JOHNT. HERRONAND ROBRRT E. Hum
1328 zero emf was not attained until concentrations X , of 29 mol % Ce in CeC13 l o and 33 mol % La in LaClB,l2 far in excess of the actual saturation limit which is very firmly established as 9 and 10 mol yoin the Ce-CeCIB 318 and La-LaCla,6JS respectively. Also, the claim is made that the interchange of a NaC1-KC1 eutectic, Clz, C electrode for the CeC13,Clz, C electrode in the cell
Mo 1 Ce (satd) CeCL, boron nitride
1 asbestos
1 diaphragm
1
CeCls 1 CIS, c
crucible produces no change in potential of the cell."
The
present study shows that a sizable effect of 0.6 V should have been noted. It is nothing less than astonishing that solubility measurements made by the Russian workers10J2using molybdenum crucibles and sampling of the salt-rich phase a t the equilibration temperature as well as quenching experiments yielded the same results as they obtained in emf and conductivity measurements where reaction with the ceramic containers undoubtedly occurred. In a recent article PolyachenokP6also takes note of the discrepancies in the saturation concentrations as determined by Smirnov, et U Z . ~ O J ~ (46) 0. G. Polyachenok, 2. Neorg. K h i m . , 13, 200 (1968).
Rates of Reaction of Atomic Oxygen (O'P). Experimental Method and Results for Some C1 to C, Chloroalkanes and Bromoalkanes by John T. Herron and Robert E. Huie Institute for Materials Research, National Bureau of Standards, Washington, D . C.
2 0 . 2 3 ~ (Receiued August 1 5 . 1 9 6 8 )
A method of measuring rate constants for the reactions of atomic oxygen (gap)with organic compounds i described, and rate constants are reported for the reactions of atomic oxygen with 12 chloroalkanes and bromoalkanes from 336 to 622'K.
Introduction
Experimental Section
The reactions of atomic oxygen (8P) are of importance in areas of atmospheric chemistry ranging from the properties of the upper atmosphere to air pollution. Although the number of reactions of known or probable importance is large, only a limited number of reliable rate constants have been reported. The only class of compounds which has been studied extensively is the olefins, for which relative rate constants have been reported by Cvetanovic and coworkers,l and absolute rate constants have been reported by Eliasa2 In the case of the alkanes reliable data exist for methane and ethanea and for n - b ~ t a n e . ~For other reactions the data come from a variety of sources, are often limited to one temperature or a small range of temperatures, or are simply unreliable, In view of the need for experimental data in this area, we have undertaken a survey of the reactivity of atomic oxygen with different classes of organic compounds, using a mass spectrometric method. In this first part we report results for a series of chloroalkanes and bromoalkanes.
The experimental apparatus has been described in detail el~ewhere.~It is basically an electrical dischargeflow system coupled to a mass spectrometer. Atomic oxygen was prepared by passing a mixture of 1-5% oxygen in an argon carrier through a 2450MHz electrodeless discharge. This resulted in the decomposition of about 20% of the molecular oxygen. In addition to ground-state atomic oxygen, the discharge may yield various metastable atomic and molecular species capable of initiating reaction. This point is discussed later. The mixed gases passed through a trap cooled with liquid nitrogen before entering the discharge zone, After leaving the discharge zone, the gas mixture flowed through the reactor which was a 20-mm i d . Pyrex tube with an overall length of about 40 cm.
The Journal of Physical Chemiatry
(1) R. J. Cvetanovic, Advan. Photochem., 1, 115 (1963). (2) L. Elias, J . Chem. Phys., 38, 989 (1963). (3) A. A. Westenberg and N.de Haas, i b i d . , 46, 490 (1967). (4) L. Elias and H. I. Schiff. Can. J . Chem., 3 6 , 1657 (1960). (5) F. S, Klein and J. T. Herron, J . Chem. Phys., 41, 1285 (1964).
5327
RATESOF REACTION OF ATOMICOXYGEN Summary of Rate Meaaurernents for the Reaction 0
Table X:
Temp,
"K
381 385 385 434 434 437 448 505 506 508 513 603 603 607 607 a
1 Torr
=
Total pressure,a N m-a
velocity, cm sec-1
302 318 297 299 299 297 306 295 297 281 291 286 299 293 302
900 700 920 1020 1040 1100 1100 2090 2090 2070 2060 3540 3440 3530 8440
+ CH&l
---10-11(0),,
115 110 123 66.2 82.6 57.8 71.2 66.1 69,5 63.7 49.2 43.8 44.8 40.7 43.4
4
OH
+ CH2CI
Reactant concn, mol lO-la(A)o
cu-g
161 233 251 179 160 128 252 154 11.4 169 129 116 72.5 118 120
lO-la(A)t
157 228 246 173 154 122 235 122 10.5 164 122
103 66.9 110 112
-
lo-%, cma mol-! sec-1
0.108 0.074 0.098 0.348 0 293 0.522 0.670 1.12 1.38 0.688 1.48 6.15 3.88 3.51 3.48 I
133.32 N mV2.
Table 11: Summary of Rate Measurements for the Reaotion 0 -/ CzHbCl-+ OH Temp, nK
Total pressure, N m-1
Velocity, cm sec-1
336 336 339 381 385 390 434 434 437 437 505 506 513 603 603 607 622
318 309 284 302 297 304 299 299 284 297 295 297 291 286 299 293 274
810 760 850 900 920 950 1020 1040 1040 1104 2092 2090 2060 3540 3440 3530 3320
----Reactant 10-"(0).r
156 127 117 112 109 99,l 80.6 79.9 54.3 55.6 66.1 67.7 50.0 43.0 44.8 37,5 22.6
Linear flow velocities varied from about 5 to 40 m/sec. (See Tables I-XII.) Reactant gas was added through a movable central inlet tube. The position of this tube defined one end of the actual reaction zone. The other end was fixed by the position of the sampling orifice of the mass spectrometer. The length of the reaction zone could be varied from about 5 to 30 cm. The reactor could be heated by means of a foursection resistance heater wound over a ceramic form which in turn fitted snugly around the reactor tube. Temperature was measured by means of a Pt-Pt-lO% R,d thermocouple inserted in the movable reactant inlet. The uncertainty in temperature due to g r a dients and in measuring the thermocouple emf was about 3". The temperature of the reactor could also be con-
+ CzH4Cl
concn,mol om-&----10 -1a (A)o lO-la(A) 6
132 222 58.6 51.7 81 .o 36.3 17.3 20.1 19.4 50.8 61.2 19.9 11.8 73.7 58.8 13.6 25.9
122 205 55.0 45.7 68.6 31.7 12.9 16.1 16.3 40.4 48.3 14.3 9.04 52.1
40.3 8.65 23.2
lO-lOk, cma mol-1 sec-1
0.240 0 323 0.264 0.632 0.684 0.815 2.43 2.27 2.09 2.78 4.70 6.47 7.04 18.1 18.6 25.8 25.6 I
trolled by circulating fluid through a jacket surrounding the reactor. The temperature of the fluid was controlled by circulation through a constant tempera ture bath. This latter arrangement, although not used in any of the measurements reported in this paper, has been used in other studies to be reported later. Its primary use was in extending measurements to lower temperatures. In this case, temperature was measured by means of a copper-constantan thermocouple immersed in the reactor jacket. The approximate ranges of temperatures used were 195-360°K for the circulating liquid system and 330-650°K for the resistance heater system. Total pressure was measured by means of a diaphragm-type manometer calibrated against a McLeod gauge. Accuracy was better than 1%. The pressure measurement was made through the reactant inlet Volume Y3,Number 6 May 1989
JOHNT,HURRONAND RQEWRT E, Hum
1328
+ i-C8H1Cl
Table 111: Summary of Rate Measurements for the Reaction 0 Temp, "K
Total pressure, N m-a
Velocity, cm am-1
336 330 339 339 843 394 429 437 451 461 451 452 612 532 595 603 606 607 621 622
281 259 267 281 306 300 318 284 295 286 290 357 274 283 307 286 277 293 286 274
1430 1500 1170 830 820 950 980 1040 1060 1860 2990 1120 2210 2180 3500 3640 2550 3530 3640 3320
r -
lO-l*(O).v
Total
pressure, N m-z
336 336 339 339 381 394 410 437 443 448 506 512 613 604 607 622
279 304 281 281 302 300 311 284 290 290 297 281 291 286 293 274
Velocity, cm sec-1
1420 800 810
840 900 950 980 1040 1800 1800 2090 2140 2060 3530 2550 3320
+ CsHsCl
15,1 14.5 4.53 18.3 16.2 2.98 11.4 3.07 8.83 17,O 19.9 7.15 5,87 19.4 20.0 3.79 3.25 17.5 11,7 4.46
+ sec-C4B&1
---
4
OB
13,4 11.4 3.58 15,O 12.7 2.11 8,37 2.07 4.68 12,9 16.8 5.24 4.00 10.1 11.5 1.95 2.09 9.64 8.04 2.63
89.9 50,2 89.0 88.4 113.6 68.7 63.2 53.1 12.9 66.8 71.9 34.0 48.4 44.2 38.7 22.4
-
lO-lok,
cmx mol-] sec-1
1.13 1.34 1.39 1,27 1.22 3.39 4,75 6.17 6.86 6.40 7.79 7.90 17.0 17.7 42.2 33.8 37.6 32.9 38.0 51.2
+ CrH,C1
Reactant concn, mol cm-&------
10-1l(O)."
(with no reactant flow), and refers to the midpoint of the reactor, The pressure drop across the reactor was never greater than about 5%. The flow rate was measured by collecting pump effluents over water or by means of a calibrated flowmeter of 0.25% accuracy. Agreement to better than 1% waa found between the two methods. Reaction time was taken to be the reactor volume divided by the volume flow rate. The gas composition in the reactor was continuously monitored by means of the mass spectrometer. Partial pressures of reactants were determined by calibrsr tion with the appropriate pure substances. Relative atomic oxygen measurements were based on the mass-16 peak at reduced ionizing energy (about 20 eV) . This The Journal of Physical Chemistry
OH
Reactant concn, mol cm-a--lO"*(A)o lO-'*(A)t
92.9 92.3 87.9 87.4 99.4 59.5 45.6 54.9 54.5 47,8 47.4 29.1 31,8 49.3 37.2 42.9 26.8 38.7 21.3 20.7
Table IV: Summary of Rate Measurements for the Reaction 0 Temp, OK
--+
10 -18 (A)o
10-1a(A)t
296 190 111 75.4 13.0 97.7 48.2 31.8 37.7 26.4 32.3 83,6 33.7 9.34 36.3 21.2
241 124 74.6 50.4 6.29 5347 20.9 13.9 13.1 14.1 10.9 42,O 13.8 3.00 x3,a 10.6
lO-IOk,
cma mol-'
am-1
2.02 1.95 2,25 2.28 3.63 5.97 8.03 9.93 9.66 10.3 19.9 29.0 24,0
57.8 56.3 72.9
was put on an absolute basis by means of the NO, titration.6 A serious experimental problem encountered in this work was that arising from the increase in background signal when the reactor was heated. The outer walls of the reactor are in direct contact with the inside of the ion source housing, so that on being heated they evolve gases which give rise to background signal. Inasmuch as we were not equipped to discriminate against background signal, we have had to program our experiments around this problem. The background spectrum resembles that of a mixture of alkanes, and is most serious below mass 60. (6)
F. Kaufman, Progr. Reaction Kinetics,
1, 1 (1961).
RATESOF REACTION OF ATOMICOXYGEN
1329
+ i-C4HsC1+ OH + C4H8Cl
Table V: Summary of Rate Measurements for the Reaction 0 Temp,
O K
336 339 339 381 390 394 434 443 443 505 506 512 513 603 607 607
Total pressure, N m-2
Velocity, cm sec-1
304 281 281 302 304 300 299 290 290 295 297 281 291 286 293 302
800 810 840 900 950 950 1020 1840 1840 2090 2090 2140 2060 3530 3550 3440
Reactant concn, mol cm-a---
10-11(0).v
102 86.7 102 107.3 92.6 50.3 63.5 129 64.9 64.7 65.9 30.2 47.6 41.6 36.5 40.7
Table VI: Summary of Rate Measurements for the Reaction 0 Temp,
O K
336 339 339 381 390 394 410 437 437 452 506 512 513 603 607 607
Total pressure, N
m-9
318 281 284 302 304 300 311 284 297 357 297 281 291 299 293 302
Velocity, cm sec-1
810 840 850 900 950 950 980 1030 1100 1120 2090 2140 2060 3440 3550 3440
Total pressure, N m-*
336 339 339 381 394 410 437 437 443 506 512 513 603 606 607
304 281 281 302 300 311 284 297 290 297 274 291 286 277 293
Velocity, cm sec-1
800 810 840 900 950 980 1030 1100 1840 2090 2210 2060 3530 2550 3550
10-18(A)t
10-18 (A) n
538 54.4 64.0 14.3 29.0 112 18.1 29.0 21.1 15.8 22.4 18.8 20.6 18.3 27.5 11.4
cma mol-1 sec-1
2.31 1.99 2.64 6.40 4.72 5.91 8.50 10.1 11.3 18.8 21.3 24.3 25.5 72.6 71.6 68.3
336 38.5 38.1 4.50 13.7 64.7 7.81 9.47 11.2 6.34 7.65 11.3 8.14 4.75 8.06 3.16
+ t-C4HsC1+ OH + CrHBCl
------Reactant
lo-"(o)Bv
159 90.4 120 111 100 59.5 63.2 53.6 58.3 37.1 68.8 29.6 48.1 43.9 37.5 39.0
Table VII: Summary of Rate Measurements for the Reaction 0 Temp, OK
lO-lOk,
7---
concn, mol cm-a-----lO-'a(A)o lO-la(A)r
85.3 100 211 91.7 47.5 188 86.0 91.3 62.3 67.3 51.9 62.6 63.8 76.1 168 63.5
+ neo-CsHI1C1
----1o-"(o).v
108 87.9 85.3 111 58.0 60.1 52.2 55.8 64.0 67.7 32.0 46.8 42.4 26.4 39.4
--$
OH
76.9 25.1 188 79.5 39.7 159 70.8 74.1 51.3 59.0 37.3 52.5 48.1 50,5 117 47.0
0.334 0.522 0.333 0.728 1.05 1.65 1.96 2.46 2.32 2.70 6.40 8.63 7,65 20.6 20.8 16.9
+ CsH&l
Reactant concn, mol cm-a------lO-'S(A)o lO-la(A)t
457 78.5 56.1 25.7 70.7 34.5 41.5 27.9 24.0 29.6 21.3 22.7 18.0 13.8 17.5
lo-lok, cma mol-1 sec-1
338 64.5 44.4 17.2 51.4 21.2 24.0 15.5 16.1 13.8 14.2 12.2 7.64 9.34 8.24
lO-lOk,
cma mol-1 sec-1
1.38 1.13 1.41 2.07 3.32 4.91 6.59 7.39 7.24 14.4 18.3 17.3 45.6 36.6 41.1 Volume Y% Number 6 May 1060
JOHN T. HERRON AND ROBERT E. HUIE Table VIII: Summary of Rate Measurements for the Reaction 0 Temp, OK
Total pressure, N m-8
385 385 429 433 434 448 505 508 595
318 297 318 299 299 306 295 281 307
Velocity, cm sec-1
7 -
lO-"(O).v
700 920 980 1040 1020 1100 2090 2070 8500
107 115 45.6 80.7 63.5 68.9 66.1 51.8 39.5
OK
336 336 339 385 385 390 433 434 448 503 505 508 600 603 607
Total pressure. N m-8
Velocity, cm sec-1
318 309 284 318 297 304 299 299 306 281 295 281 281 299 302
810 760 850 700 920 950 1040 1020 1100 3180 2090 2070 3590 3530 3440
pressure, N m-9
Velocity, cm sec-1
336 336 339 339 385 385 390 433 434 437 443 505
318 309 281 284 318 297 304 299 299 284 290 295 297 791 299 293 302
810 760 840 850 700 920 950 1040 1020 1040 1840 2090 2090 2060 3440 3536 3440
Total
506
513 603 607 607
Reactant concn. mol cm-------10-18 (A)o lO-l*(A) e
587 507 58.8 194 494 183 360 203 32.0
Reactant concn, mol cm-a 10-1~(O)nv 10-1a(A)o
158 126 118 106 111 98.9 79.3 -64.7 68.2 90.9 66.1 47.7 56.3 43.7 42.6
lO-lOk, cms mol-1 eec-1
573 497 57.4 189 482 177 346 199 29.6
0.096 0.113 0.347 0 201 0.270 0.382 0.781 0.603 5.10 a
-
147 191 414 140 51.8 119 98.6 82.1 26.5 24.3 28.6 44.1 16.3 20.2 18.3
7
lO-l:(A)t
138 180 391 125 46.9
108 77.4 69.5 21.5 19.0 22.3 38.5 11.5 14.4 13,5
lO-lQk, cm: mol-1 sec-1
0,198 0.239 0.265 0.492 0.522
0.581 2.01 1.69 2.13 5.33 4.99 3.74 13.7 16.9 15.6
+ i-CsH7Br-+ OH + CaHeBr
,-10-"(0).y
157 123 84.9 115 103 102 98.5 78 .O 63.2 56.0 67.4 63.5 69.7 47.3 42.9 40.4 42.6
In practice, therefore, it is very difficult to measure rate constants at elevated temperatures for any of the lower members of the alkane series. In some cases deuterium substitution permits the reactant to be distinguished from background signal. The Journal of Physical Chemistry
+ CHaBr
7---
Table X: Summary of Rate Measurements for the Reaction 0 Temp. OK
OH
+ CIHsBr -+ OH + CpHlBr
Table IX: Summary of Rate Measurements for the Reaction 0 Temp,
+ CHaBr -+
Reactant concn, mal cm-~--------? IO-l:(A)o 10-qA)r
74.1 210 126 174 123 99.5 91.9 59.4 73.8 117 49.6 63.4 44.0 66.0 56.2 44.8 36.1
58.8 175 107 140 85.5 74.9 63.6 35.3 \48.1 75.1 36.3 38.5 25.1 43.9 31.9 26.0 20.3
IO-lQk, cm* mol-: 8ac-1
0.753 0.785 0.992 0.965 1.55 1.61 2.18 6.76 4.43 5.17 5.43 1.0.6 10.5 11.6 29.1 28.5 29.1
In the case of the alkyl halides reported here, this problem is much less serious. However, for some of these compounds the parent ion in the mass spectrum was too small to be used, and it was necessary to use a fragment ion to monitor the partial pressure. In
RATESOF REACTION OF ATOMICOXYGEN
1331
Table XI: Summary of Rate Measurements for the Reaction 0 Temp,
O K
336 336 339 385 385 390 433 434 448 603 605 508
600 603 607
Total pressure, N m-*
318 309 284 318 297 304 299 299 306 281 295 281 291 299 302
Velocity, cm 8ec-1
+ n-CdHeBr -Reactant
I
Temp, OK
336 336 339 385 385 387 429 448 503 508 595
309 259 284 318 297 299 318 306 281 281 307
lO-lOk,
10-1:(A)t
cma mol-1 sec-1
158 120 108 101 101 89.7 74.2 62.1 67.1 87.1 63.5 39.1 53.4 39.7 41.1
37.4 125 135 74.9 47.9 77.1 26.0 31.4 28.9 10.1 14.0 27.4 13.8 17.2 26.6
22.7 85.4 87.7 34.5 25.9 36.1 8.38 12.9 11.6 4.35 4.48 15.3 4.37 5.20 8.62
1.67 1.62 2.01 3.41
+ i-C4HeBr
--$
,-----
760 1500 850 700 920 930 980 1100 3180 2070 3500
concn, mol cm-r-lO-*O(A)o
810 760 850 700 920 950 1040 1020 1100 3180 2090 2070 3590 3530 3446
Velocity. cm sec-1
+ CdHBBr
10-11(Oh"
Table XII: Summary of Rate Measurements for the Reaction 0 Total Pressure, N m-8
OH
--.)
OH
3.53 4.86 9.71 9.34 9.36 19.4 23.8 19.2 49.3 67.1 39.0
+ ClHsBr
Reactant concn, mol c m - a 10-10 k,
10-11 (O),V
114 274 77.8 64.7 90.6 71.8 75.9 61.6 34.9 104 20.7
121 88.0 113 99.7 85.4 64.5 41.4 67.5 85.1 56.4 37.1
this case the situation could again be unfavorable if the fragment ion was of low intensity and unfavorably located in the mass spectrum. For example, in the case of isobutyl chloride, measurements were based on the mass-56 peak which is not only a low intensity peak in the spectrum but corresponds to hydrocarbon background.
Nature of the Reactive Species Although ground-state atomic oxygen (03P) is the principal reactive species produced in our discharge system, we must be certain that other potentially reactive species produced in the discharge do not affect the rate measurements. A t the outset we can probably eliminate metastable argon atoms by noting that discharged argon by itself is totally unreactive, The only metastable oxygen species which have been observed any distance from the discharge zone are O,(IAg)and 0~('Zg+)with excitation energies of 0.98 and 1.64 eV mol-', respectively. Such species are not sufficiently energetic to initiate reaction.7 Other potentially important species are 0 (ID) and
10-1s (A)o
-
10-18 (A)t
cms mol-1 sec-1
73.5
1.87 2.95 2.47 3.57 4.12 4.35 10.1 9.79 27.0 19.2 72.9
254
45.1 29.7 49.4 43.6 41.6 19.9 11.1 44.2 7.36
O(lS) which are both metastable. However, these species were not detected in a mass spectrometric study of the products of discharged oxygen.s Furthermore, nitrous oxide, which probably reacts on every collision with 0 (ID) ,* does not react to any measurable degree with the products of a microwave discharge in oxygen.8 It seems reasonable to conclude that the rate data reported here refer only to reactions of ground-state atomic oxygen,
Rationale of the Rate Measurements
+
For the general reaction 0 A 3 products, the integrated form of the rate expression may be written as
where (A)oand (A) t are the concentrations of reactant (7) Computed rate constants for hydrogen atom abstractions by these species are given by 9. W. Mayer and L. Schieler; J . Phys. Chem., 7 2 , 26.28 (1968). ( 8 ) J. T. Herron and H. I. Schiff, Can. J . Cham., 3 6 , 1159 (1958). (9) H. Yamazaki and R. J. Cvetanovic, J. Chem. Phys., 39, 1902
(1968). Volume 78, Numhr 6 May 1080
JOHNT. HERRON AND ROBERT E. HUIE
1332 at times zero and t, respectively, and (0) is the concentration of atomic oxygen. Only relative values for (A)o and (A) are required. This simple rate expression is valid if reactant is not reformed in subsequent steps, and is consumed only through reaction with atomic oxygen, i.e., reaction products such as atomic hydrogen or hydroxyl radicals do not react to any appreciable degree with the reactant. To ensure that this is the case, the experiments have been carried out under conditions of excess atomic oxygen, Furthermore, the amount of reactant A consumed has been kept as small as possible. Typically (see Tables IXII) (0)B V / ( A = ) ~ 100 and (0),,/A(A) ‘v 500. If we assume to a first approximation that each reactant molecule lost leads to one reactive product species, then the total number of such species is about 346 of the number of oxygen atoms. Species of like reactivity such as atomic hydrogen will make no contribution to the observed rate. On the other hand, hydroxyl radicals are about 100 times more reactive than oxygen atoms at room temperaturelo and could thus possibly interfere with the rate measurements. If we consider the simple reaction scheme O+A--tOH+R
+A OH + 0
OH
--+
+R +H
H2O
+ 0 2
(1)
(2)
v = 1460 cm s-8 T ~ 3 3 9K
7 * 4xlo-*
-
0.8
-
Q7 c
-
V
* 3320 cm s-1
5
=
0
0.6
%,
3 0.5
- 0.4
622 K
y = 2.5~
1 3 O3
0
0.2
2
4
6
B
IO
TIME, IOW3S
Figure 1. Loss of atomic oxygen by surface recombination. Open circles give (0)us. t , solid circles log (0)us. t. The dashed line represents the value of (0)predicted from the log plot.
(3)
and use values of IC1 ‘v lolo, kz N 10l2, and ks ru 1013, all in cm3 mol-1 sec-’, then a steady-state treatment leads to (OH) N kl(A)/k3 N 1OdS(A)= 10-6(0). At this concentration, hydroxyl radicals can have no effect on the rate measurements. Inasmuch as hydroxyl radicals are almost certainly the most reactive products to be found in these reactions, it seems safe to conclude that if A(A)/(O) is kept small, we may discount the effects of secondary reactions on the rate measurements. This point is considered further by Elias.2 If secondary reactions were important, then the measured rate constant would no longer be a constant, but rather would vary with (A/O)o. We observe no such dependency over about a fourfold change in (A/O)o. (See Table I11 in particular). These experimental conditions are such that addition of reactant does not appreciably alter the oxygen atom concentration. If there were no other loss mechanism for atomic oxygen
problems associated with adding excess NO2 through the central inlet tube. These data can be interpreted in terms of a pseudofirst-order loss mechanism for atomic oxygen, for which In (0) = k,t. A plot of log (0) vs. t may then be used to predict the oxygen atom concentration at t = 0 and to derive the rate constant k,. The latter is related to the recombination coefficient y by y = Dk,/v, where D is the diameter of the reactor and v is the root-mean-square molecular velocity. The values of y derived in this manner are shown in Figure 1. They refer to a poorly characterized surface which we can best describe as “dirty” Pyrex. Nonetheless, the values are reasonable, being about a factor of 2 lower than those for clean quartz surfaces.” Referring again to Figure 1,we may note that a plot of (0)vs. t is also essentially linear, and its extrapolation to t = 0 does not differ significantly from the values obtained from the log plots. This leads to the further simplication
( ( 0 ) dt = ( 0 ) t
[(O) However, there is loss of atomic oxygen due to surface recombination, which can be anywhere from 10 to 50% of the initial atom Concentration, depending on temperature and flow rate. This can be measured by titration with KO2. Typical data of this kind are given in Figure 1. Measurements were not made close to the base of the reactor in order to avoid streaming The Journal of Physical Chemistry
dt = (0)0.d
where (Ojo.a is the oxygen atom concentration at the midpoint of the reactor. (10) N.R. Greiner, J. Chem. P h y s . , 46, 3389 (1967). (11) P. G. Dickens and M. B . Sutcliffe, Trans. Faraday Soc., 6 0 , 1272 (1964).
RATESOF REACTION OF ATOMICOXYGEN
I
1
1333 This procedure may be repeated for a series of different reactants. Finally the mass-16 peak is measured with no reactant added, and the atomic oxygen is titrated with NOz at various positions along the reactor. A straight line is drawn through a plot of atomic oxygen concentration us. distance and the median value (0)0.6 is read off.
1
Results and Discussions
IO9
t
1.5
I
I
2.0
2.5 IO)/T,
,I
3 .O
K-'
Figure 2. Arrhenius plots for the reactions of atomic oxygen with CHaCI, CzH&I, and i-CaH7Cl.
Measurements were carried out from 336 to 622'K as summarized in Tables I to XII. Velocity refers to the linear flow velocity in the reactor. From a plot of log IC us. 1/T, shown in Figures 2 to 4,the Arrhenius parameters and rate constants at 298°K were derived as shown in Table XIII. Uncertainties are standard errors of the reported values based on a least-squares treatment of the data. For comparison, data on the equivalent alkanes are included. The methyl bromide data were the least reliable of the measurements. Because of the large uncertainties in log A and E/R, the data have not been plotted, and only approximate values of log A , EIR, and IC (298'K) are given in Table XIII.
In practice an experiment is carried out as follows. Reactant is added, the peak height for the particular compound is measured with the discharge on and off, giving (A)o/(A)~. Then the mass-16 peak is measured (at 20 eV) with the discharge on, giving a measure of the relative oxygen atom partial pressure. The flow of reactant is then stopped while monitoring the mass-16 peak to be sure that no change in the atomic oxygen partial pressure has occurred.
lo':
10"
e
5,
, I
13
'O"
Y
10'0
io*
ro3/~,K-I Figure 3. Arrhenius plots for the reactions of atomic oxygen with t-CdHsCI, neo-CsHlIC1, sec-CdH9C1,and i-C4H9Cl,
Figure 4. Arrhenius plots for the reactions of atomic oxygen with CeHbBr, i-CaH?Br, n-CtH9Br,and i-C4HSBr. Volume 79, Number 6 May 1080
JOHN T. HERRONAND ROBERTE. HUIE Table XIII: Log A, E/R, and k(298"K) for the Reactions of Atomic Oxygen with Some Alkanes, Chloroalkanes, and Bromoalkanesa Reactant Methaneb Methyl chloride Methyl chloridec Methyl bromide Ethane* Ethyl chloride Ethyl bromide Isopropyl chloride Isopropyl bromide n-Butaned n-Butyl bromide sec-Butyl chloride Isobutyl chloride Isobutyl bromide &Butyl chloride Neopentanee Neopentyl ohloride
Log A ,
10-9 k(298'K),
cma mol-1 sec-1
E/R
cms mol-' sec-1
13.23 13.40 1 0 . 2 0 13.26 n~ 1 3 . 0 13.26 13.68f0.09 13.49 10.11 13.42 f0.07 13.37 f0.09 13.48 13.64f0.08 13.55 f0.07 13.5610.11 13.53 f 0 . 2 1 13.42f0.11 13.77 13.50 f 0 . 0 8
4380 3880 f220 3990 -3500 3070 3330 190 3260 f 110 2620 f70 2700 f90 2110 2680 f90 2530 f70 2520 1110 2510 -i. 190 3010 1110 2920 2670 f80
0.024 0 056 0.007 NO.1 0.60 0.67 0.56 4.1 2.8 25.0 5.5 7.2 7.7 7.4 1.1 3.3 4.0 I
a This work except as noted. Uncertainties are standard errors of the reported values baRed on a least-squares treatment. b A. A. Westenberg and N. deHaas, J. Chem. Phys., 46, 400 (1967). The high-temperature points were not used in computing t.he Arrhenius parameters. Using all data, we compute an average E/R for ethane of 3470. W. E. Wilson, Jr., and J. T. O'Donovan, J. Chem. Phys., 48,2829 (1968). Based on relative rate measurements with respect to methane. Also gives rate constants at 613 and 966°K for methyl bromide. L. Elias and H. I. Schiff, Can. J. Chem., 88, 1657 (1960). Unpublished data from this laboratory.
The principal source of uncertainty in our data is in the measurements of (A)o/( A) t . Sources of systematic error, such as the measurement of pressure, flow, temperature, and reactor geometry, are small by comparison. The estimated imprecision in measuring (A)o/(A)tis closely reflected in the imprecision of the final results. Obviously the closer (A)o/(A)~ approaches unity, the poorer the measurement becomes. This explains the relatively poor precision in the case of the methyl chloride and bromide data. The absolute accuracy of the rate constants is probably within *,he range of twice the standard deviation. Although we have made no attempt to determine reaction mechanisms from a study of the reaction products, it seems reasonable to believe that they all involve the abstraction of a hydrogen atom and are thermoneutral or exothermic in nature. The corresponding chlorine or bromine atom abstraction reactions are from 60 to 120 kJ mol-' endothermic and hence are unimportant under our experimental conditions. Examination of Figures 2 to 4 indicates that the Arrhenius plots may not be exactly linear in all cases. If the reactions are hydrogen atom abstractions, then we would anticipate curved Arrhenius plots if more than one kind of C-H bond were involved. Comparing the results for the alkyl halides and the alkanes, it is apparent that the principal differences between the two classes of reactants are in the activ& tion energies rather than the preexponential factors, and that at least two opposing effects arise due to halogen substitution. In those cases where a primary hydrogen atom is replaced, and all other C-H bonds The Journal of Phveical Chemistpy
are primary, the overall activation energy is unchanged or reduced, e.g., methane, ethane, and neopentane (see footnote 6 of Table XIII) . On the other hand, where nonprimary hydrogen atoms are replaced or nonprimary C-H bonds are present, the overall activation energy is increased. Qualitatively, these effects can be related to the weakening of the C-H bonds on carbon atoms to which the halogen substituent is attached and the polar effects of the halogen substutient, respectively. These reactions are thus analogous to the abstraction of hydrogen atoms from the alkyl halides by halogen atoms.12 These effects are not predictable, however, so that in general the reactive sites are not known. In certain cases, e.g., the methyl and t-butyl halides, there is only one kind of C-H bond and therefore only one possible radical product. It may also be noted that the inductive effect in t-butyl chloride appears to be quite small since E / R is almost identical with that for neopentane. The magnitude of E / R for the ethyl and neopentyl halides suggests that for these compounds the reactive site is the carbon atom to which the halogen is attached. For the other reactants there are probably multiple reaction sites, and no detailed predictions can be made. For larger molecules, the polar or inductive effect becomes less important. For example, we find that for. n-heptyl chloride, lc(298"K) 8 X 1O'O cm8 mol-l sec-', which is comparable to Ic(298"K) for nheptane and much greater than any of the alkyl halide rate constants given in Table XIII.
-
(12) J. M.
Tedder, Quart. Rev. (London), 14, 338 (1980).
INTERACTION VIRIALCOEFFICIENTS IN FLUOROCARBON MIXTURES Some qualitative observations may also be made about some other reactions. Thus, methyl fluoride and cyanide have reactivities not much different from methyl chloride or bromide, On the other hand, the alkyl iodides react very rapidly a t room temperature, although it was not possible to measure rate constants since reaction products deposited on the walls of the reactor. In the case of methyl iodide, formaldehyde (but not formyl iodide) was found to be a major product along with a yellow-brown hygroscopic deposit. An aqueous solution of the latter was acidic and precipitated silver iodide on addition of silver nitrate, which suggests that the wall deposit was a complex oxide of iodine.13 The greater reactivity of the alkyl iodides suggests
1335
that these reactions may involve the abstraction of an iodine atom rather than a hydrogen atom. If this is the case it might help resolve the question as to the bond dissociation energy of IO, for which two values have been proposed, 174 f 19 kJ m ~ l - ~ and , ' ~ 238 f 24 k J mo1-1,16the latter being the only one compatible with the iodine atom abstraction mechanism.16 (13) I). I. Walton and L. F. Phillips, J.P h y s . Chem., 70, 1317 (lees), in a study of the atomic oxygen-iodine reaction describe a similar deposit, which they identifled as being principally IzOS. (14) E. A. Durrie and D. A . Ramsay, Can. J. Phys., 36, 35 (1958). (15) L. F. Phillips and T. M. Sugden, Trans. Faraday Soc., 57,
914 (1961). (16) The C F d reaction is also fast, and leads to the formation of CF20, probably through the secondary reaction 0 C F s - CE.20
+ F.
+
Interaction Virial Coefficients in Fluorocarbon Mixtures by E. M. Dantzler and C. M. Knobler Contribution No. 2276, Department of Chemistry, University of California at Los Angeles, Los Angeles, California 9 0 0 . Q (Received September 4, 1 9 6 8 )
Measurements are reported of the excess second virial coefficient, E [=BIZ- )(B11 1- &)] and interaction second virial coefficients are calculated for binary mixtures of the normal perfluorocarbons,Ferfluoromethane through perfluorohexane, at 50 and 100'. The results are well described by a Kihara spherical core potential using the geometric mean of the energy parameters. The applicability to these mixtures of a three-parameter corresponding states equation originally developed for hydrocarbons is discussed. The principle of congruence is approximately valid for the excess volumes and heats of mixing in these systems.
Introduction This is the second in a series of three papers dealing with the low-density equation of state of binary mixtures of simple nonspherical molecules, We have previously described measurements on the normal alkanes, methane through hexane,' and the present communication is devoted to similar studies of the normal perfluoroalkanes. I n a later paper alkaneperfluoroalkane mixtures will be investigated. With this plan in mind it is desirable to formulate some consistent picture of the interactions between these molecules which is equally applicable to both the hydrocarbons and the fluorocarbons. I n I we were able to show that the behavior of the hydrocarbon mixtures was accurately described by a three-parameter corresponding states formalism. The interaction virial coefficients of the mixtures could be predicted with the equation proposed by McGlashan and Potter2 if proper combining rules were chosen for the pseudocritical constants. It will be shown here that a three-parameter corresponding-states approach
is also applicable to mixtures of fluorocarbons, but it is necessary to base the description of the virial coefficients on a more general formulation than the McGlashanPotter equation. The previously utilized combining rules are equally effective.
Experimental Section Materials. Table I lists the sources and purities of the materials used in these studies. The analyses were performed by gas-liquid partition chromatography using a 42-ft Chromosorb P 60-80 column with SE 30 silicone oil stationary phase. The perfluoropentane and perfluorohexane were degassed before use. Apparatus. The apparatus used has been previously describedla and the details of the data analysis have been given in I. I n the experiments the pressure change, AP, on mixing two gases a t constant tem(1) E. M. Dantzler, C. M. Knobler, and M . L . Windsor, J. P h y s . Chem., 72, 676 (1968);hereafter referred to as I. (2) M. L. McGlashan and D. J. B. Potter, Proc. R o y . Soc., A267, 478 (1962). (3) C. M.Knobler, Rev. S c i . Znstr., 38, 184 (1967). Volume 79, Number 6 May IgBB
