79
THEKINETICSOIFDISSOCIATION OF HYDROGEN FLUORIDE
The Kinetics of Dissociation of Hydrogen Fluoride behind Incident Shock Waves by Jay A. Blauer Air Force Rocket Propulsion Laboratory, Research and Technology Division, Air Force Systems Command, Edwards, California 93683 (Received April 80, 1967)
The rate of dissociation of H F behind incident shock waves has been studied in the temperature range of 3700-6100°K. The course of dissociation was followed by monitoring the emission intensity of the 1-0 band of H F centered at 2.5 p . The experimental results gave values of Icl = (0.47 X 1019)T-' exp( - 134,10O/RT) and kz = 2 X 10l2exp( -35,000/ F M and H RT) for the bimolecular rate constants of the reactions H F M = H H F = H2 F, respectively. The rate of the fluorine exchange reaction H F F = H Fz was found to be of no consequence to the present study, even though large amounts of Fzwere added to the gaseous mixtures used.
+
+
Introduction The dissociation of H F can proceed by at least three separate paths
+M = H +F +M H F + H = H2 + F H F + F = H + Fz
€IF
(1)
(2) (3)
The exchange reactions 2 and 3 will of course be followed by the reactions
I&+
M =H
+H +M
(4)
and
Fz
+M =F+F +M
(5)
+ +
+
+ +
system, their data have not yet been reduced to actual rate constants for any of the reactions listed above. The choice of an infrared-emission technique for following the course of the dissociation was made possible by the results of Ma1kmuslo concerning the infrared emissivity of H F at temperatures between 1800 and 7000"K, and optical densities between and 10 atm cm. Calculations based upon these results show that if the experimental conditions are held within the boundaries for temperature and optical density of 35007000°K and 0.01-0.5 atm cm, respectively, the emission intensity of the gas will be directly proportional to the partial pressure of H F in the reacting mixture. Furthermore, this proportionality is found to be independent of temperature in the range cited. The maximum error incurred in the assumption of this proportionality within the prescribed range of experimental conditions is 15%.
Data concerning the rates of reactions 4 and 5 have appeared in published form in numerous and were not subjected to further investigation in the Experimental Section present study. The shock tube designed by Avco is of stainless steel When this investigation was initiated, no published and has an inside diameter of 1.5 in. The over-all data were available concerning the dissociation rate of length of the test section is 25 ft, and the entire inside HF, although two other studies were under way in other laboratories.*I6 The results of Jacobs, et aL13 have since appeared in published form. Jacobs pro(1) J. P. Rink, J . Chem. Phys., 36, 262 (1962). ceeded by adding various amounts of Hz to the gaseous (2) G. Careri, ibid., 2 1 , 749 (1953). mixtures under investigation, with the intent of ob(3) T. A. Jacobs, R. R. Geidt, and N. Cohen, ibid., 43, 3688 (1965). (4) D. Britton and C. D. Johnson, J . Phys. Chem., 64, 742 (1960). taining definitive values for the rates of reactions 1, 2, (6) R. L. Oglukian, Technical Report No. AFRPL-TR-65-152, and 4. In the study being reported herein, various Oct 1965. amounts of Fz were added to the gaseous mixtures in (6) J. F. Spinnler, "Quarterly Progress Report in Interior Ballistics," an attempt to situdy not only reactions 1 and 2, but also No. P-64-22, Defense Documentation Center No. AD355643, Rohm and Haas Co., Huntsville, Ala., Nov 1964. reaction 3. Jacobs has suggested' that, based upon (7) Personal communication. his results, the addition of large amounts of F2to the (8) J. B. Levy and B.K. Copeland, J . Phys. Chem., 67, 2156 (1963). gaseous mixture should permit a study of reaction 3. (9) J. B. Levy and B. K. Copeland, ibid., 69,408 (1965). Although Iiwy and CopelandspQhave reported in(10) W. Malkmus, General Dynamics-Convair Report, No. ZPhvestigations of the kinetics of the hydrogen-fluorine 119, 1961. Volume 76,Number 1 January 1968
JAYA. BLAUER
80 surface is finished to a grade-8 smoothness. The observation port is equipped with sapphire windows held in compression by close-tolerance brass collets. Window-shock tube sealing is effected with indium wire gaskets. The driver, having an over-all length of 66 in., was separated from the downwind section by means of scribed diaphragms of cold-rolled steel. The downwind section was in turn separated from a 55-gallon dump tank by means of a thin sheet of Mylar." Shock detection was by means of ionization probes having a spatial resolution of 1 mm and placed at intervals of 30 in. along the entire length of the downwind section. Shock detectors were also placed 5.25 in. from the center, on each side of the observation port. The outputs of all the probes were fed to a Tektronix-535 oscilloscope, equipped with a raster sweep and Radionics Model TWM crystal-driven timing generator. Infrared radiation was detected and its intensity measured by means of an indium antimonide detector12 used in a photoconductive mode, Signal amplification consisted of two stages of capacitance-coupled cascade amplification with cathode followers for interstage coupling and as output stages.13 Spectral resolution was by means of a Perkin-Elmer prism monochromator. Wire screens of known blocking efficiency for radiant energy were placed between the monochromator and a blackbody source of radiant energy to test the linearity of the response of the analytical system to radiant intensity. The linearity of response was found to be good within 5% over the entire range of radiant intensity considered in this study. After amplification the detector output was fed to a Tektronix Model 535 dual-trace oscilloscope. The data were recorded on 3000-speed Polaroid film by means of a Fairchild Type F29.6 camera. Time markers and calibrated voltage markers were placed on the film by means of a Radionics Model TWM crystal-driven timing generator and an SRC (Systems Research Corp.) Model 3512B dc power supply, respectively. Argon, hydrogen, and hydrogen fluoride, having stated purities of 99.998%, 99.998%, and 99.9%, respectively, were purchased from Matheson and used without further purification. Mass spectrometric analysis, using argon as an internal standard, indicated the presence of less than 0.01% Oz, 0.03% Nz, and trace amounts of SiF, as the only impurities in the HF used, and only trace amounts (ca. less than 0.01%) of Hz0 and M2 in the Ar and Hzused. Gaseous Fzwith a minimum purity of 98.2% was purchased from Allied Chemical Corp. A mass analysis revealed the presence of 0.7% O2 and 0.2% HF as the only significant impurities. After passage through a column of NaF pellets, the gas was used without further purification. Mixtures of Ar, HF, H2, and Fz which contained 04% HF, 0-0.32% H2, and 0-6% Fzwere prepared and stored in stainless steel tanks, which had been passiThe Journal of Physical Chemistry
vated with gaseous F2 a t 20 psi for 1 week. Heise gauges, whose scales could be read to 0.1% of full scale, were used in all mixing operations. Immediately before using a gaseous mixture in a test, its H F and Fz contents were determined by reading its optical densities at 2.5 and 0.285 p , respectively, on a Beckman DK-2 spectrophotometer, equipped with a stainless steel absorption cell having sapphire windows. Calibration of the spectrophotometer was accomplished by determining the optical densities of freshly mixed samples of Fz, HF, and Ar. Beer's law described the results, provided the partial pressure of H F was kept below 100 torr in all operations. Prior to each test it was found necessary to condition the walls of the shock tube and the absorption cell of the Beckman DK-2 to the presence of HF. This was accomplished by passing the test gas through the entire apparatus until the optical density at 2.5 p was reproducible over a period of 10 min. Only by following this procedure could reproducible results be obtained. The apparatus was then reevacuated, and a fresh sample was admitted to the desired test pressure (ca. 10-40 torr). The gaseous pressure within the tube was measured by means of a Wallace and Tiernan, 0-100 torr, gauge. The test was then conducted using a helium driver.
Results and Discussion On the assumption of transparent gas radiation, the concentration of HF in the reacting mix is related to signal output of the analytical system by means of the equation (HF)o (v) (HF) = -
vo
where the subscript refers to the conditions immediately behind the shock wave. The value of was found via a solution of the Rankine-Hugoniot equations. Due to the extreme instability of F2 under the experimental conditions,*!5it was assumed totally dissociated within the shock front itself. The value of V o was found by simple logarithmic extrapolation of the oscilloscope trace to the initial conditions; see Figure 1 and Figure 2. These initial trace heights are shown in comparison to the initial concentrations of H F in the reacting mix in Table I. The constancy of the ratio of T'o/(HF)o as tabulated in the last column of the table demonstrates the validity of our calculations based upon the results of n!falkmus.10 Resort was made to the method of initial slopes for the evaluation of the rate constant of reaction 1. The results are illustrated in Figure 3 as a function of tem(11) Trade name. (12) Purchased from Santa Barbara Research Center, Goleta, Calif. (13) The analytical system was designed by William Netusil of
Rocketdyne, Ino., Canoga Park, Calif., Contract AF 04(611)-8502.
THEKINETICSOF DISSOCIATION OF HYDROGEN FLUORIDE
81
Figure 1. Reaction profile for: (a) shot no. 71; 2.06% HF, 0.375% Hz, 3231"K, 10-psec time markers, and 0.2-v ordinate divisions; (b) shot no. 99: 2.23% HF, 1.48% Fz,5367"K, 5-psec time marks, and 0.1-v ordinate divisions; (0) shot no. 58: 2.35% HF, 1.80% Fz,4871"K, 10-psec time marks, 0 . 2 4 ordinate divisions; (d) shot no. 60; 2.47% HF, 1.80% Fz, 5862"K, 10-psec time marks, 0.1-v ordinate division.
'0 O ',
, O
'
0
0, 0,
0
. 0
L
1
10
' 10
'
'
30 IPI.4
I 40
' ' I $0
O
O
'
'
Figure 2. Logarithmic extrapolation illustrated for shot no. 52: 5.84% Fz,3.02% HF, 5283OK.
Table I : Internal Consistency of Initial Emission Intensities (HF)o X lCP, mole/co
Run no.
T. "K
49 53 50 54 51 52 47
4175 4264 4312 4426 4784 5283 561.3
First Set" 0.238 0.285 0.185 0.426 0.176 0.112 0.200
71 69 70 72 66 67 68
3231 3932 3961 4280 6105 6276 6365
Second Set 0.086 0.081 0.102 0.185 0.098 0.077 0.077
1.04 1.54 0.95 2.23 0.89 0.59
0.87
4.4 5.4 5.1 5.2 5.1 5.3 4.4
0.82 0.71 0.92 1.37 0.68 0.82 0.76
9.5 8.8 9.0 7.4 7.0 10.6 9.9
Refers to individual settings for bias current and amplifierplate voltage. (I
perature and are tabulated in Table 11. The relationship
(7) gives the best simple fit to all of the experimental data, although the cascade model of Benson and Fueno14 does reproduce the data very well from a standpoint of
Figure 3. Temperature dependence of initial rates.
temperature dependence and absolute magnitude.I5 The activation energy is assumed identical with the endothermicity of the reaction. The result given in eq 7 agrees favorably with the result obtained by ;.e. Jacobs, et
kl =
'*
'O"
T
e-134,100
IRT
The absence of any pronounced effect of large amounts of atomic fluorine upon the initial slope (see Figure 3) suggests that reaction 3 is either insignificant in comparison to reaction 1 over the entire ranges of temperature and concentration utilized or identical with the reaction
F
+ HF = H + F + F
with the efficiency of F atoms as a third body being roughly equivalent to that of argon. The over-all reduction of the rate data proceeded by matching observed and calculated reaction profiles. The computed profiles were obtained by means of a nonequilibrium computer program'o that involved an (14) S. W. Benson and T. Fueno, J. Chem. Phys., 36, 1957 (1962). (15) Our calculations utilize a collision diameter for the collision complex of HF which is just half that given by the procedures of ref 14. This is to account for the difference in relative efficiency of energy transfer for collisions occurring with the heavy F end and the light H end of the complex. (16) Furnished by Dr. T. A. Jacobs of Aerospace Corporation, El Segundo, Calif.
Volume 76,Number 1
January 1968
82
JAY A. BLAUER
Table 11: Compositions and Shock Parameters for Individual Shots Shot no.
40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 0
mm
mm/paec
Ti,OKC
Tf,OKd
( H F ) ~x 107, moles/cc
kl x 10-11, cc/mole 8ec
0.00 0.00 0.00
31.5 31.5 31.5 31.5 40.0 40.0 40.0 32.0
2.455 2.410 2.410 2.363 2.324 2.019 1.983 2.477
5563 5370 5379 5174 4970 3822 3696 5613
5053 4872 4881 4676 4386 3613 3525 4917
1.44 1.43 1.43 1.43 2.55 2.43 2.43 2.00
0.0282 0.000539 0.0337 0.0201 0.00953 0 000574 0.000233 0.0278
5.84 5.84 5.84 5.84 5.84 0.00 0.00 0.00 1.80 1.80 1.80 1.80 0.00 0.922 0,922 0.922 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.375 0.00 0.00 0.00 0.00 0.00 0.00 2.97 2.97 2.97 2.97 2.97
40.0 32.0 24.0 15.8 40.0 54.1 12.0 40.0 31.5 18.9 31.5 18.9 18.9 18.9 18.9 18.9 18.9 24.0 18.9 18.9 18.9 24.0 18.9 40.0 40.0 40.0 40.0 32.0 24.0 18.9 40.0 24.0 24.0 16.0 24.0
2.270 4138 2.272 4145 2.437 4784 2.469 4914 2.262 4110 2.191 4426 2.555 5905 2.278 4499 2.346 4824 2.358 4871 2.551 5672 5862 2.595 6314 2.641 2.589 6022 6320 2.656 2.150 4218 2.474 5613 6102 2.583 2.624 6276 6365 2.646 3932 2.050 3961 2.058 1.849 3231 4260 2.140 4425 2.161 3379 1.871 3068 1.775 4768 2.250 5150 2.339 5216 2.359 4089 2.171 4700 2.330 4522 2.285 Velocity trace failure 2.042 3621
4053 4046 4410 4414 4011 4039 4996 3794 4469 4473 5161 5350 5479 5739 6054 4041 5146 5623 5808 5909 3711 3747 3203 3992 4166 3323 3043 4528 4851 4966 3989 4385 4426
2.38 1.85 1.76 1.17 2.85 4.26 0.963 1.74 1.59 0.949 1.71 1.01 1.37 0,570 0.559 0.546 0.766 0.977 0.770 0.771 0.807 1.02 0.790 1.71 0,808 0.792 0.784 0.681 0,508 0.400 1.75 1.06 1.06
0.000539 0.00109 0.00430 0.0105 0.000666 0.00333 0.0679 0.0000 0.00694 0.0161 0.0244 0.0635 0.0693 0.0822 0.127 0.00305 0.0371 0,0741 0.140 0.116 0.0000 0.0000 0,0000 0.0000 0.00320 0.0000 0.000 0.00784 0.0213 0.0278 0.000614 0.00355 0.00256
1.04
0.0000
* , .
...
3603
2.97 2.97 2.97 2.97 0.00 0.00 0.00
24.0 24.0 24.0 12.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 24.0 31.9 31.9 16.0
2.554 2.526 2.442 2.663 2.382 2.489 2.553 2.418 1.716 2.374 2.592 2.366 2.354 2.138 2.452 2.463 2.543
5635 5513 5155 6109 5320 5784 5977 5390 2849 5206 6153 5300 5124 4274 5367 5413 5756
5209 5089 4768 5669 5096 5558 5432 4859 2816 4687 5602 5300 4614 3931 4893 4938 5253
1.00 1.01 1.01 0.510 0.452 0.455 0.992 0.991 0.940 0.989 0.998 0.000 0.983 0.976 1.47 1.46 0.736
0.0266 0.0256 0.0229 0.104 0.0341 0.0469 0.0930 0,0333 0.0000 0.0185 0.0775
Pi,a
2.17 0.00 2.12 0.00 2.12 0.00 2.12 0.00 3.03 0.00 2.95 0.00 2.95 0.00 2.95 0.00 Velocity trace failed 2.52 0.00 2.45 0.00 3.12 0.00 3.02 0.00 3.02 0.00 3.75 0.00 3.75 0.00 2.02 5.00 2.36 0.00 2.35 0.00 0.00 2.52 2.47 0.00 3.57 0.00 1.34 0.00 1.39 0.00 1.39 0.00 1.92 0.00 1.92 0.00 1.92 0.00 1.92 0.00 2.07 0.00 2.06 0.00 2.06 0.00 2.06 0.00 0.984 0.00 0 984 0.00 0.984 0.00 0.00 1.03 1.02 0.00 1.02 0.00 0.00 2.05 0.00 2.05 0.00 2.05 2.05 0.00 0.00 2.05 Defective oscillogram 1.93 0.00 1.95 0.00 1.95 0.00 1.95 0.00 0.852 0.00 0.911 0.00 1.96 0 323 1.97 0.323 1.97 0.323 1.97 0.323 1.97 0.323 0.00 0.00 0.00 0,232 1.97 0.323 2.23 0.00 2.16 0.00 2.16 0.00 I
Initial total pressure.
%Fn
% H2
%HF
I
b
0.00 0.00 0.00
0.00 0.00
...
...
0.00 0.00 0.00 0.00
0.00 0.00 0.00 1.48 1.48 1.48
Shock velocity.
The Journal of Physical Chemistrpl
0
...
...
U,b
...
...
Temp of gas immediately behind shock wave.
...
...
...
d
...
...
...
I
...
...
...
...
0.0183 0.00149 0.0166 0.0146 0.0466
Temp of gas a t equilibrium.
83
THEKINETICS OF DISSOCIATION OF HYDROGEN FLUORIDE . 46
45 98
10
.
3515 3614 3931 4166 4518 4681 4863 5558
-. 00 00 031 00
00 032 0 0 00
1
08
9
73 16 94 41 PO
I
L
3691 3821 4214 4360 4789 5106 5379 $?a4
0
06
-
0
2
0
L
I
0 I
I
10
30
I
0
0
10
I
I
40 50 LABORAIORV TIME(
U
I
I
I
I
60
10
80
90
0
SECI
Figure 4. Illustrittion of the dependence of the reaction profile on the value of ICZ: 1.97% HF, 0,32% Ht, 4274°K (initially), shot no. 98.
10
40
60
BO
100 1'20 140 LA00RAlORY IIMEIP S E C I
I
D
5106
94
180
200
Figure 7. Comparison of computed and observed reaction profiles for mixes containing no added fluorine; ka has the best value.
SYMBOL
10
160
4687
RUN
l l ~ n t l ~ dI l l i n e l l
XI 2
A B
53 70
4110 3961
3979 3141
504 030
E 1
51 65
4024 5613
4469 5146
038
031) 180
I-
S
\
04
.o.%.O
L 40
10
0
'
60
L L 00 100 110 140 LABORAIORY I l M l IClSECI
160
,
100
100
Figure 5. Comparison of computed and observed reaction profiles for mixes containing no added fluorine; IC2 is too large.
Figure 8. Comparison of computed and observed reaction profiles for mixes containing added fluorine.
SYMBOL RUN Ilinirmll Illinol) XI41 A
I C D
45 98 I6 94
3614 3931 4518
3811
41?4 4769 5106
4601
00 012 00 031
k2 = 1 X 1018e-35*000/RT
(9)
Here, as before, the activation energy is approximated by the endothermicity of the reaction. The procedure was an iterative one involving variations in only one of the rate constants at a time, Figure 4 illustrates the effect upon one reaction profile of shifting the value of kz while the value of kl is held fixed. The value of kl is shown in this figure as
d 0
10
40
60
BO 100 110 LABORAlORV IlMEl fi
140
160
180
100
1.0
Figure 6. Comparison of computed and observed reaction profiles for mixes containing no added fluorine; kz is too small.
integration technique, which utilized both temperature and time as independent variables. The initial estimate of reaction 2 was taken from ref 3 as
due to the fact that a T-2 temperature dependence seems to fit the data for mixtures not containing Fz slightly better than a T-' dependence, whereas the reverse is true for mixes containing FB. The difference between expressions 7 and 10 is not statistically significant. The best fit to the experimental data is given for
Volume 73,Number 1 January 1068
F. CRAMAROSSA, E. MOLINARI, AND B. ROIO
84 A comparison of the observed and computed reaction profiles for 15 separate tests are shown in Figures 5-8. The results confirm eq 11. The values given by eq 11 are one-fifth as large as the results of Jacobs, et aZ.;s see eq 9. Although this difference is statistically significant (see Figure 4),there is no satisfactory means of making an a priori judgment between the two results.
Conclusions In conclusion, the rate constants for reactions 1 and 2 have been evaluated in the temperature range of 3700 to 6100°K in FrHF-H2-AR mixes. The results are in reasonable agreement with the results of Jacobs, et aL3 The presence of large amounts of atomic F in the reacting mix has no pronounced effect upon the rate of
dissociation, other than for the simple mass action suppression of reactions 1 and 2. The third body eficiency of atomic F for recombination of H and F is not grossly different from that of Ar. The cascade model of Benson and Fueno for calculating atomic recombination rates reproduces the absolute magnitude of these rates very well, although little can be said regarding the temperature dependence due to the degree of scatter in the data.
Aclnowledgments. We wish to thank Dr. T. A. Jacobs and Dr. N. Cohen of Aerospace Corp of El Segundo, Calif. for assistance in the planning stage of this research and for providing the nonequilibrium computer program used in the computation of reaction profiles.
Rates of Reactions of Oxygen Atoms with Solid and Liquid
Sulfur (and Selenium) by F. Cramarossa, E. Molinari, and B. Roio Istituto di Chimica Generale e Inorganica dell' Universit;, Bari, Italy Accepted and Transmitted by The Faraday Society
(May 2 , 1967)
The kinetics of the heterogeneous oxidation of solid and liquid sulfur by gaseous oxygen atoms has been studied in the temperature region 50-160". Rates of selenium oxidation have been determined between 200 and 330". Primary reaction products are SO and SeO which undergo further oxidation in the gas phase. The dependence of the rates on the concentration of oxygen atoms is different below and above the melting point, and the dependence on the temperature is complex. The chemical and the evaporation regimes of the reaction have both been observed. The kinetics is affected by the interplay of homogeneous and heterogeneous reactions.
The interaction of gaseous atoms with surfaces has been the subject of a number of investigations in recent times, mostly in connection with the catalytic behavior of surfaces toward atom recombination. A variety of chemical reactions of atoms with solids is reported in the literature. Kinetic studies of these reactions are, however, limited to a few systems only: and Mo, W 0.5 The C H,' C 0,a,3 Ge common feature of these systems is that the reaction products are volatile a t the temperatures of the experiments. Rate-limiting conditions imposed by diffusion of the reactants across the surface layer formed by the products are therefore absent.
+
+
+
The Journal of Physical Chemistry
+
In the present work the kinetics of the reaction of oxygen atoms with solid and liquid sulfur has been studied between 50 and 160'. Rate measurements of the oxidation of selenium, performed between 200 and 330", will also be reported. However, the kinetics of (1) A. B. King and H. Wise, J. Phys. Chem., 67, 1163 (1963). (2) D.E.Rosner and H. D. Allendorf, AIAA J . , 3, 1523 (1965). (3) J. D.Blackwood and F. K. McTaggart, Australian J . Chem., 12, 114,553(1959). (4) K . M . Sancier, S. R. Morrison, and H. U. D. Wiesendanger, J . Catalysis, 5,361 (1966). (5) D. E. Rosner and H. D. Allendorf, J. Chem. Phys., 40, 3441 (1964); J . Electrochem. Soc., 114, 306 (1967).
