. Shock detection was by means of moderate response (ca. 7 psec) piezoelectric detectors6 having a spatial
resolution of 2 mm and placed a t intervals of 76.2 cm along the entire length of the downstream section. The detector outputs were displayed on a Tektronix Model 535 oscilloscope equipped with a raster sweep and a Radionics Model TWM crystal-driven timing generator. The observation port was equipped with sapphire windows held in compression by close-fitting brass collets. Window to shock tube sealing was effected with indium wire gaskets. The windows were polished to a tolerance of 1wavelength of the Na D line. The course of the dissociation was followed by means of a once-through, single-light-path, ultraviolet absorption spectrorneter.‘j The ultraviolet source was a Beckman deuterium arc lamp. Spectral isolation was by means of a Baird 250 8. atomic interference filter centered a t 2200 Detection was by means of a Type 1P28 photomultiplier tube. The instrument has a spatial resolution of 2 mm and an overrelaxation time of about 3 psec. Argon having a stated purity of 99.998% was purchased from Matheson and subjected to mass analysis, which confirmed high purity. Gaseous OFz,purchased from Allied Chemical Corp., was filtered through a column of NaF pellets (previously purged of H F by maintaining above 350” for several hours) and condensed in a prepassivated Monel flask a t 77°K. A vacuum was then applied to the material for a t least 1 hr, after which a 0.9 fraction of the residue was slowly distilled into a second Monel flask held a t 77°K. Mass analysis of the purified gas with argon as an internal
*
(1) W. Koblitz and H. J. Schumacher, 2. Physik. Chem. (Leipzig), BZ5, 283 (1934). (2) L. Dauerman, G. Salser, and Y. A. Tajima, private communic&
tion. (3) K. J. Laidler, “Chemical Kinetics,” 2nd ed, McGraw-Hill Book Co., Inc., New York, N. Y., 1965, pp 170, 171. (4) A. F. Trotman-Dickenson, “Gas Kinetics,” Butterworth and Co. Ltd., London, 1955, p 80. (5) Purchased from Kistler Instrument Corp., Model No. 601. (6) Furnished by Rocketdyne, Canoga Park, Calif., under Contract NO. AF 04(611)-5963.
Volume 72, Number 7 July 1068
2308 standard indicated 0.7% 0% and 1.8% N, as the only significant impurities. Mixtures of OF, and Ar were prepared by pressure difference utilizing Bourdon gauges.' Prior to each test, the OF, content of the sample was determined by measurement of its optical density at 220mp with a Beckman DIG2 spectrophotometer. The results were closely approximated by Beer's law. No change in concentration was observed on standing, even for several days. All gaseous mixtures were used within 1 meek of their preparation.
Data Analysis A typical absorption trace is shown in Figure 1. These traces were extrapolated linearly to the shock front to obtain the optical densities under conditions of negligible dissociation, which conditions were described by the Rankine-Hugoniot relations. The concentration of OFt immediately behind the shock front was varied by a factor of 9 (see Table I). Beer's law describes these results closely, the temperature dependence of the extinction coefficient being illustrated in Figure 2. A small initial "blip" lasting for 30-50 psec was observed for data taken below 900°K (see Figure 1). Its presence was ignored in the extrapolation.
Table I : Incident Shock Parameters and Initial Reaction Rates OF,. %
lO'(0FiIt.'
PI,
mol/em*
etm
38 44 4G 47 48 51 55
10.0 5.0 5.0 2.5 2.5 2.5
0.1804 0.0898 0.0890 0.0434
GI 6R
5.0 5.0
12.5 14.8 14.8 14.8 19.0 12.1 12.9 11.8 10.2
Shot 00.
7.5
0.0457
0.0408 0.1320 0.OR30 0.0480
T..
lour),
' K
mOl/Cm*
R5G
0.1624 0.171 0.169 0.1G9 0.178 0.159 0,163 0.158 0.001
1003 1012 1041 1264
oar, 892 863 1290
The subscript 2 refers to the initial state of the gas (after passage of the shock wave).
During the course of the dissociation, gaseous Fzis formed. Since fluorine resulting from dissociation also absorbs radiation in the range considered, its presence must he accounted for. This was accomplished in two ways. Binary mixtures of F2 and Ar were shocked to temperatures covering the entire range of interest. This procedure gave an extinction coefficient of 9370 + 600 cma/mol, which was found to be statistically independent of temperature. A second procedure involved use of the equilibrium conditions behind the shock wave. A t all temperatures considered, OF, mas completely dissociated.8 As a consequence, the extinction coefficient for Ft can be obtained from the equilibrium voltage of the oscillogram. This procedure gave a The Journal of Physic01 Chmietru
600
800
,,OD
,000
llD0
,600
7%
Figure 2. Temperature dcpeialenee of molar extinetion coeffirients f o r I??Oat 220 mp.
*
value of 10,390 7.50 cma/mol for the extinction coefficient which was also independent of temperature. The slightly higher values in this instance can be ascribed to a slight absorption owing to O?. Other possible molecules which may absorb radiation in the spectral range of interest are 0212?, OIF, and OF. The large first-order rate constant for the decomposition of 02F2* = 5.9 x ~ o 1 ? ~ - 1 7 . K W l f sTc c - ~ assures that its conccntr:ition will remain negligibly small. Arkellla has prepared OF in an Ar matrix at liquid He temperatnres. When the system mas allowed to warm above 40°1loq6 mol/cm*). The chain termination probably occurs by recombination of F atoms and/or by interaction of OF radicals. It must be concluded that until the real nature of this decomposition has been demonstrated, no reliance should be placed upon the activation parameters derived for step 1.
Acknowledgment. The authors wish to acknowledge the useful technical criticism of Dr. Larry Edwards which was received during the course of this study. This work was accomplished as part of an in-house project of the Propellant Division, Air Force Rocket Propulsion Laboratory, Edwards, Calif. (1,) R. Gatti, E. Staricco, J. Sirce, Chem. (Leipzig), 35, 343 (1962).
H,Bohumacher,z. physik.
