RELATIVE RATECONSTANTS
Relative Rate Constants for 0
+ HCO
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2215
OH
+ CO and 0 + HCO --+ H + CO,
by A. A. Westenberg* and N. deHaas Applied Physics Laboratory, Johns Hopkins University, Silver Spring, Maryland
20910
(Received February 14, 1972)
Publication costs assisted by the Bureau of Naval Ordnance
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A simple analysis is given showing how the measurement of 0 atoms consumed and C02 formed in the 0 H2C0 reaction system, with (0)o>> (HzCO)~, can be related to the rate constant ratio k&z for the alternate pathways 0 HCO -t OH CO (kz) and 0 HCO -+H COZ (k3). Experimental results in a fastflow system at room temperature give the value k& = 0.73 i 0.15.
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I n a prcvious paper’ we pointed out how the measurement of the absolute concentrations of “steady-state” intermediates could be used t o determine the relative importance of various reaction rates involving the HOz radical. A somewhat different approach is exploited here in regard to the HCO radical, the actual intermediate concentrations not being required for the desired result. When 0 atoms react with HSCO in a fast-flow system the primary step is surely the simple abstraction
0
+ HzCO +OH + HCO
(1)
which is then followed by the fast steps
0
+ HCO/- O HH+ C+OCOS 0 + OH +H ----t
--it 0 2
(4)
This sequence has been discussed by Niki2 in connection with his measurement of kl. We shall be interested only in the situation where the initial concentration (0)ois in large excess compared to (HsCO)~,and in this case one can reasonably argue that other conceivable secondary reactions are unimportant. The reaction H HzCO -t Hz HCO is about half as fast as (1) a t room temperature3p4and could not compete in excess (0). While H HCO -+ Hz CO should have a rate constant comparable t o kz and k3, it also would be unlikely t o compete appreciably in excess (0).Furthermore, Hz is not found as a product.2 The reaction OH HsCO 4 HzO HCO was supposedly ruled out by Niki2 in contributing t o the HzO formed, although it has since been found to be very fast5 (k N 10la cm3 mol-‘ sec-I). Nevertheless, neither it nor OH HCO .+ HZO CO would be expected t o compete for OH removal with the still fastere reaction 4 in excess (0). Using reactions 1 4 , the only source of COS is (3), OH + COZ H (k v 1011) being the reaction CO negligible in excess (0). Assuming steady-state concentrations of HCO and OH it is straightforward to derive thP relation
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where (0)r and (COz)t refer to concentrations existing after all the HzCO has been consumed and both (0) and (COJ attain constant “final” values (ie., “final” as far as these four reactions are concerned). Measurement of the left-hand quantity above would then permit the ratio k3/kS t o be determined. As a check on the validity of the analysis using only reactions 1-4, the “complete” mechanism including the other four possible reactions discussed above in addition to (1)-(4) was programmed for rigorous solution on a computer; ie., the set of simultaneous rate cquations for all specics (without the steady-state approximation for the intermediates) was solved using known or estimated values for the rate constants. The rigorously computed quantity [(O), - (0)r]/(COz)f was 3k3/ shown to be within 10% of the quantity [4 k2]/(k3/lc2)for any ratio (0)o/(H2CO)o > 4. An alternate method is possible in principle, since the same analysis is easily shown to lead to the relation [(O)o (O)fI/(HzCO)o = [4 3k~/h1/[1 (h/kz)l. This is far less desirable, however, since the measurable quantity on the left only varies in the range 4 + 3 as ks/kS takes on the possible values 0 4 03. I t is, therefore, very insensitive compared to the COS method in which the measurable quantity varies as 00 4 3 as k3/kz = 0 4 m . Experiments were performed in our usual discharge flow system7 with a fixed esr cavity to measure (0)and a mass spectrometer inlet with continuous sampling
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(1) A . A. Westenberg and N. deHaas, J . Phys. Chem., 76, 1586 (1972). (2) N.Niki, J , Chem. Phys., 45, 2330, 3468 (1966). (3) W. Brennen, I. D. Gay, G. P. Glass, and H. Niki, ibid., 43, 2568 (1965). (4) A. A. Westenberg and N. deHaas, J . Phys. Chem., 76, 2213 (1972). ( 5 ) E. D. Morris, Jr., and H. Niki, J . Chem. Phys., 55, 1991 (1971). (6) A. A. Westenberg, N. deHaas, and J. M. Roscoe. J. Phys. Chem., 74, 3431 (1970). (7) A . A . Westenberg and N. deHaas, J . Chem. Phys., 47, 1393 (1967).
The Journal of Ph&cal Chemistry, Vol. 76, N o . 16, 1979
A. A. WESTENBERG AND N. DEHAAS
2216 Table I : Summary of Xeasurements of k 3 / k z a t Various (O)O/(H~CO)~ Ratios (All Runs Were a t Room Temperature in 97-98y0 Argon Carrier) P.
(Oh,
(COdf,
Torr
mol/cma
mol/cma
1.11 1.80 1.87 1.87 1.88 1.49 1.49 1.90 1.65 1.49
8.4 X 12.3 X 14.8 x 14.3 x 13.4 X 12.6 X 6.1 x 13.5 X 14.4 X 12.3 X
10-l" 1O-lo 10-1O 10-lo
0.58 x 0.63 x 0.74 x 0.31 x 0.45 x 0.53 X 0.31 x 0.43 X 0.51 x 0.25 x
l0-lo 10-lo 10-lo 10-1O 10-1O 10-10 10-lo 10-10 10-10
(O)f/(O)O
(O)o/(HzCO)o
ka/k~
0.38 0.62 0.53 0.66 0.64 0.68 0.63 0.74 0.70 0.85
4.7 5.1 5.9 6.7 8.0 9.4 9.6 10.6 16.7 19.9
0.68 0.92 0.63 0.32 0.52 0.85 0.96 0.80 0.72 0.86
Av
downstream of the cavity for the COz produced. 0 atoms were furnished by a microwave discharge in argon carrier containing 1-2% of 0 2 . HzCO was generated from paraformaldehyde heated to 60-80" and metered directly to a movable injector through a needle valve. The HzCO flow was measured after each run by transferring it to a calibrated volume and timing the pressure rise. No special problems with metering and handling the HzCO were encountered, except that cleaning the needle valve was occasionally necessary. Steady flows of HzCO could bc readily introduced to the main flow system in this way, as was proved by monitoring the m/e 30 peak in the mass spectrometer. The COzformed was monitored at m/e 44, and the mass spectrometer was calibrated after each run by adding a metered flow of COZ to the main flow. Absolute (0) values were measured by esr in our usual way,8 using a separate filling of pure O2 for calibration. For each run the flows and injector position were adjusted so that the (H&O) was completely consumed by the time the mixture reached the esr cavity, as evidenced both by attainment of a constant 0 signal upon moving the injector farther away from the cavity, and the disappearance of HzCO in the mass spectrometer. This 0 signal was proportioned to (O)f, and the corresponding m/e 44 peak to (CO,)f. The H&O was then bypassed out of the reactor and an esr integral taken, which gave (0)o. These were converted to absolute concentrations by the above calibration procedures, and a value of k3/kZ was determined. Results of a number of such runs are given in Table I. A limited total pressure range could be covered due to physical limitations of the system while still meeting the basic requirement that there be no further reaction
The Jotlrnal of Physical Chemistrg, Vol. 76, No. 16, 1972
0.73f0.15
beyond the cavity. The more important variable, the initial (0)o/(H3CO)oratio, was varied over a rather wide range of 4.7-19.9. The average of 10 runs gave a valuc k / l c z = 0.73 Jr 0.15 at room temperature, with no significant trend as (0)o/(HZCO)owas varied. The simple abstraction reaction 2, which is 73 kcal/ mol exothermic, is thus slightly faster than the substitution reaction 3, which has an exothermicity of 99 kcal/mol. The latter represents a somewhat more complex process, so that we do not consider the result lcz > IC3 to be surprising, with the aid of hindsight. HOz This is similar to the results obtained' in the H HOz ---t reaction, where the substitution path H HzO 0 was found to be slower than either of the alternate abstraction reactions. The only other value for k3/1cz in the literature is that of Niki, et aZ.,9 in a study of the 0 CzH4 reaction, where HzCO and then HCO are formed in the secondary reactions. From their mechanism they derived (CO,) f / (H)f = (1c3/k2)/2[1 (lc3/lcz)],and from the measured quantity on the left they obtained Ic3/k, = 0.25 f 0.16. This is lower than our ratio, although not much beyond the respective quoted error limits. Since the measur0.5 as able quantity (COz)f/(H)f varies only as 0 k3/k2 = 0 -t w , this is a very insensitive method and based on a more complex system. This, plus the fact that their mass spectrometric (H) measurement caliNOz titration was not vcry brated against the H reliable (as they note), leads us to prefer the present value for k3/h2.
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(8) A . A . Westenberg and N. deHaas, J . Chem. Phgs., 40,3087 (1964). (9) H. Niki, E. E. Daby, and B. Weinstock, "Twelfth Symposium on Combuntion," The Combustion Institute, Pittsburgh, Pa., 1969, p 277.
