6937
J. Am. Chem. SOC.1980,102, 6937-6941
Kinetic Study of the Equilibrium H 0 2 and the Thermochemistry of H 0 2
+ NO 3c OH + NOz
Carleton J. Howard Contribution from the Aeronomy Laboratory, NOAAIEnvironmental Research Laboratories, Boulder, Colorado 80303. Received May 2, 1980
-
+
+
-
+
Abstract: Rate constants for the reactions H 0 2 + NO OH NO2 (kF) and OH NO2 HOz NO ( k ~ have ) been measured a t high temperatures by using laser magnetic resonance detection of HOz and OH reactants in a flow tube reactor. exp[(+240 30)/acm3 molecule-' s-l for 232 < T < 1271 K, including earlier The results are k F ( q = (3.51 0.35) X low-temperature measurements and k , ( n = (3.03 f 0.60) X lo-" exp[-(3360 & 1 2 5 ) / q cm3 molecule-l s-I for 452 < T < 11 15 K. These data a r e combined with other thermochemical data to calculate the heat of formation of the H 0 2 radical, m f 0 2 9 8 = 2.5 f 0.6 kcal mol-'. Other measurements of this quantity and the thermochemistry of H 0 2 are discussed.
*
1. Introduction The hydroperoxyl radical, HOZ,is an important intermediate in atmospheric chemistry,' in combustion,2 a n d in most chainreaction mechanisms involving oxidation of molecules which contain hydrogen. I t s thermochemistry, specifically t h e heat of formation, MHfo? is a critical factor in evaluating possible reactions of H02 in t h e atmosphere because reactions t h a t are endothermic by more t h a n a b o u t 3 kcal mol-' are usually t o o slow to b e significant. Similarly, in combustion, thermochemical d a t a are t h e basis for evaluating initiation mechanisms, branching ratios, and equilibrium constants. T h e currently accepted value of the heat of formation of H02$ M f 0 2 9 8 ( H 0 2 )= 5 f 2 kcal mol-', is based upon d a t a from t h e , ~ used electron-impact classic s t u d y of Foner a n d H u d ~ o n who ionization with a modulated molecular beam mass spectrometer. Recently Kochubei and Maid have challenged t h e accuracy of this value, a n d from kinetic studies of the temperature dependence of reaction 1, w h e r e X = C1, Br, or I, they have deduced HX
+ 02+H02 + X
(1)
AHt298(H02)= 0.9 kcal mol-'. The maximum difference between these two measurements including the uncertainty in each is about 8 kcal mol-'. This large range of values does not allow a definitive assessment of t h e likelihood of several important atmospheric HOz reactions such as eq 2, 3, and 4. If one assumes Mf"(HO2) =
+ NO H02 + C10 HOz + C1HO2
-
+0 2 ClOO + OH
+
HNO
C10
+ OH
(2) (3) (4)
AHfo(HOz)= 0.9 kcal mol-', reactions 2-4 are endothermic by 1.3, 5.3, a n d 3.8 kcal mol-', respectively. If on t h e other hand, AHH,"(HO~) = 5.0 kcal mol-', then reactions 2 a n d 4 a r e exothermic and (3) is only slightly endothermic (- 1.2 kcal mol-'). In this work t h e value of A H f o ( H 0 2 ) is determined from measurements of the temperature dependence of the rate constants for the forward a n d reverse reactions (1) Duewer, W. H.; Wuebbles, D. J.; Ellsaesser, H. W.; Chang, J. S . JGR, J Geophys. Res. 1977,82, 935-942. (2) Benson, S . W.; Nangia, P. S. Acc. Chem. Res. 1979, 12, 223-228. (3) Throughout this paper the standard state for thermochemical data is 298 K and 1 atm unless otherwise noted. Bond dissociation energies refer to the AIPZ9*for the reaction which breaks the indicated bond, e.g., D(H-O2) = AHfDm (HOZ)- AHf0298(H) - AHfoB8(02).All thermochemical data from JANAF.~ (4) Stull, D. R., Prophet, H., Eds., "JANAF Thermochemical Tables"; National Bureau of Standards: Washington, D.C.; compiled and calculated by the Dow Chemical Co., Midland, MI, including revised tables issued through 1979. (5) Foner, S. N.; Hudson, R. L. J. Chem. Phys. 1962, 36, 2681-2688 and references cited therein. (6) Kochubei, V. F.; Moin, F. B. Dokl. Akad. Nauk S S S R 1974, 219, 141- 144.
This article not subject t o
US.Copyright.
HO2
+ NO
OH
+ NO2
The equilibrium constant K, = k(forward)/k(reverse) is combined with thermodynamic data, iw,"and So,for NO, OH, and NO2 t o obtain M f " ( H 0 , ) . The s t a n d a r d entropy of HOz is also reevaluated by using new spectroscopic d a t a . 2. Experimental Section The kinetic measurements are made by using a discharge-flow reactor coupled with a water-vapor laser magnetic resonance (LMR) spectrometer for detection of H 0 2 , OH, NO, and NO2. A detailed description of the apparatus' and a recent low-temperature study of the forward (HO2 NO) reaction" are published. Only a brief description of the measurement techniques is given here. The LMR spectrometer is used to detect paramagnetic radicals with rotational transitions that are nearly coincident with the laser wavelength, in this case X = 118.6 km. Resonant absorption of the laser radiation is achieved by passing the radicals through the laser cavity and by Zeeman tuning a component of the rotational transition to the fixed laser frequency. When the radicals absorb the laser radiation, their concentration is proportional to the fractional change in the laser output power. Some aspects of the LMR spectroscopy of H 0 2 ? NO,'O OH," and N02I2 are described in previous publications. The method is sensitive and specific for the detection of the ground vibrational and electronic states. A special high-temperature flow reactor designed for this work is shown in Figure 1. The quartz flow tube is 25.0-mm i.d. and about 102 cm long. The He carrier gas (299.99% purity) enters the tube near the top. The H 0 2 radicals are made in a high-pressure source* (p 20 torr) by the reaction H O2+ M HOz M ( k = 5 X cm6 molecule-2 s-I).l3 O H radicals are made in the flow tube by the reaction H + NO2 -OH NO ( k = 1.4 X cm3 molecule-' s-I).14 The reactant, H 0 2 or OH, mixes with the carrier gas and enters a 60-cm long section of the flow tube that is surrounded by a furnace. The furnace consists of a No. 24 strand of nichrome wire wrapped uniformly with about 8-mm spacing around the flow tube. Thermal insulation is provided by (a) 3 layers of 1.6" thick asbestos tape, (b) 4 layers of alternate AI foil and 1.6" thick glass cloth, and (c) a 6-mm layer of glass braid. The gas stream flows about 10 cm in the temperature controlled region before it reaches the first of 6 reactant gas ports spaced at 7-cm intervals. Each port is attached by stainless-steel tubing to a valve connected to a manifold that supplies the NO or NO2 reactant. The reaction begins at the point of addition of this reactant to the gas stream and continues to the detection region at the bottom of the flow tube. The detection region is located at the intersection of the flow tube and the laser cavity. It is defined by the homogeneous magnetic field indicated by the dashed circle on Figure 1. A stream of N 2 is injected into the laser tube to flush the reactants
+
+
-
+
+
(7) Howard, C. J.; Evenson, K. M. J. Chem. Phys. 1974,61, 1943-1952. ( 8 ) Howard, C. J. J. Chem. Phys. 1979, 71, 2352-2359. (9) Hougen, J. T.; Radford, H. E.;Evenson, K. M.; Howard, C. J. J. Mol. Spectrosc. 1975, 56, 21C-228. (10) Mizushima, M.; Evenson, K. M.; Wells, J. S . Phys. Rev. A 1972, 5, 2276-2287. (11) Evenson, K. M.; Wells, J. S.; Radford, H. E. Phys. Reu. Lett. 1970, 25. 199-202. (12) Curl, R. F., Jr.; Evenson, K. M.; Wells, J. S. J. Chem. Phys. 1972, 56, 5143-5151. (13) Kurylo, M. J. J. Phys. Chem. 1972, 76, 3518-3526. (14) Michael, J. V.;Nava, D. F.; Payne, W. A,; Lee, J. H.; Stief, L. J. J . Phys. Chem. 1979, 83, 2818-2823.
Published 1980 by t h e American Chemical Society
6938
J. Am. Chem. SOC.,Vol. 102, No. 23, 1980
Howard
-
Table I. Summary of Rate Constant Measurements for the Forward Reaction HO, + NO
CARRIER GAS ADDED GAS
ADDED GAS
FURNACE
-
e REACTANT
MEASUREMENT
d',
[MI 1016 molecule cm-3 1.61 1.05 1.13 1.88 3.37 2.35 2.47 3.41 3.32 2.62 2.99 2.40 2.27 2.05 2.52 2.04 7
' R A M A L SOURCE
F
GAS
DETECTION REGION
no. of expt 4 6 7 5 7 3 3 3 3 4 4 4 4 4 4 4 a
Figure 1. Schematic of high-temperature flow-tube reactor. from the laser cavity. The usual moveable inlet method of reactant addition could not be used at high temperatures because the lubricant coating on the inlet reacts at high temperatures with 0, and NO, to generate large concentrations of radicals. The flow-tube pressure is measured with a capacitance manometer ( f l % accuracy) near the center of the reaction zone. The flow-tube temperature is measured with three chromel-alumel thermocouples located on the quartz tube about 7, 32, and 56 cm from the bottom of the furnace. The digital temperature readout was calibrated with an ice point compensated microvolt potentiometer by using NBS tablesI5 and found to be accurate to within the resolution of the thermometer ( f l "C) over the range 300-1300 K. The temperature profile along the flow-tube axis was measured with a thermocouple attached to a quartz rod and is uniform (*5 "C) along the middle 45-cm section of the reactor. Although normally either H3B03or H3P04coatings are applied to the flow-tube interior to inhibit O H and H 0 2 wall destruction, they cannot be used here because they are pyrolyzed at high temperatures. The flow-tube surface was conditioned by flowing high concentrations of OH, -5 X 10l2 molecule c m 3 , and 02,-5 X 10l6 molecule through the tube for several hours at 7'N 1200 K. After this treatment the first-order wall reaction rate constant for both OH and HOz is small,
