- Email: [email protected]
Rate constant for the reaction BrO + NO → Br + NO2
Volume 61. number 2
RATE CONSTANT
CHEMICAL PHYSICS LEl-fERS
FOR TIIE REACTION
15 February 1979
BrO + NO + Br -I-NO,
Ming-Taun LEU Jet Propulsion Loboratov. Gzlifornia Institute of Technolo~, Pasadena,Glifomia 91103. USA Receiwd 16 October 1978
The rate constant for the reaction BrO + NO -+ Br f NO2 has been determine4 over the tempemture range 230 I( to 425 K in a discharge flow system using a mass spectrometer as a detector_ Results, expressed in the Arrhenius form kr = (7.11 5 0.23) X 10-t* eup[(296 f 10)/T] cm3 s-t, are compared with previous measurements.
I. Introduction There are two reasons to study the reaction Icl
BrO+NO+Br+NOa.
(1)
First, concern over possible ozone depletion in the stratosphere due to bromofluorocarbon compounds (e.g., CBrF3. CBrF2CBrF2) has generated considerable interest in bromine chemistry [l-3] _ Reaction (1) is important in determining the efficiency of the BrO, catalytic cycie. Second, recent studies of the reactions [4,5]
spectrophotometry with a multi-path cell to monitor BrO. Relatively high BrO concentrations (~101~ cmm3) were used. Second, Clyne and Watson [P] reported kl = (22 2 0.4) X lo-l1 cm3 s-l using the discharge flow/mass spectrometer technique_ Both studies were made at 298 K. Since the temperature dependence of k, is required to assess the effect of ozone depletion and to determine the Arrhenius parameters, in this paper we report the measurement of kl over the temperature range 230.3 to 424.6 K in a discharge flow system using a mass spectrometer as a detector.
2. Experimental Cl0 + NO 2 Cl + NO-,
(2)
and 16,771 HOz i- NO “,3 OH f NO2
(3)
indicate that kz and k3 have weak negative temperature dependences- The cause of the negative temperature dependence is not known with certainty. It is believed that the kinetics of reaction (1) are similar to those of reactions (2) and (3), and therefore it is of interest to determine if this reaction also shows a negative temperature dependence. Reaction (1) has been s&died by twc groups in recent years. First, Clyne and Cruse [S] obtained kI = 2.5 X lo-l2 cm3 s-l using ultraviolet absorption
method
The apparatus and the experimental principle used for this research have been described in detail in previous publications [lo,1 l] _All rate constant measurements were made by observing the decay of BrO (nz/e = 95) in a large excess of NO in a Pyrex flow tube 120 cm in length and 2.50 cm inside diameter_ In a side arm of the flow tube oxygen atoms were generated by passing a trace of oxygen in a helium carrier through a quartz discharge tube, with approximately 40 W of microwave power. The discharge tube was not coated. The atomic oxygen concentrations were measured by using the discharge on/off method, [O] = 2( [Oz] ott - [02] on)_ The BrO radicals were produced by admitting an excess of bromine [(3-10) X 1013 cme3] into the flow tube, leading to the reaction 27.5
Volume 61, number 2
- k4 O+Br,+BrO+Br_
CHEMICAL
PHYSICS
(5)
However, we assume the initial BrO concentrations were approximately equal to the atomic oxygen concentrations- The reactant NO. premixed with helium, was then admitted into the flow tube through a sliding Pyrex injector (6 mm o-d_) with muItipIe holes around the tip. The flow tube was coated with phosphoric acid aged under vacuum conditions. Flow fates of gases were measured by linear mass Rowmeters (Teledyne-Hastings-Raydist) Flowmeters were calibrated by a Hastings bubble-type calibrator (model HBM-1). The flow rate of the premixed NO/He mixture was cahbrated by the method ofpressure drop at constant vohrme and temperature_ The total pressure of the reaction zone was measured by a calibrated MKS Baratron pressure meter (model 310 Al-E-IO) which was calibrated against an Octoil manometer. The temperature of the flow tube was controlled by a high-capacity Haake circuIator with fluids flowing through the jacket and measured by a chrome1 -constantan thermocouple in the middle of the cooling jacket_ Most of the gases used for this research were supplied by Matheson Gas Products, inchnling hehum (UHP), oxygen (Research Grade) and nitric oxide (CP Grade). Helium was further purified by passage tbrougb a molecular sieve trap at liquid nitrogen temperature, wbicb was believed to be effective in removing Hz0 and Oz. Nitric oxide was purified by passing the gas through another molecular sieve trap at dry ice temperature to remove higher oxides of nitrogen_ The mass spectrometer was used to prove the absence of NO, (m/e= 46). Bromine (99.7%) was supplied from the J.T. Baker Chemical Company and further purified by passing through P,O, to remove water and vacuum distillation at 77-K and 196 K_
276
15 February
1979
3_ Results and discussion (4)
The rate constant k4 has been measured by CIyne and co-workers [ 12,13]_ The BrO then travelled approximately 10 ms to allow compIete reaction. A small amount of Br-0 radicals reacted with oxygen atoms by the side reaction [12,13] ks O+-BrO+Br+O1_
LEl-i-ERS
In the present experiment the gas concentrations in the reaction zone were adjusted such that 4 X 10’” <[He] <9X 10f6,2X 101oGIBrO] <4X loll, and 2-77 X lOl2 Q DO] d 16.6 X 1012 (cm-3). The initial BrO concentration [(l-4) X lo1 1 cmM3] was at least one order smaller than the NO concentration to satisfy the pseudo first order conditions_ Also, interference from the reactions BrO + BrO 2 2Br + 0,
(6)
and BrO + NO, + He 2 BrONO, + He
(7)
is believed to be negligibie based on the reported rate constant k6 [9,14] and an upper limit for k,. Under the experimental conditions the effective rate of 0.6 to 2.4 S-I for the reaction BrO + BrO is much smaller than that of 48 to 324 s- l for reaction (1). Also, in unpublished resuhs we have found the rate constant k7 i 10m30 cm6 s-l for M = He at 298 K using the same experimental apparatus. The effective rate for the reaction BrO + NO, + He is less than 0.1 s-l _ The radicals BrO were detected at nz/e = 95 amu by a mass spectrometer. The sensitivity for BrO detection was found to be 3 X 109 cmS3 for a signal to noise ratio of one, with 100 s integration. In all experiments 100 s was used for integration at every position of the sliding injector. The wall loss of BrO was checked and is negIigibIe (< 5 s-l). The BrO signa was found to be approximateIy constant when the injector was puked out without the reactant NO. Fig. 1 shows examples of the experimental data, taken under the following conditions: T = 298 K, P = 1.92 torr, F = 1850 cm/s, and [BrO] o = 2.8 X 1011 molecules cmm3. The concentrations cf nitric oxide were varied from 3.26 X IO*? to 7.63 X lOI2 molecules cmM3. [BrO] was observed to decay exponentially and was defmed as
lBti1 f = PrOI,-, exp(--kI1r),
03)
where ki is the first order rate constant for reaction (1) and t is the reaction time. kf was then calculated from the slopes of the straight lines in fig. 1_ The bimolecular rate constant k, was determined as
Volume
61, number
2
CHEMICAL
2I IO’ 9 0
LETTERS
15 February
27x
1.92,o,r
p =
!
1=298K ~=lBOCJ&U [ero]o= 2.8x 10"
"
1979
a NO2
BrO + NO--E,
L\
PHYSICS
cm-3
0.0
Fig_ 3. Arrhenius plot of kr versus 1/T of our data. The Lncertainty bars represent lo of the data. The numbers mentiozred
~IN0,=7_&3r1012,-3 below the data are the numbersof experiments. IO”,
I
t
I
I
10
in
30
Lo
I 50
f.on Fig. 1. BrO decays resulting from the reaction BrO f NO -t Br f N02_ I is the sliding injector position_
k, = k;[[NO]
_
(9)
All data at T = 230.3 K, 298.0 K, and 424.6 K are summarized in fig_ 2, which shows the variations of kf versus [b?O] _ The data can be represented approximately by a straigbt line with zero intercept at each temperature_ The average of these data at 298 K is I
I
BrO+NO-SrlN02
(I -89 -+0.16) X 10-l: cm3 s-l _ The uncertainty represents the first standard deviation of k, . All data were corrected for the effect of concentration gradient due to axial diffusion [ 151 with the maximum correction being about 2.5%. The correction for the effect due to radial diffusion was small and was not made (=Z5%) because of the imperfect knowledge of the radral concentration gradient [16]_ It should be noted that a gaseous diffusion coefficient given by DP = SO1-) torr cm?- s-l was assumed for the BrO radicals in helium at 300 K [ 173 _ A temperature dependence of T3” for DP was also assumed [ 17]_ The results of a total of 64 experiments taken in the temperature range of 230.3 K to 424.6 K are shown in tig. 3 and are summarized in table 1. A least squares fitting computer program was used to obtain the Arrhenius expression k, =(i’_llCO_23)X
lO-~~e(~96*1c)~T cm3 s-l_
Table 1 Summary of measurements
20
Fig_ Z Plot of ki versus [NO] 424.6 K..
of X-1
TW
kr(lo-” ---
230.3 261.5 298.0
2.55 f 0.19 a? 2.23 % 0.08 1.89 c 0.16
353.8 384 6
1.68 t 0.07 1.53 f 0.11
4 5
cm3 s-1)
424.6
1.41 t 0.13
10
at T = 230.3 I;, 298.0 K, and a) The uncertainty
(10)
Number of experiments 8 7 30
represents 10. 277
Volume 61. number 2
CH=tICAL
15 February 1979
PHYSICS LETTERS
TabIe 2
Comparison of measurements of X-t --_ ___.298 ii (cm3 5-l ) I
___-
25 x lo-‘2 (22 i O-4) x lo-” (I.89 t O-16) x IO-” -
Table 3 Temperature dependence for RO + NO -
Technique
Ret
dischatge flowlw absorption discharge flow~tnass spectrometer discharge Row/mass spectrcmeter
Clyne and Cruse ]8] dyne and Watson [9] this work
R + X02 reactions (R = Br. Ct end OH) Arrhenius expressions (cm3 s-’ )
TeN
~perimentf
7.11 x 10-t2e.\p(296/T)
Teobg6
discharge tloa f mass spectrometer
this uork
discharge Row/ mass spectrometer
Leu and DeMore [5]
dischargeflow/
Zahniser and Kaufman [4]
Reaction
Temperature range (EC)
BrO+h’O+Br+h’02
230-42s
aO+NO~Cl+No~ (AH = -9-14
217415
5.72
230-298
1.13 x IO-“e~p(20O/T)
H02+MO+OH+N02 -4.86
resonance Ruorescence (relative to the reaction CI+O3+ClO+O2) 3.3
x IcfraeIp(254/7-)
T-o-83
discharge flow/ iaser magnetic resonance
Howud
270-425
S-7
X 10-tze~p(130/f)
p-o.4
discharge flow/ resonance fluorescence
Leu [7]
kcal)
10-11(T/29S)-o-g6’o-04
cm3 s-l_
(11)
The systematic error of all data was estimated to be IS%, which inchrdes the uncertainties of absolute pressure, flow rates, the effect of axial and radial diffusions, the geometry of the flow tube, and the position of the sliding injector. Table 2 summarizes the comparison of the present results and those of previous measurements of 11-t_Our data at 298 K is in good agreement with that obtained by Clyne and Watson [9] _ However, the rate constant reported by Clyne and Cruse [S] is much smaller than the other three measurements_ interference by the reaction BrO f BrO + 2Br + 0, may have been serious, because they used very high BrO concentrations (==1013 cmm3)_ The negative temperature dependence observed in 27-S
Ref_
230-400
Again the uncer&ties represent the first stsndard deviation of the-4-factor a&d the acrivation temperature_ An altemarive expression obtained by the same computer prog-am is - _ __, k, = LS9X
T-o-75
kc@
+ NO4 OH + NO+ (AH = 4.86 kcal) (AH =
+o
x lo-“exp12961T)
kul)
ao+NodatNoP (AH = -9.14
H%
.
techniques
[6]
this work is compared in table 3 with those obtained for the reactions of this type RO t NO + R + NO, where R = Br, Cl and OH.
Acknowledgement The author wishes to thank Dr. W-B_ Dehlore for valuable discussions in the course of this work. This work was supported by the National Aeronautics and Space Administration under contract No. NAS7-100.
References ] P. Cmtzen. Gn- J- Chem. 52 (1974) 1569_ [2] S.C. Wofsy. h1.B. McEtory and Y-L. Yung, Geophys. Res. Letters 2 (1975) 215. 131 RT- Watson. CIAP hfonogmph l(1975) ch. 5, p. 125. [4] M-S. Zahniser and F_ Kaufman, J.
[l
3673_
Volume 61. number 2
CHEMICAL
PHYSICS LETTERS
[5] M-T. Leu and W-B. DeMore, J. Phys. Chem. (1978). to be published. [6] C-3. Howard, WMO Symposium on the Geophysical Aspects and Consequences of Changes in the Composition of the Stratosphere, Toronto, June 26-30,1978. 171 M.-T. Leu. J. Chem. Phvs. (1978). to be published. i8 j M.A.A. C&e and H_\V:C&e, T&s. F&day Sot. 66 (1970) 2214. ]9] hLA_A. CIyne and RT_ Watson, J. Chem- Sot_ Faraday Trans. 171(1975) 336_ [IO] hf.-T. Leu &d %.R. DeMore. Chem. Phys. Letters 41 (1976) 121.
15 February 1979
[ 11 ] M--T. Leu, CL. Lin and W-B. De&fore, J. Phys. Chem. 81 (1977) 190[12] M.A.A. CIyne and H-W_ Cruse, Trans. Faraday Sot. 67 (1971) 2869. 1131 M.A.A. CIyne. P-B. hlonkbouse and L-W_ Townsend, Intern. J_ Chem. Kinetics 8 (1976) 42.5. [14] N. Basco and SK Dogra, hoc. Roy. Sot. A 323 (1971) 401,417. 11.51F_ Kaufman, Progr- Reaction Kinetics 1 (1961) 1. 1161 R-V. Pohier and R-W. Carr Jr., J. Phys. Chem. 75 (1911) 1593. [:7] T-R. hbxrreroand E-A_ Mason, J. Phys. Chem. Ref. Data 1 (1972) 3_
279
RATE CONSTANT
CHEMICAL PHYSICS LEl-fERS
FOR TIIE REACTION
15 February 1979
BrO + NO + Br -I-NO,
Ming-Taun LEU Jet Propulsion Loboratov. Gzlifornia Institute of Technolo~, Pasadena,Glifomia 91103. USA Receiwd 16 October 1978
The rate constant for the reaction BrO + NO -+ Br f NO2 has been determine4 over the tempemture range 230 I( to 425 K in a discharge flow system using a mass spectrometer as a detector_ Results, expressed in the Arrhenius form kr = (7.11 5 0.23) X 10-t* eup[(296 f 10)/T] cm3 s-t, are compared with previous measurements.
I. Introduction There are two reasons to study the reaction Icl
BrO+NO+Br+NOa.
(1)
First, concern over possible ozone depletion in the stratosphere due to bromofluorocarbon compounds (e.g., CBrF3. CBrF2CBrF2) has generated considerable interest in bromine chemistry [l-3] _ Reaction (1) is important in determining the efficiency of the BrO, catalytic cycie. Second, recent studies of the reactions [4,5]
spectrophotometry with a multi-path cell to monitor BrO. Relatively high BrO concentrations (~101~ cmm3) were used. Second, Clyne and Watson [P] reported kl = (22 2 0.4) X lo-l1 cm3 s-l using the discharge flow/mass spectrometer technique_ Both studies were made at 298 K. Since the temperature dependence of k, is required to assess the effect of ozone depletion and to determine the Arrhenius parameters, in this paper we report the measurement of kl over the temperature range 230.3 to 424.6 K in a discharge flow system using a mass spectrometer as a detector.
2. Experimental Cl0 + NO 2 Cl + NO-,
(2)
and 16,771 HOz i- NO “,3 OH f NO2
(3)
indicate that kz and k3 have weak negative temperature dependences- The cause of the negative temperature dependence is not known with certainty. It is believed that the kinetics of reaction (1) are similar to those of reactions (2) and (3), and therefore it is of interest to determine if this reaction also shows a negative temperature dependence. Reaction (1) has been s&died by twc groups in recent years. First, Clyne and Cruse [S] obtained kI = 2.5 X lo-l2 cm3 s-l using ultraviolet absorption
method
The apparatus and the experimental principle used for this research have been described in detail in previous publications [lo,1 l] _All rate constant measurements were made by observing the decay of BrO (nz/e = 95) in a large excess of NO in a Pyrex flow tube 120 cm in length and 2.50 cm inside diameter_ In a side arm of the flow tube oxygen atoms were generated by passing a trace of oxygen in a helium carrier through a quartz discharge tube, with approximately 40 W of microwave power. The discharge tube was not coated. The atomic oxygen concentrations were measured by using the discharge on/off method, [O] = 2( [Oz] ott - [02] on)_ The BrO radicals were produced by admitting an excess of bromine [(3-10) X 1013 cme3] into the flow tube, leading to the reaction 27.5
Volume 61, number 2
- k4 O+Br,+BrO+Br_
CHEMICAL
PHYSICS
(5)
However, we assume the initial BrO concentrations were approximately equal to the atomic oxygen concentrations- The reactant NO. premixed with helium, was then admitted into the flow tube through a sliding Pyrex injector (6 mm o-d_) with muItipIe holes around the tip. The flow tube was coated with phosphoric acid aged under vacuum conditions. Flow fates of gases were measured by linear mass Rowmeters (Teledyne-Hastings-Raydist) Flowmeters were calibrated by a Hastings bubble-type calibrator (model HBM-1). The flow rate of the premixed NO/He mixture was cahbrated by the method ofpressure drop at constant vohrme and temperature_ The total pressure of the reaction zone was measured by a calibrated MKS Baratron pressure meter (model 310 Al-E-IO) which was calibrated against an Octoil manometer. The temperature of the flow tube was controlled by a high-capacity Haake circuIator with fluids flowing through the jacket and measured by a chrome1 -constantan thermocouple in the middle of the cooling jacket_ Most of the gases used for this research were supplied by Matheson Gas Products, inchnling hehum (UHP), oxygen (Research Grade) and nitric oxide (CP Grade). Helium was further purified by passage tbrougb a molecular sieve trap at liquid nitrogen temperature, wbicb was believed to be effective in removing Hz0 and Oz. Nitric oxide was purified by passing the gas through another molecular sieve trap at dry ice temperature to remove higher oxides of nitrogen_ The mass spectrometer was used to prove the absence of NO, (m/e= 46). Bromine (99.7%) was supplied from the J.T. Baker Chemical Company and further purified by passing through P,O, to remove water and vacuum distillation at 77-K and 196 K_
276
15 February
1979
3_ Results and discussion (4)
The rate constant k4 has been measured by CIyne and co-workers [ 12,13]_ The BrO then travelled approximately 10 ms to allow compIete reaction. A small amount of Br-0 radicals reacted with oxygen atoms by the side reaction [12,13] ks O+-BrO+Br+O1_
LEl-i-ERS
In the present experiment the gas concentrations in the reaction zone were adjusted such that 4 X 10’” <[He] <9X 10f6,2X 101oGIBrO] <4X loll, and 2-77 X lOl2 Q DO] d 16.6 X 1012 (cm-3). The initial BrO concentration [(l-4) X lo1 1 cmM3] was at least one order smaller than the NO concentration to satisfy the pseudo first order conditions_ Also, interference from the reactions BrO + BrO 2 2Br + 0,
(6)
and BrO + NO, + He 2 BrONO, + He
(7)
is believed to be negligibie based on the reported rate constant k6 [9,14] and an upper limit for k,. Under the experimental conditions the effective rate of 0.6 to 2.4 S-I for the reaction BrO + BrO is much smaller than that of 48 to 324 s- l for reaction (1). Also, in unpublished resuhs we have found the rate constant k7 i 10m30 cm6 s-l for M = He at 298 K using the same experimental apparatus. The effective rate for the reaction BrO + NO, + He is less than 0.1 s-l _ The radicals BrO were detected at nz/e = 95 amu by a mass spectrometer. The sensitivity for BrO detection was found to be 3 X 109 cmS3 for a signal to noise ratio of one, with 100 s integration. In all experiments 100 s was used for integration at every position of the sliding injector. The wall loss of BrO was checked and is negIigibIe (< 5 s-l). The BrO signa was found to be approximateIy constant when the injector was puked out without the reactant NO. Fig. 1 shows examples of the experimental data, taken under the following conditions: T = 298 K, P = 1.92 torr, F = 1850 cm/s, and [BrO] o = 2.8 X 1011 molecules cmm3. The concentrations cf nitric oxide were varied from 3.26 X IO*? to 7.63 X lOI2 molecules cmM3. [BrO] was observed to decay exponentially and was defmed as
lBti1 f = PrOI,-, exp(--kI1r),
03)
where ki is the first order rate constant for reaction (1) and t is the reaction time. kf was then calculated from the slopes of the straight lines in fig. 1_ The bimolecular rate constant k, was determined as
Volume
61, number
2
CHEMICAL
2I IO’ 9 0
LETTERS
15 February
27x
1.92,o,r
p =
!
1=298K ~=lBOCJ&U [ero]o= 2.8x 10"
"
1979
a NO2
BrO + NO--E,
L\
PHYSICS
cm-3
0.0
Fig_ 3. Arrhenius plot of kr versus 1/T of our data. The Lncertainty bars represent lo of the data. The numbers mentiozred
~IN0,=7_&3r1012,-3 below the data are the numbersof experiments. IO”,
I
t
I
I
10
in
30
Lo
I 50
f.on Fig. 1. BrO decays resulting from the reaction BrO f NO -t Br f N02_ I is the sliding injector position_
k, = k;[[NO]
_
(9)
All data at T = 230.3 K, 298.0 K, and 424.6 K are summarized in fig_ 2, which shows the variations of kf versus [b?O] _ The data can be represented approximately by a straigbt line with zero intercept at each temperature_ The average of these data at 298 K is I
I
BrO+NO-SrlN02
(I -89 -+0.16) X 10-l: cm3 s-l _ The uncertainty represents the first standard deviation of k, . All data were corrected for the effect of concentration gradient due to axial diffusion [ 151 with the maximum correction being about 2.5%. The correction for the effect due to radial diffusion was small and was not made (=Z5%) because of the imperfect knowledge of the radral concentration gradient [16]_ It should be noted that a gaseous diffusion coefficient given by DP = SO1-) torr cm?- s-l was assumed for the BrO radicals in helium at 300 K [ 173 _ A temperature dependence of T3” for DP was also assumed [ 17]_ The results of a total of 64 experiments taken in the temperature range of 230.3 K to 424.6 K are shown in tig. 3 and are summarized in table 1. A least squares fitting computer program was used to obtain the Arrhenius expression k, =(i’_llCO_23)X
lO-~~e(~96*1c)~T cm3 s-l_
Table 1 Summary of measurements
20
Fig_ Z Plot of ki versus [NO] 424.6 K..
of X-1
TW
kr(lo-” ---
230.3 261.5 298.0
2.55 f 0.19 a? 2.23 % 0.08 1.89 c 0.16
353.8 384 6
1.68 t 0.07 1.53 f 0.11
4 5
cm3 s-1)
424.6
1.41 t 0.13
10
at T = 230.3 I;, 298.0 K, and a) The uncertainty
(10)
Number of experiments 8 7 30
represents 10. 277
Volume 61. number 2
CH=tICAL
15 February 1979
PHYSICS LETTERS
TabIe 2
Comparison of measurements of X-t --_ ___.298 ii (cm3 5-l ) I
___-
25 x lo-‘2 (22 i O-4) x lo-” (I.89 t O-16) x IO-” -
Table 3 Temperature dependence for RO + NO -
Technique
Ret
dischatge flowlw absorption discharge flow~tnass spectrometer discharge Row/mass spectrcmeter
Clyne and Cruse ]8] dyne and Watson [9] this work
R + X02 reactions (R = Br. Ct end OH) Arrhenius expressions (cm3 s-’ )
TeN
~perimentf
7.11 x 10-t2e.\p(296/T)
Teobg6
discharge tloa f mass spectrometer
this uork
discharge Row/ mass spectrometer
Leu and DeMore [5]
dischargeflow/
Zahniser and Kaufman [4]
Reaction
Temperature range (EC)
BrO+h’O+Br+h’02
230-42s
aO+NO~Cl+No~ (AH = -9-14
217415
5.72
230-298
1.13 x IO-“e~p(20O/T)
H02+MO+OH+N02 -4.86
resonance Ruorescence (relative to the reaction CI+O3+ClO+O2) 3.3
x IcfraeIp(254/7-)
T-o-83
discharge flow/ iaser magnetic resonance
Howud
270-425
S-7
X 10-tze~p(130/f)
p-o.4
discharge flow/ resonance fluorescence
Leu [7]
kcal)
10-11(T/29S)-o-g6’o-04
cm3 s-l_
(11)
The systematic error of all data was estimated to be IS%, which inchrdes the uncertainties of absolute pressure, flow rates, the effect of axial and radial diffusions, the geometry of the flow tube, and the position of the sliding injector. Table 2 summarizes the comparison of the present results and those of previous measurements of 11-t_Our data at 298 K is in good agreement with that obtained by Clyne and Watson [9] _ However, the rate constant reported by Clyne and Cruse [S] is much smaller than the other three measurements_ interference by the reaction BrO f BrO + 2Br + 0, may have been serious, because they used very high BrO concentrations (==1013 cmm3)_ The negative temperature dependence observed in 27-S
Ref_
230-400
Again the uncer&ties represent the first stsndard deviation of the-4-factor a&d the acrivation temperature_ An altemarive expression obtained by the same computer prog-am is - _ __, k, = LS9X
T-o-75
kc@
+ NO4 OH + NO+ (AH = 4.86 kcal) (AH =
+o
x lo-“exp12961T)
kul)
ao+NodatNoP (AH = -9.14
H%
.
techniques
[6]
this work is compared in table 3 with those obtained for the reactions of this type RO t NO + R + NO, where R = Br, Cl and OH.
Acknowledgement The author wishes to thank Dr. W-B_ Dehlore for valuable discussions in the course of this work. This work was supported by the National Aeronautics and Space Administration under contract No. NAS7-100.
References ] P. Cmtzen. Gn- J- Chem. 52 (1974) 1569_ [2] S.C. Wofsy. h1.B. McEtory and Y-L. Yung, Geophys. Res. Letters 2 (1975) 215. 131 RT- Watson. CIAP hfonogmph l(1975) ch. 5, p. 125. [4] M-S. Zahniser and F_ Kaufman, J.
[l
3673_
Volume 61. number 2
CHEMICAL
PHYSICS LETTERS
[5] M-T. Leu and W-B. DeMore, J. Phys. Chem. (1978). to be published. [6] C-3. Howard, WMO Symposium on the Geophysical Aspects and Consequences of Changes in the Composition of the Stratosphere, Toronto, June 26-30,1978. 171 M.-T. Leu. J. Chem. Phvs. (1978). to be published. i8 j M.A.A. C&e and H_\V:C&e, T&s. F&day Sot. 66 (1970) 2214. ]9] hLA_A. CIyne and RT_ Watson, J. Chem- Sot_ Faraday Trans. 171(1975) 336_ [IO] hf.-T. Leu &d %.R. DeMore. Chem. Phys. Letters 41 (1976) 121.
15 February 1979
[ 11 ] M--T. Leu, CL. Lin and W-B. De&fore, J. Phys. Chem. 81 (1977) 190[12] M.A.A. CIyne and H-W_ Cruse, Trans. Faraday Sot. 67 (1971) 2869. 1131 M.A.A. CIyne. P-B. hlonkbouse and L-W_ Townsend, Intern. J_ Chem. Kinetics 8 (1976) 42.5. [14] N. Basco and SK Dogra, hoc. Roy. Sot. A 323 (1971) 401,417. 11.51F_ Kaufman, Progr- Reaction Kinetics 1 (1961) 1. 1161 R-V. Pohier and R-W. Carr Jr., J. Phys. Chem. 75 (1911) 1593. [:7] T-R. hbxrreroand E-A_ Mason, J. Phys. Chem. Ref. Data 1 (1972) 3_
279