T H E
J O U R N A L
OF
PHYSICAL CHEMISTRY Registered i n U. S. Patent Office 0 Copyright, 1977, by the American Chemical Society
VOLUME 81, NUMBER 3 FEBRUARY 10, 1977
Measurement of the Rate Constant of the Reaction of Nitrous Acid with Nitric Acid E. W. Kaiser' and C. H. Wu Chemistry Department, Engineering and Research Staff, Ford Motor Company, Dearborn, Michigan 48 121 (Received July 6, 1976) Publication costs assisted by the Ford Motor Company
The rate constant of the reaction of nitrous acid with nitric acid, HONO + HN03 = 2N02 + H20 (kl), has been measured in the gas phase in a Pyrex reactor at 300 K using a mass spectrometer detector. A reactant stoichiometry of unity was confirmed during these experiments, and the observed rate constant was k l = 1.55(0.3) x cm3/molecules. Evidence was also obtained that this reaction can under certain conditions be sensitive to the reactor surface condition. Thus, the above value of k l must rigorously be regarded as an upper limit to the homogeneous rate constant.
I. Introduction We have directly measured the rate of the gas phase reaction of nitrous acid (HONO) with nitric acid (HNOJ at 300 K. To our knowledge, this is the first direct observation of the reaction HONO
k
+ HNO, = 2N0, + k -1
H,O
(1)
although an indirect estimate of the rate constant has been published previously.' These molecules are present in the polluted atmosphere, and a measurement of this rate constant is, therefore, important in the modeling of atmospheric chemistry.2 In our experiments, both HONO and HN03 were unambiguously monitored by a quadrupole mass spectrometer, and the rate constant of the reaction was deduced from concentration-time profiles of both species. The stoichiometry of the reaction was also determined during the course of the study. 11. Experimental Section These experiments were performed in a static-cylindrical Pyrex reactor having a total volume of 750 cm3 and a surface-to-volume ratio of 0.63 cm-'. Samples were injected through a 2.54-cm diameter Pyrex sphere with 50 holes in its surface located at the center of the reactor. The total dead volume in this injection system was 12 cm3. This type
of injection produces a well-mixed sample immediately after introduction of the reactants. The contents of the reactor were continuously sampled through a 0.0025-cm diameter nozzle located at the apex of a cone which penetrated approximately 2 cm into one end of the reactor. This continuous sampling from the vessel resulted in a steady pressure drop of 0.3% /min within the reactor. The detector was located in a differentially pumped chamber separate from the chamber containing the reactor. The detector consisted of an Extranuclear Laboratories quadrupole mass filter (Model 324-9 with D2 high Q head) with a Type I1 high efficiency electron impact ion source. This ion source was operated at 70-eV electron-bombardment energy during the experiments. The HONO and HN03 concentrations were monitored at the parent mass peaks of each species (47 and 63 amu, respectively). The parent mass of HONO contributed more than 19% of the total ion current from all HONO fragments and, therefore, provided a sensitive technique for monitoring the HONO concentration. The observed relative ion intensities in the fragmentation pattern of pure HONO are: Z(47 amu) = 1.0; Z(46 amu) I 0.23; 1(30 amu) I4.0. Only upper limits can be placed on the values for 46 and 30 amu because of unavoidable contamination of the sample by NO and NOz. The parent mass of HN03 contributed only 1.6% to the total "OB ion current, however, and the sensitivity for HNOB detection was substantially lower than that for HONO.
188
E. W. Kaiser and C. H. Wu
I INJECTION OF HONO
I
MIXTURE
W
2
0.125
0
I
0
5
I
I
I
IO 15 20 TIME (SEC)
I
25
A
0
I
'300
Flgure 1. Experimental data obtained while monitoring the HONO concentration at 47 amu. (a) Curve 1 shows the HONO decay rate in the absence of "OB. The individual points (A)represent the best computer fit to the data obtained using k2 = 1.4 X cm3/molecule s. (b) Curve 2 shows the decay curve of HONO in the presence of HN03 under the conditions of sample 5 in Table I. Computer fits to the data are shown as individual points: (0)best fit using the parameters (0) same except given in Table I; (V)same except ki = 1.95 X kl = 0.87 X (0) same except k3 = 3.0 X IO-*".
Equilibrium samples of HONO were prepared by mixing measured amounts of NO, NO2, and H 2 0 in a 2-1 Pyrex storage bulb at a total pressure of approximately 1 atm. The equilibrium HONO concentration in the mixture a t 300 K was calculated from the following equilibria using the indicated equilibrium c o n ~ t a n t s : ~ t ~ NO t NO, t H,O = 2HONO K e q = 1.42 x Torr-' NO t NO, = N,O, K,, = 6.43 X Torr-' NO,
+ NO, = N,O,
K e , = 7.39 x
Torr-'
The HONO concentration in these mixtures was varied between 9 and 19 Torr by changing the initial NO, NO2, and H 2 0 concentrations in the bulb. HN03 (purity >go%) was freshly prepared in vacuo prior to each experiment by the addition of KNOB (Reagent ACS) to concentrated H2S04(Reagent ACS) as described by Forsythe and Gia ~ q u e .The ~ HN03 was injected into the reactor immediately after formation to avoid decomposition. The experiments were performed using the following procedure. A measured amount of HNO, was first introduced into the reactor. The initial HNO, concentration was varied between 0.05 and 2 Torr as measured by a pressure transducer a t the higher concentrations and by the calibrated mass spectrometer a t the lower. A 1228-cm3volume of the equilibrium NO, NOz, H20, HONO mixture was then injected producing a total pressure of from 3 to 30 Torr in the vessel. The reaction between HONO and "OB was observed by monitoring either the HONO or the HNO, ion current at the appropriate parent mass. 111. Results
Pure H N 0 3 was stable in the reactor for time periods greater than 10 min. However, when a sample of the HONO mixture in the absence of HN03 was introduced into the reactor, the superequilibrium HONO concentration, which is a result of the decrease in mixture pressure from 1atm to a few Torr after sample injection, decayed at a measureable rate to the new equilibrium value determined by the reaction HONO
+ HONO
k
k -2
NO t NO,
-t
H,O
The Journal of Physical Chemistry, Vol. 8 1, No. 3, 1977
(2)
1
I
0
5
I
I
I
IO 15 20 T I M E (SEC)
Flgure 2. Experimental data obtained while monitoring the HN03 concentrationat 63 amu under the conditions of sample 6 in Table I. Computer fits to the data are shown as individual points: (0)best fit using parameters given in Table I; (V)same except kl = 1.95 X (0) same except ki = 0.87 X (0) same except = 3.0 X IO-*'.
Curve 1 in Figure 1 shows a typical concentration-time profile of HONO in such an experiment. The HONO signal rises rapidly immediately after injection of the mixture and then decays by a reaction mechanism which we have determined to be heterogeneous and very sensitive to the condition of the surface. The decay curve of HONO in the absence of HNO, was frequently measured during the course of the experiments to obtain the best value of k z for each determination of llzl using a rate law based on a simple interpretation of the mechanism represented by eq 2. The true rate law is not fully understood but is more complicated than this appr~ximation.~ A typical HONO decay curve in the presence of HN03 is presented in curve 2 of Figure 1. Note that the initial decay rate is increased by approximately a factor of 5 in comparison to the self-decay rate, demonstrating that a reaction does occur between HONO and HN03. Figure 2 presents the results obtained while monitoring the HNO, ion signal using initial concentrations which are virtually identical with those used in generating curve 2 of Figure 1. As discussed in section 11, the signal-to-noise ratio is substantially lower when HN03 is monitored because of the unfavorable fragmentation pattern. In addition, the HN03 signal is disturbed for a period of approximately 2-3 s after injection of the HONO sample mixture because of the sudden pressure rise in the reactor. We have established that this is only a temporary effect and that the ion signal returns to near its original value after the 2-3-9 interval if pure nitrogen is injected into the reactor at 12 Torr. This effect produces the sharp drop in the ion signal observed during the first second in this figure. Figure 3 shows the decay observed when a portion of the HONO mixture is injected into a sample of HN03 whose concentration is a factor of 4 greater than the HONO concentration. The sharp initial drop in HN03 signal is again the result of the pressure disturbance. Injection of 13 Torr of Nz into 1.2 Torr of H N 0 3 confirms that the HNO, signal returns to its original value within 3-4 s after the N2 is injected. Therefore, the net drop observed in Figure 3 after approximately 6 s have elapsed is primarily a result of the HONO-HN03 reaction. We have determined that the HONO is almost totally consumed during this period. The presence of the slower decrease in HN03 concentration after 10 s suggests that HNOB also reacts with one or more other components in the mixture but at a much slower rate. Modeling calculations to be described
189
Reaction of Nitrous Acid with Nitric Acid
TABLE I: Measured Rate Constants for t h e Reaction of HONO with HNO, _ I _
Sample no. 1 2 3 4 5 6 7
8 9 10
HONO
HNO
Total reactor pressure, Torr
0.24a 0.24 0.12b 0.24 0.24 0.24 0.46‘ 0.46‘ 0.30 0.71d
0.14 0.13 0.068 0.17 0.28 0.27 0.50 0.50 1.20 2.00
12.3 12.3 3.1 12.3 12.5 12.3 22.1 22.1 14.1 33.0
Initial reactant concn, Torr
Rate constants used to fit t h e data, cm3/molecule s x 1 0 1 1
k, 1.50 1.40 2.20 1.40 1.30 1.30 1.50 1.80
e e
Species monitored HONO HNO , HONO HONO HONO HNO HONO HNO HNO HNO
k2
0.092 0.092 0.190 0.140 0.140 0.140 0.145 0.145 0.120 0.100
Measd stoichiometry ( A HNO ,/ AHONO)
1.o 1.o
a T h e HONO was injected into t h e reactor a t known concentration from an an equilibrium mixture of NO, NO,, H,O, and HONO prepared as described in t h e text. Unless otherwise noted, the initial concentrations of NO, NO,, and H,O in t h e samples were: NO = 10.9 Torr; NO, = 0.75 Torr; H,O = 0.15 Torr. The value of k , in all cases was 1.5 X lo-” c m 3 / Initial concentrations: NO = Initial concentrations: NO = 2.0 Torr; NO, = 0.8 Torr; H,O = 0.15 Torr. molecule s. Initial concentrations: NO = 28 Torr; NO, = 1.9 Torr; H,O = 0.38 Torr. 19.5 Torr; NO, = 1.36 Torr; H,O = 0.28 Torr, e At these high initial concentrations, t h e reaction proceeds t o o quickly t o permit an accurate determination of k , . However, t h e decay curves were compatible with t h e rate constant determined at lower concentrations.
‘
INJECTION OF HONO MIXTURE
to both NO and HN03 concentrations. The reaction was heterogeneous in our reactor, and the true rate law was more complicated than that represented by a simple interpretation of the above reaction mechanism. A value of k3 was obtained from these measurements which fitted the data reasonably well under the experimental conditions present in the reactor during the measurements of kl. This measured value was k3 = 1.5 X cm3/molecule s. The initial consumption of HN03 by this reaction is a factor of 20 smaller than the observed HONO or HN03 decay rates during the HNOB HONO reaction. The HN03 NO reaction does satisfactorily account for the slower decay in HN03 concentration which is observed to occur in Figure 3 after the rapid reaction between HONO and HN03 nears completion. The rate constant k1 was determined for each set of data by integrating the following rate expressions:
+
I
I 30
0
I 60
I 90
TIME (SECI
Figure 3. Experimental data obtained while monitoring the HN03 concentration at 63 amu under conditions of sample 9 in Table I. The best computer fit to the data using unit reactant stoichiometry is shown as individual points (0)in the figure (k,= 1.5 X lo-”, k2 = 1.00 X k3 = 1.5 X lo-’*). The points, (a),were obtained using a HN03/HON0stoichiometry of 2.0, while the points, (A),were obtained using a stoichiometry of 0.5.
later show that the steady-state HONO concentration formed by reactions 1and 2 is 10 times smaller than would be necessary to account for this latter decay. These data confirm that a reaction does occur between HONO and HN03 which is faster than any of the competing reactions in the system. In fact the rate is sufficiently fast to drive the HONO concentration below the final equilibrium value as demonstrated in curve 2 of Figure 1. After the HN03 is consumed, the HONO equilibrium is reestablished by the formation of HONO via the much slower reaction -2. No such subequilibrium HONO concentration is observed when the self-decay of HONO in the absence of HN03 is measured. We have established that HN03 does not react significantly with either NO2 or HzO in our reactor a t the concentrations and time scales used in these experiments. However, a reaction does occur between HN03 and NO possibly by a mechanism involving reaction 3 followed by k HNO, t N O 3 HONO k -3
+ NO,
(3)
reaction 1. This reaction was determined in separate experiments to be approximately first order with respect
+
d(HONO)/dt = k-1(NOZ)’(HzO) 2h-,(NO)(NO2)(H2O) h,(HNOj)(NO) - k,(HONO)(HN03) - 2hz(HON0)2 - h-,(NO,)(HONO) d(IINO,)/dt = h-1(N0,)2(HzO) + h-,(NOz)(HONO) - k 1 (HONO)( ”0,) - h3( HNO,)(NO)
+
+
The independently measured values of k2,h3, and the initial reactant concentrations were used as input to the integration program, which generated concentration-time profiles for comparison with experimental data. The reverse rate constants were calculated from the known equilibrium con~tants,~,~ Kl = 1960.8 Torr, KZ = 704.2 Torr, K3 = 2.78, and the measured forward rate constants. These reverse reactions are unimportant at the beginning of an experiment but control the final equilibria. In the above kinetic expressions, we have assumed that the simplified mechanisms for reactions 2 and 3 are correct. As was previously stated, both of these reactions have more complicated true rate laws which are not fully understood. However, because reaction 2 contributes only 20-25% and reaction 3 only 5% to the initial decay rates observed during the HONO + HN03 reaction, use of the simplified mechanism produces negligible error in the data analysis and is fully justified. In Figures 1-3, the best computer fits to the observed experimental data are plotted as individual points. Also included in Figures 1and 2 are concentration-time profiles The Journal of Physical Chemistw, VoL 81, No. 3, 1977
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 to 1.4 X cm3/molecule s as the water vapor concentration 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 at the Jet 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-
