F
Registered in U . S. Patent Ofice @ Copyright, 1973, by the American Chemical Society I____-
VOLUME 77, NIJ
VEMBER 8, 1973
A. P. Modica Avco Sysrems Division. Wiirningfon, Massachusefts 07887 (Received Aprii 30. 1973) Pubiication costs assisted by the Space and Missile Systems Organizafion,
U.S.Air Force Systems Command
Measurements of SF6 and SF&l decomposition were conducted in argon shock waves at total pressures of 1-3 atm and temperatures from 1700 to 2300°K. Similar experiments of SFa decomposition were carried out with hydrogen. A Bendix time-of-flight mass spectrometer was used to identify the important reaction products behind reflected SF6-argon shock waves a t 1800°K. Equilibrium ultraviolet absorption of SF4 a t 2660 A was monitored in the SF6, SF5C1, and SFa-Hz decomposition studies. Thermochemical shock tube calculations are fitted to experimental SF4 equilibrium concentrations. Data analyses suggest a AHf0(SF5)of -241.7 kcal mol-1, and bond energies of (SF5-F) 65.2 and (SF4-F) 87.9 kea1 mo1-l for the SF6 molecule.
I. Introduction A shock tube and a thermochemical equilibrium shock wave computer program have been used in previous studies to determine band system oscillator strengths of the 6 2 and CF radicals from fluorocarbon decomposition1 and to evaluate the chemical kinetics of the CFa radical in fluorocarbon chemistry.2 Currently, there is interest in the high-temperature chemistry of sulfur hexafluoride because of its application to electron attachment processes in air plasma^^,^ and its stable source of fluorine atoms for shock-induced laser experiments with h y d r ~ g e n .Studies ~ of the chemical kinetics of SF6 and SF4 dissociation have recently yieided probable bond energies from limiting first-order dissociation rate constants.6,7 The present investigation deals with the thermal decomposition of SF6 molecules, SF5C1 molecules, and SFs-Hz mixtures studied in argon shock waves. A Bendix time-of-flight mass spectrometer coupled to a shock tube allowed the important decomposition products of shock heated SF6 to be identified for the (chemistry input to the thermochemical equilibrium program. The SF&1 molecule provided a source of SF5 radicals in the study. Mixtures of SF6 and hydrogen were used to generate MF in the SF6 decomposition chemistry. In the three sets of experiments, the equilibrium ultraviolet absorption of 2660 A of the SF4 decomposition produce was monitored behind the shock wave. The
infrared emission of the MF vibrational band of 2.5 p was measured to determine the H F concentration in the shocked gas for comparison with the thermochemical calculation. XI. ~
x ~Procedure e ~ ~ ~ ~ ~ Shock Tube. The SF6 decomposition experiments were conducted behind incident shock waves in an optical spectroscopy shock tube and behind reflected shock waves in a shock tube coupled to a time-of-flight mass spectrorneter. The optical shock tube is a 1.5 in. i.d. stainless steel tube with a 3.5 ft driver section and a 10 f t driven section. The mass spectrometer shock tube apparatus is constructed from 1 in. i d . Pyrex glass pipe arid has a 5 ft driver section and a 1.5 ft driven section. A Bendix (Model 14-206) time-of-flight mass spectrometer is attached to the glass driven section through the “fast reaction chamber.” The shock velocity in the two facilities was measured with a series of platinum heat transfer gauges. The output of these gauges was displayed on a time-mark folded oscilloscope sweep. operated by a Radionics (Model N-2A) triangular wave and marker timing generator. Transit times between stations were measured to within f l psec. The maximum error in the shock velocity was about h0.370 and introduced an uncertainty of less than 3~15°Kin the calculated incident shock temperature. The 2713
t
~
2714
A. P. Modica
0
MASS SPECTRA SFs -ARGON BEHIND SHOCK WAVE
SF6 MASS SPECTRA
f 2 v1 0
51
--)+*27 MASS PEAKS
I
1
127
108
I
11
I
89 81
40 38
MASS PEAKS
Figure 2. Mass spectra of sF6 decomposition behlnd 1:lOO SF6-argon reflected shock wave: time between spectral scans, 25 psec; electron beam voitage, 30 eV; ion peaks, SF5+ (1279, SF4+ (108), SF3- (89), SO3+ (80) impurity, Ar+ (40) off scale, Ar+ (38) isotope; sample pressure 0.33 atm; sample temperature 1860°K.
108
89
70
51
32
Mass PEAKS
Figure 1. Mass spectra of SF6 and SF4 molecules: time between soectral scans. 25 usec: samole temoerature. 298°K: electron beam voltage, 50 e+; ion peaks, SFs"(127), SF4+ (108), SF3+ (89), SF2+ (70), SF+ (51), andSC (32)
error in the reflected shock temperature could have been about *40"K based on ideal reflected shock calculations for a chemically relaxed gas. Schematics of the optical and mass spectra facilities may be found in previous reports.8 Reagents. The SFs and SFBCl test gases were diluted separately in 1:100 mole ratios with research grade argon. Mixtures of SFs and hydrogen were prepared in mole ratios of 1:0.5:100, 1:1:10@$and 1:5:100 SFG-Wz-argon for study. A 1:5:100 HF-H2-argon mixture was used to calibrate the 2.5 p infrared radiometer signals with known shock concentrations of HF molecules. Ultraviolet absorption measurements of SF4 were made in 1:100 SF4-argon shock heated samples. All reagents were obtained from commercial suppliers and used without further purification. The stated purities of the test gases were Matheson SFs (98% purity), SF4 (94% purity), Hz (99.98% purity), HF (99.9% purity), and argon (99.999% purity). Su!fur chloride pentafluoride, SF5C1, was obtained from Peninsular Chemresearch, Tnc. Samples of SFe were analyzed for ultraviolet absorption with a Bausch & Lomb Spectronic 505 spectrometer and were found to he optically pure in the wavelength region 2000-3200 .A> indicating virtual absence of any SF4 or possibly thionyl fluoride, SOF2, impurity. Commercial samples of SF5C1 showed ultraviolet absorption bands a t 2880 and 2400 A similar to SF4 spectra.
111. Mass Spectrometer Measurement Mass spectral recordings were taken of room temperature samples of SFs and SF4 to identify their respective positive ion mass cracking patterns (Figure I). The timeresolved ion peaks, taken a t 25-psec scan intervals, were displayed on a Tektronix 535A oscilloscope in combination with a Type CA preamplifier and multiscan generator. The positive ion spectra correspond to mass peaks of SFs+ The Journal of Physical Chemistry, Vol. 77, No. 23, 1973
3400
3100
3000
2800
2600
2400
2900
A,:
' h 1660 22.0
SF6
5 zooi
0
DATA
SF4 D A T A
leob
-LEAST
SQUARES
i
1 6 0 1
i
4
4.0 2.0
LU/-d 500
IO00
1500
2000
2500
TEMPERATURE, OK
Figure 3. SF4 ultraviolet absorption showing (a) spectral sorption coefficient from radiometer data taken in shocked decomposition mixtures and from SF4 spectrophotometer sorption spectra and (b) temperature dependence of SF4 sorption coefficient at 2660 A.
abSF6 abab-
(127), SF4t (108). SF3+ (89), SFz+ (70), SFt (51), and S+ (32). In the SFe spectra the parent positive ion peak SFs+ (146) does not appear because of a more favorable ionization mechanism, namely, the electron resonance
2715
SFs and SFsCl Equilibrium Decomposition
i
I
0
25-
I
Y
w -
2
I
I
I
-
OPTICAL PATH, 3.97cm 2 5 p RADIOMETER
+
HF CONCENTRATION
T
1
a
I O Q t
A
0 7 /
c 0
tI
245.7
2
0 5
f '
1500
L
1600
ol-
I 5
6
1700
1900
1800
2000
2100
1900
-
U J 2100 2 3 0 0 TEMP' K
Figure 5. Experimental and thermochemical SF4 equilibrium concentrations behind chemically relaxed 1:lOO SF6-argon shock waves.
I00 SFs - H p -AR
0
I
1600
I
1800
I
2000
O K
1
I
1600
I800
IL1---J
2000
IT00
1600
I800
2300
Figure 6. Experimental and thermochemical SF4 equilibrium concentrations behind chemically relaxed 1 :100 SF5CI-argon shock waves.
P S F :0.01-0.015 ~ atm I 0.5 100 SF6 -ti? -AR
Teq
1900 2100 TEMP' K
I 2000
O K
Figure 4. Infrared emission of the H F fundamental vibrational band of 2.5 + behind argon shock waves showing (a) comparison of experimental and theoretical absolute intensities from shocked 1:5:100 HF-He-argon mixtures and (b) 2.5-p emission normalized to mole ratio of HF and initial SFS concentration in
0.9
-
shocked SF,j-H2-argon reaction mixtures: 0 , ir experimental data: solid lines, thermochemical calculation.
Pi
-
capture reaction forming SF6- ions, and the electron disSFs- + F) above sociative attachment of SF6 (SF6 + e electron accelerating voltages near 0.01 eV.9 The dominant ion peak in the SFs mass spectra is the SF5- (12'7) positive ion. The SF3+ mass peak is observed to be greater than the SF4- ion peak characteristic of the parent cracking pattern of SF6. For SF4, the normal parent SF4 positive ion peak dominates the spectra with decreasing abundance of the SF3+, SF2+, and SF+ cracking pattern ions. A mass spectral record of SF6 decomposition at 1860°K behind a reflected argon shock wave is shown in Figure 2. The electron beam voltage of the experiment was adjusted to 30 eV low enough so that the cracking patterns were reduced to essentially mass peaks of parent molecules in the reaction mixture. The dominant ion peak of the SF6 decomposition mixture is found to be that of the SF5+ ion.
:4.0
cm Hg
A H f ( S F g 1, kcal malo-1 - 241 7
0 3
0.I
1.25
1.30
I700
1.35 1.40 U s ,rnm/,usec 1800
1900
1.45
2000
I50
2100
TEMPERATURE, " K
Figure 7. Experimental and thermochemical SF4 equilibrium concentrations behind chemically relaxed 1:0 5.100 SFs-H2argon shock waves Minor ion peaks for SF4+ and SF3- are discernable. A mass peak near 80 appears in the background spectra and is believed to be SOa- formed by an ion-molecule reacThe Journal of Physicai Chemistry, Voi. 77, No,23, 1973
216 6
A . P. Modica
.
.
-
_ I _
_.
.
.
. -
-. . ..
.
.
-
.
4
HOLE F R A C T I O N
ie9I581fi-02 9e440046-Q3
-
4.342111)
l 4 C I O L N T hLB(#bk b l V E S P E C I E S MEAN MOLECULAR b E l G I i 7 OF M l X T U R E 4i.0Z0Q4r) O l E P A R T l C b E S I C C SPECIE MOLE F R A C l l C M P A R T I C L f S I C C S P E C I E 2 ~ 1 9 9 2 2 01 7 F L595210-06 1.831190 13 F2 1mOe3650 L9 I F 4 2-707750-04 3.188310 1 5 5 F 5
m
00
2 ~ ~ ~ ~ -$g , p3k ? 3D2 0- 0~0 ~ i.262260 00
S.BL9530 Of2 #OLE FRACVlOY ~ ~ 9.711280-01
~
R P 6R ~ SPECIE ~ ~ ~2 ~ 123 SF@ 4 A.PP419D 19 A
A ~
~ € ~ ~~
~
~ 4
'
Hea! of forma!ion of sulfur iiuorine species ir calculat,on 1 H f (0°K) SFS -288 4 kcal/rnol, SF5 241 7 Kcalirnoi, SF4 -172 2 kcdl/mol Shock quantities are US shock velocity in mrn/ysec, T2 sl-ack temperature ( O K ) , T S / T l temperature ratio across shock, RHO2 shock density in grn/crn3 RH02/RHO1 density ratio across shock t i 2 shock enthalpy in kcal/gm, U2 qas velocity in mm/psec -I
tion of oxygen and sulfur impurities trapped in the drift tube of the mass spectrometer. The SFs+ peak is believed to be that of SFs raclxals in the reaction mixture, since SFe decomposition experiments in the infrared and thermochemical equilibrium calculations show that the SF6 molecule a t 1860°K is dissociated.6 However, the presence of the SFs ion as the dominant peak and the SF4+ ion as a minor peak in the spectra of shock-heated SFG is in complete discord with the thermochemical calculation reported by Bott and Jacobs (ref 6 ) . Their thermochemical calculation shows that for a 0.5% SFs-99.5% Ar mixture between 1600 and 1850"K, SF4 is the dominant chemical species at equilibrium and that its concentration is nearly constant over the temperature range. The SFs concentration initially is about one-third of SF4 a t 1600°K and decreases monotonically by a factor of 40 below SF4 at 1750°K The SF5 concentration begins about an order of The Journa: of F'hysical Chemistry, Voi, '77,
No. 23, 7973
magnitude lower than SF4 and continues to decrease by another factor of 10 near 1'975°K. The Bott-Jacobs thermochemical. calculation also included the SF3 species and shows that its concentration a t around 1850°K increases to about one-third of SF*. In the present study, the mass spectrometer data (Figure 2) indicated only a very minor response to the SF3f ion peak for decomposed SFe a t 1860°K.
PV. Ultraviolet A ~ ~ ~ ~ Spectral absorption coefficients of the SF4 molecule were obtained over the wavelength region 3400-2400 A from photometric measurements of dissociat,ed samples of SFs, analyzed according to Beer's law. The ultraviolet radiometer consisted of an Engis SO502 grating monochromator (30-A triangular band pass at ha!f-peak-~reight),an xenon-mercury continuous ultraviolet light source (Hano-
~
2717
SFs and SF&l Equilibrium Decomposition TABLE I I: Shock Tube Equilibrium Calculation for 1: l o 0 SFsCI-Argon Mixture ...PZ/Pl nm-LS .- -1 2 12/Tl p2 1.3118Q
00
COLE F P A C l l f l Y 5.45C230-03 1.37493D-03-_ -3.988510-05 9.79t900-01 LS
1.368490 -
_-
OC
MCLE F R A C T I O N 6.65924(1-03 ' 8.341050-04 .- 2 . 0 4 3 5 3 0 - 0 5
5.772550-01
1.700000
1.423060 -
JO
MCLE F P A C l l O N 7.614650-0'3 4.95134D-01 7. b21580-065
-
5.74P260-01
1.800000
1.472350
03
PARTICLES / 5.201500 6.515160 1.596200 7.b33300
1.9OOOOD
6.010270
CC 16 15 14 18
00
03
--
2.134820-06
-- S.73CO2O-Ol-_. US
__
__
.
00
(1
00 2 . 4 2 5 5 5 0 -
i.914910
-
60
_pz -
5.26b710-04
3.964770
9
---
2.0%2-2QO
_ _-____
00
2.63745D-Cl
-__
- --
-_
-.
S.-411920-04 -- -
__
00
1.b811ZD-01
-4.074C80 --
00
--
-
2.003000 -
03
6.711410
P A R T I C L E S / CC 6.938990 16 2.375200 1 5 1.752270 1 3 7L98b'!5D-18
00 -
.
03
PARTICLES I 8.575120-03 1,444290 bb73320-04__ 10387910 5.117390-07 4.244550 9.715400-01 8.061620
uz
u2mi
00
7 . 0 4 6 9 e ~ 00
1.023330
1.236850
00
2.832080 -.
-
.
RHU2
(1
.
5,510391)-04
RHOZlRHDl
4.148210
9
00
uz
nun1
2.038021)-01
-8.630090
00
1.073760
UZ
HtlHl
2.203820-01
1.196701)-03
OB
P A R T I C L E S I CC S P E K I E 9.318830 10 S C L F S 4.907010 1 2 F Z
HZ
I.017511) 00 MOLE F R A C T I O N 4.113400-09 6.196320-OY
00
P A R T I C L E S / CC S P E C I E 2.111010 11 S C L F S 3.3698813 1 2 F 2 3.319111) 16 SF5
00
-9.332200
L.L1741D
00
P A R T I C L E S / CC S P E C l E 3.376300 5,085970 10 12 D F ZC L F S
9.822590
IS SFS
~
12/11
12
SPECIE SCLFS FZ SFS
~
P2/Pl
~ N C I D E N T NWUL WAVE SPECIES M E A N M O L E C U L A R WEIGHT O F M l X r U C E = 4.046480 01 1 SPECIE M O L E F R A C T l O N P A R l l I L E S / CC S P E C I E 6.961100-04 5. i13710 1 5 CLF CL 6. 127950 16 F EL2 - 7.a31270-a3 8 . 5 3 0 8 C O 0 3 7 . C5213O 16 S F 4 SF6 A
9.70359W01
- _.
P2
12/11
C A R I I C L E S I CC 3.922171) 11 1.561240 12 4.549160 1 6
HZ
__
__
-
OD
-9.118780
le859kqD-01 - 1 . 8 1 3 8 3 0
1.013141) 00 MOLE F R A C T I O N 2.102130-08 4e31429D-01 4.249290-03
ntioz/nHoi
RUOL - ..
PZtPl -
.
U2
n2mi
.
l N C l D E N 1 AORPAL WAVE S P L C I E S M E A N MOLECULIR n E i c n r OF MIXTUFE = 4.0522bU OA 1 = 1.015671) 00 P A R T I C L E S / CC S P E C I E M O L E F R A C T I O N - P A R T l U E S / CC S P E C I E MOLE F R A C l l O N 6.175260 16 CL 1.082320-03 8 . 108700 15 CLF 1.1581 5 0 - 0 8 6.)75_560-03 4. '690_5L 16 F - - . _6 . 0 9 8 4 6 0 - 0 1 3.984810 15 C L 2 7.264720-03 5. H703D 16 b F 4 2.47288C-03 b.13739R 1 3 S F 6 J784)150 1 6 A *-------.--.... -
- 1 . 5 1 5 ~ ~ 00 ~ 1 ?.iooooo -- - __ COLE F R A C T I O N
3.841300
5.102700-04
I
6.175840
-
2.893140-04
00
12/11
-. -
MCLE FPPCTlOW 8.45?870-03
--
__
12
._ -
(1
2.21394i
-._ - -
_ I
I H C l D E N T NORbAL W A V E S P E C I E S M E A N M O L E C U L A R W E I G H T OF M I I T U H I 4.0623SD 01 i M O L E F R A C T I O N P A R T I C L E S I CC S P t C I E SPECIE 1.444090-03 1. u 7 9 n o 1 6 CLF CL 4.036420-03 3. 152830 I 6 F CLZ _ _ SF6 5;501al0-03 4. r8744D 16 SF4 A _ -
. .
LS
-__ .
1.74i051)-00
---.
nz
__
12
- -
00
nHoz/nmL---
I N C l O E N l NORbAL WAVE S P E C I E S M E L N M O L E C U L A R - U E I G H T OF M I X T U K 9 4 . 0 7 2 4 7 D 01 2 9 1.010631) 00 P b R l l C L E S I CC S P E C I E M O L E FRICTION - P A R T I l L E S /_CC bPECPE M O L E FRAC."Y 4.114330 1 b C L 1.59577D-03 1. i o 4 6 4 0 1 6 C L t 5.19568U-08 1.037920 16 C L Z 2 . 0 9 3 6 6 0 _ - 0 3 1. !80600- 16 f -- 2.~141670- 17 3.729720-03 2. t i 5 5 3 0 16 SFlt b.026250-03 3.010890 14 S F 6 7.395590 18 A P2/PL RnUZ nnozinnoi HZ 12 12ITl P2
us
.- -
0 3 - 5.704700
-
_ I _ _
P2 2.372370
nnw
PZIPI
00
3.005000
_ .
(i
5.5623+~-04
_.
__
~ n o 2 1 ~ n1 a 4 . ~ 7 3 2 0 00
INCIDENT NORIAL YAVE SPECIES M E A N M O L E C U L A R Y E I G H T OF M I X T U R E 9 4.04033D 01 i CC S P E C I E M O L E F R K T I O N P A R T I l L E S / CC S P L C I E 1 6 CL 4.086240-04 3. 189200 15 C L F 15 CLZ B079C750-03 7. i 9 1 3 6 0 l b F 9.201510-03 1 s t 3 2 0 6 0 16 SF4 12 SFb 18 A
via arc lamp, type D-901B01), and a 1P-28 photomultiplier. The light beam was collimated perpendicular to the shock tube axis by two sets of slits, each 1 mm in width. SF4 concentrations in the shocked reaction mixtures were calculated from absorption data taken a t 2660 A (wavelength of optimum signal-to-noise ratio). The spectral absorption coefficient for SF4 and the temperature dependence of the 2660-A absorption coefficient are shown in Figure 3. Measurements with SFe near 2100°K were corrected for incomplete dissociation (85% SFs dissociated). The absorption coefficient a t 2660 A was determined both from shocked sample of SF4 and from SFe samples considered to be completely dissociated to SF4. The spectral absorption data at room temperature were obtained from a Bausch & Lomb spectrophotometer SF4 spectra.
V. Infrared Measurements An indium. antimonide detector (Philco Type ISC-301) was filtered a t 2.5 p to monitor the H F fundamental emis-
-
nZ
z . 3 5 3 5 ~ 0 - 0 1 - 9 . 9 6 6 ~ 1 0 00
L.018b60 00 MOLE F R A C T I O N 1.313770-09 5.122720-07 5.219170-04
uz
nzini
1.153940
00
P A R T I C L E S / CC S P E C I E i o o n w I~ O SCLFS 4.248960 I 2 F Z 4.328960 I S SFS
sion band in shock-heated H F samples and shocked reaction mixtures of SFs and hydrogen. The infrared radiation from the high-temperature test gases was viewed through barium fluoride windows, orthogonal to the shock tube axis. The 2 . 5 - 1 radiometer was calibrated against a standard black body source (Barnes Engineering Model 11201-1) for absolute intensity measurements. A Tektronix 556 dual-beam oscilloscope displayed the radiometer signals. Absolute intensities of the H F fundamental band in emission were measured in shocked 1:5:100 HF-Hz-argon gas samples and were compared to theoretical estimates based on H F line intensities for the ( 1 , O ) and ( 2 , l ) fundamental transitions.1° Steady-state infrared emission from H F molecules formed by the reaction of SFs with hydrogen was observed and normalized in terms of the mole ratio of HF to initial SFs behind the shock wave. The results of the H F absolute intensity measurements and the normalized H F emission data in the SFe-hydrogen reaction study are given in Figure 4. The Journal of Physicai Chemistry. Vol. 77. No. 23, 1973
2718
I. ~ ~ ~ ~ Concentrations of the SF4 decomposition product from SFs, SF5Cl, and SFe-hydrogen shocked experiments were determined from the measured steady-state ultraviolet absorption behind the shock wave and the SF4 absorption coefficient: E \ Z ~ ~ G O . in Figure 3 at the equilibrium shock temperature. A shock tube thermochemical equilibrium computer programii was used to calculate the equilibrium cornposition of the shocked gas mixtures. The computer program in general couples the Rankine-Hugoniot equations with a Newton-Kaphson iteration procedure t o minimize the Gibbs free energy of the shocked gas composiThe Journal of Physical Chemistry, Vol. 77, No. 23. 7973
A. P. Modica
tion, The free-energy functions for the sulfur fluorine species ~ were taken from~ the Aerospace othermochemical~ tabled2 mentioned in ref 6. Free-energy functions of all other species considered in the equilibrium calculations were obtained from JANAF thermochemical tables.13 The results of equilibrium calculations with the Aerospace tables for the sulfur fluorine species predicted that over the shocked temperature range of the study ( 16OO-24OO0K), the SFs and SF5C! molecules would be virtually dissociated completely to the SF4 finsl decomposition product. However, in the present study, measured ultraviolet SF4 concentrations and the mass spectrometer data showing the dominant ion peak to be SF5+ in shock-heated SFs
2719
SFS a n d SFBCl E q u i l i b r i u m D e c o m p o s i t i o n 105
L.
T R R K R A T E ' INTEGRAL E ~ ~ - ~ , E ~ : c ~o i o~n oQi eZ O ._ A 3 : 0 . Z 5 IBOTT. JACOBS1 LOC.' E - 6 5 2 0 0 c o l / m o l c Ao:O.O% IT315 W O R K 1
7
I
-
io^^^^ Sr6 S:7 b .
until calculated SF4 concentrations were in agreement with experiment. Also the Aerospace free-energy function for SF4 was modified to the original JANAF table SF4 heat of formation j.Nf0(SF4), -172.2 kcal/mol. Typical shock tube equilibrium computer output fer the SFs, SF5Cl, and SFG-hydrogen mixtures tested in the study are presented in Tables I, IT, and 111. A comparison of experimental and calculated SF4 concentrations for the shocked mixtures shows that the data independently are best fitted with a SFS heat of formation taken to be j.Hf"(SFa), -241.7 kcal/mol (Figures 5 ! 6, and 7).
VII. Discussion
4.8
5.2
5.0
5.4
5.8
5.6
oooo ,*
6.0
K-l
T CURVES ARE CALCULATED WITH RRK RATE INTEGRAL
*,THE
C O L L I S I O N FREQUENCY. GIdEN B Y w.m2
1Oir)l'Z PkT
0
Figure 8. Theoretical curve fits to Bott-Jacobs e x p e r i m e n t a l limiting first-order rate c o n s t a n t s f o r S F 6 d i s s o c i a t i o n . TABLE I V : Comparison
of T h e r m o c h e m i c a l Heats of
F o r m a t i o n for
Sulfur Fluorine Species
__
-
Ref
Heat of Formation !O"K), kcal!moi
Bond energy kcalimol
Mass spectrometer and ultraviolet absorption dara on SE's decomposition have indicated for the temperature interval of the present study an equilibrium reaction between the SF5 radical and SF4 molecule. Thermochemical shock tube calculations with the proposed equilibrium mechanisms are also found to fit the experimental steadystate SF4 absorption data of the SF5Cl thermal decompo.sition reactions, Infrared emission measurements on the formation of HF from the SF6 reaction with hydrogen showed that the hydrogen atoms were reacted stoichiometrically with equal numbers of SF6 fluorine atoms. Equilibrium SF4 and H F concentrations were therrnochemically calculated for shock SFC-hydrogen mixtures and were comparable to the experimental SF4 u1travio:et absorption and MF infrared emission data. The SF5 heat of formation, l H t o ( S F 5 )= -241.7 kcal/mol, and the SF4 heat of formation, AiWf0(SF4)= -172.2 kcallmol, taken from the first edition of the JANAF Table were found to give a best fit to all the data of the present study. A cornaarison of the heats of formations used in Bott-Jacobs thermochemical calculatior! and those used in the calculations of this work are gi.ven in Table XV. The first and second bond dissociation energies in SFs are derived from the appropriate heats of formation in both studies and are also pre-sented. It is also shown here that the value of Uo(SF5-F) = 65.2 kcal/mol can yield good equivalent RRK integral curve fits to the Bott-Jacobs limiting first-order SFs dissociation rate constants (Figure 8). References and Notes
Bott--Jacobsa ( r e f 6)
This w o r k b
SF6 SF5
-289.0
SFa
-185.1
SF6
-288'4 -241'7
SF'
SF,
-232.5
Do (SF5-F) 74.9 Do(SF4-F) 65.7
Do (SF5-F) 65.2 Do(SF4-F) 87.9
-172.2
a Based on ref 12. The 1 f f f o ( S F 4 ) value has been adopted in ?he second edition of tho JANAF Tables, ref 13. The Affr"(SF5) vaiue is obtained by curve fitting the equilibrium SF4 ultraviolet aDsorption data. The AHf"(SF4) value is taken from the first edition of the JANAF Tables.
were in disagreement with Aerospace thermochemical data. Assuming SF4 was the final product of SFe and SF&! decomposition reactions, shock tube equilibrium calculations were repeated varying the heat of formation of the SFs radical in the Aerospace free-energy function
( 1 ) J, A. Harrington, A. P. Modica, and D . R. Libby. J. Chem. Phys.. 44,3380 (1966), (2) A P. Modica and S. J. Sillers, J. Chem. Phys.. 48, 3283 (1968) (3) A. J. Carswell and G.G.Cioutier. Phys. Fluids. 7. 602 (1964). (4) K . E. Starner, H l A A J . . 7 , 2357 (1964). (5) D. J. Spencer, T . A. Jacobs, b1. Mirels. ana R. W. F. Gross. inf. J . Chem. Kinet., 1, 493 (1969). (6) J. F. Bott andT. A . Jacobs, J. Chem. Phys.. 5 0 , 3850 (1969). (7) J F. Boit. J . Chem. Phys., 54, 181 (1371). (8) A. P. Modica and J. E. LaGraff, J. Chem. Phys., 43, 3383 (1965). (9) W. M . Hickarn and R. E. Fox, J. Chem. Pirys., 25,642 (1955). (10) L. E. Young (Pvco Everett Research Lab). private communication. i l i ) R. Colirang and K. 0 . Friderichs, "Supersonic Flow and Shock Waves," Interscience, New York. N . Y., 19.48: R. D . Gillespie and J. Warga, "A Program for Computing Thermochemlical Equilibrium BeWnd a Moving Shock Wave." Avco Report No. RAD TM-63-65 (Sept 1963);J. Warga, J. SOC.ind. Appl. blath.. 4 1 , 594 (1963) (12) R. L. Wilkins, "Thermodynamics of SF6 and Its Decomposition and Oxidation Products," The Aerospace Corporation, TR-0158 (324020)-19 (1968) (13) "JANAF Thermochemical Tables, ' D. R. Stull, Ed.. Dow Chemicai Go., Midland, Mich., 1961 -1366.
The Journal of Physica! Chemistry, Vol. 77, h o . 23. 7573
