M. T. Leu, C. L. Lin, and W. 9.DeMore
190
calculated with different values of kl and k3 in order to show the sensitivity of the data to these rate constants. The stoichiometry of reaction 1 was verified to be unity (HNO,/HONO) within experimental error by three methods. First, as presented in Table I, the calculated values of k l obtained while monitoring HN03 are identical with those obtained when HONO is monitored. Such a result is possible only if the stoichiometry is unity. Second, model calculations were carried out using stoichiometries different from unity. Figure 3 presents the results of the calculations carried out on sample 9. It is evident that only unit stoichiometry fits the observed data. Finally, the amount of HN03 and HONO consumed during a number of experiments was directly measured by following the HONO concentration and immediately measuring the H N 0 3 concentration when the initial fast reaction was over. These experiments were carried out using a variety of initial HONO and HN03 concentrations. In all cases, the stoichiometry was 1.0 f 0.2 after suitable small corrections were made for reactions 2 and 3. Thus, we believe that the reactant stoichiometry is well established. The results in Table I show that the reaction is first order in HONO and HN03 concentrations with a rate constant at 300 K of kl = 1.55(0.3) X 10.'' cm3/molecule s. The number in parentheses represents one standard deviation from the arithmetic mean of the eight sets of data presented in Table I. This value is in fair agreement with the value of kl = 0.97(0.05) X cm3/molecule s determined indirectly by England and Corcoran' under different experimental conditions. This agreement suggests that these values could represent measurements of the homogeneous rate constant of the reaction. However, in later experiments using a new reactor whose surface had
not been well passivated, we observed rate constants for reaction 1 which were up to a factor of 3.5 greater than those presented in Table I. This proves that reaction 1 can be surface catalyzed. Adding between 0.3 and 5.0 Torr of water vapor to the reacting mixture slowed this catalyzed rate. The observed value of klwas reduced gradually cm3/molecule s as the water vapor conto 1.4 X centration was increased from 0.1 to 1.0 Torr. No furthur reduction was observed a t water concentrations between 1.0 and 5.0 Torr. The existence of this plateau in the water vapor effect proves that the observed rate constant reduction is not a result of an increase in the reverse reaction rate. The most probable explanation for the effect is that the reactor surface is passivated by addition of water vapor. The ultimate rate constant observed when the effect reaches a plateau is identical with that presented in Table I, which was obtained a t low water concentrations. This lends support to the suggestion that the value of k l reported in Table I could be a measurement of the homogeneous rate constant. However, we have not varied the surface-to-volume ratio of the reactor to verify the homogeneity of the reaction. Thus while the reaction could be homogeneous under the conditions in which the data of Table I were obtained, the value of kl determined in these experiments must rigorously be regarded as an upper limit to the homogeneous rate constant. References and Notes (1) C. England and W. H. Corcoran, Ind Eng. Chem, Fundam., 13, 373 (1974). (2) B. J. Finlayson and J. N. Pitts, Jr., Science, 192, 111 (1976). (3) P. G. Ashmore and B. J. Tyler, J. Chem. Soc., 1017 (1961). (4) W. R. Fwsythe and W. F. Giauque, J Am Chem Soc.,64, 48 (1942). (5) E. W. Kaiser and C. H. Wu, manuscript in preparation.
Rate Constant for Formation of Chlorine Nitrate by the Reaction CIO M. 1. Leu;
+ NO2 + MI
C. L. Lin, and W. B. DeMore
Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 9 1103 (Received August 11, 1976) Publication costs assisted by Jet Propulsion Laboratory
A discharge flow/mass spectrometer apparatus has been used to measure rate constants for the reaction C10 + NOz + M. The results are: (cm6 molecule-'^-^) k(M = He) = (2.66 f 0.35) X exp[(ll40 f 40)/Tj (248-417 K, 1-9 Torr); k(M = Nz)= (3.69 f 0.24) x 10 '"exp[(ll50 f 2 0 ) / q (298-417 K, 1--6Torr); k(M = Ar) = (1.15 f 0.10) X (298 K, 1-4 Torr). The results are compared with other current measurements of this reaction rate.
Introduction Chlorine nitrate (C1ONOZ)is currently thought to play a significant role in stratospheric chemistry related to the destruction of ozone.' Association of C10 and NOz by the process C10
+ NO, + M
--*
ClONO,
+M
(1)
has been found to affect the rate of ozone depletion in model calculations.2-5 In the present study we have undertaken the measurement of the rate constant for reaction
t This paper presents the results of one phase of research carried out a t the J e t Propulsion Laboratory, California Institute of Technology, under Contract No. NAS7-100, sponsored by the National Aeronautics and Space Administration. The Journal of Physical Chemistw, Vol. 81, No. 3, 1977
1 for M = He, Ar, and Nz in the temperature range 248-417 K by monitoring the pseudo-first-order decay of C10 in a large excess of NOz in a discharge flow/mass spectrometer apparatus. Experimental Section The apparatus used for this research has been described in a previous publication.6 Briefly, all rate constant measurements were made by observing the pseudofirst-order decay of C10 ( m / e = 51) in a large excess of NOz in a Pyrex flow tube of 50 cm length and 2.5 cm i.d. (Figure 1). In the side arm of the flow tube, chlorine atoms were generated by passing a trace of molecular chlorine (0.01%) in a helium carrier (-10% of total flow) through a microwave discharge with approximately 20 W of mi-
191
Rate Constant for Chlorine Nitrate Formation HELIUM AND CHLMIINE MIXTURE
powder without colored impurity, indicating the absence of NO. Chlorine nittate, used in mass spectrometric checks described later, was prepared from C120 and Nz05, as described in%he literature.'O Concentrations in the reaction tube were adjusted such that 10" I[ClO] I5 X lo',, 4 x loL4I[NO,] I6 x 1015, ) . interference and 4 X 1 O I 6 I[MI I4 X 1017( ~ m - ~The of side reactions such as
,*
/
W SPECTROMETER SS
Z
I
I
o
~
N
-
*
o
a0 + c10 'c10, + c1 'CI, + 0 , and
I t '
a0 + 0,
PUMP
Figure 1. Schematic diagram of discharge flow/mass spectrometer apparatus.
crowave power. The C10 radicals were produced by admitting ozone into the flow tube, leading to the reaction
cl + 0, 'c10 + 0 ,
(3)
(2)
The rate constant for reaction 2 has been measured previously in our laboratory and elsewhere.' The ozone concentration was monitored a t m l e = 48, and the C1 concentration was adjusted to maintain about a tenfold excess of 03.The bulk of the He flow entered by way of the carrier gas inlet, thereby minimizing the possible production of impurity radicals such as atomic oxygen. There was in fact no evidence that atomic oxygen was present in significant concentration, since rate constants were found to be unchanged when all the carrier gas was flowed through the discharge region. The reactant NO2 was admitted into the flow tube through a sliding Pyrex injector (0.6 cm i.d.). Both the inside surface of the flow tube and the outside surface of the injector were coated with phosphoric acid, aged under vacuum conditions, to inhibit surface reactions. Flow rates of gases (except NO2) were measured by linear mass flowmeters (Teledyne Hastings-Raydist.). Flowmeters were calibrated by a Hastings bubble-type calibrator (Model HBM-1). The flow rate of NO2 was measured by the method of pressure drop at a constant volume and temperature. A Haake circulator was used to regulate the temperature. The effect of dimer N204was accounted for by using the equilibrium constant measured p r e v i o ~ s l y .The ~ ~ ~total pressure (1-9 Torr) in the reaction tube was monitored with a calibrated MKS Baratron pressure meter (Model 310 AHS-1000). The temperature of the flow tube was controlled by a high-capacity Haake circulator with fluids flowing through the jacket (see Figure 1) and measured by a chromelconstantan thermocouple dipped inside the circulator bath. The fluids used for the temperature ranges 220-300, 300-370, and 370-430 K were ethyl alcohol-dry ice, distilled water, and ethylene glycol, respectively. The temperature inside the flow tube, with flowing He, was checked by inserting a small thermocouple with radiation shield into the reaction tube through the injector. Both measurements agreed within about 1 K. Most of the gases used for this research were supplied by Matheson Gas Products, including helium, argon, oxygen (UHP), nitrogen (HP), chlorine (research grade), and NOz (99.5%). All samples except NO2 were used without further purification. Ozone was prepared in an oxygen carrier by means of a small laboratory ozonizer. To remove NO from the NO2,the gaseous sample was treated with approximately 200 Torr of oxygen in the constant volume gas holder over a period of 16 h, and subsequently purified by vacuum distillation a t 77 and 196 K. After treatment with 02, the frozen sample was a white solid
+
OClO
+ 0,
was negligible, based on reported rate data for these rea c t i o n ~ .In ~ unpublished experiments, we have confirmed that no C10 loss due to reaction 4 is observed a t O3 concentrations even one order of magnitude higher than in the present experiments. The NO2 concentration was calculated from the relationship NO, flow rate [NO,] = [total concn] X total flow rate The validity of this method was checked by a photometric method, as follows. An evacuated IO-cm quartz cell was used to sample the helium-NO2 mixture at the downstream end of the flow tube. The sample was then analyzed by a Cary 15 spectrophotometer using an absorption cross section of 6.8 X cm2at 4000 A," The measured NO2 concentrations were in good agreement (&lo%)with the calculated values. Throughout this experiment, except for M = Ar, the electron gun was operated at 20 eV to ionize the gases in the molecular beam. The measured rate constants were found to be independent of the electron energy from 20 to 100 eV. For M = Ar, 40 eV was used in order to reduce spurious signals due to background modulation, which is a phenomenon observed with Ar but not with He. Possible interference of Clot from fragmentation of C10N02 by electron impact was checked a t 20 eV. The sensitivity for C10+ from ClONO, was found to be about 1/30 the sensitivity for C10+ production from C10 itself. Calculations showed that this effect would cause the measured rate constant to be approximately 7% too low, for C10 decay over one order of magnitude. Such an effect was actually observed by admitting a large amount of NO,, which gave a large fractional decay of C10. The decay was found to be not exponential and to curve upward, consistent with interference from chlorine nitrate fragmentation. For this reason, all data were taken with C10 decay less than one order of magnitude. No attempt was made to correct for this effect. Results and Discussion A. M = He. Plots of C10 decay expressed in terms of the sliding injector position (measured from the injector tip to the mass spectrometer sampling hole) are shown in Figure 2. The linearity of the logarithmic decay profiles verifies the pseudo-first-order nature of the process. Without NO2, the profile of C10 concentration remained constant, indicating that the effect of injector surface is negligible. A series of experiments was taken by varying NO2concentrations at the Same conditions. The fmt-order rate constant (kJ vs. NO2concentration was approximately linear with zero intercept kI = kII[N021 (6) Plots of k~ vs. NO2 concentration are shown in Figure 3. The Journal of Physical Chemlstv, Vol. 8 7, No. 3, 7977
192
M. T. Leu. C. L. Lin, and W. 6.DeMore
1.62 A 1.98
a 5 -
0 A
>
