RATECONSTANT FOR H
+ HzCO
2213
paramagnetic species are produced via an oxidation of the peroxy complexes of the metal ions by both HOZ and .OH radicals. The complexed radicals produced from different peroxy complexes of the same metal ion are (except for ZeIV) indistinguishable by esr spectroscopy. TiIV, ZrIV, and HfIV exhibit similarity by yielding
Measurement of the Rate Constant for H
two different complexed radicals each on addition of HOz to the metal ion, even in the absence of HzOz. This is probably due to different hydrolyzed or dimeric forms of the metal ions. NbV and Vv do not form complexed radicals through the reaction of the free metal ions but via their peroxy complexes only.
+ H,CO
---t
H,
+ HCO at 297-6520K
by A. A. Westenberg* and N. deHaas Applied Physics Laboratory, The Johns Hopkins Umiversz'ty, Silver Spring, Maryland .%?OB10 (Received February 14, 1973) Publication costs assisted by the Bureau of Naval Ordnance
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Absolute measurements of the rate constant for the reaction H HzCO+ Hz HCO (ICl) over the temperature range 297-652'K are reported. A fasbflow reactor with esr detection for H atom decay was used. The Arrhenius expression obtained was IC1 = 1.35 X 10" exp(-3760/RT) cma mol-' sec-1. Comparisons with other data are made.
The rate constant of the reaction
H
+ HzCO +Hz + HCO
(1)
has been measured directly only a t room temperature,' while IC1 relative to other reactions has been determined by more complex photo1ysis2va and explosion limit4 studies at higher temperatures. We report here the direct measurement of kl over a temperature range 297-652' K. The technique was that used in several previous studies from this laboratory, which has been adequately described.6-7 It is a fast-flow system with movable stable gas (HzCO) injector and a fixed esr cavity (outside the heated reactor) to monitor H decay, the latter being furnished by a microwave discharge in a trace of Hz (95%). Values of kl have been corrected for axial diffusion.
1
I
I
I
1
0
3
2 103/T
(OK)
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Figure 1. Arrhenius plot of data on kl for H HzCO -+ HZ HCO. The data obey kl = 1.35 X 1018 exp( -3760/RT).
excellent agreement with that reported by Brennen, aL,l who also found kl = 2.6 X lolo. Their experiment was essentially the same as our own, except that mass spectrometric detection for H was used. Of the high temperature studies in the literature, McNesby, et ~ l . combined , ~ their photolysis results on HzCO-D2CO mixtures over the range 520-670°K with analogous results by Klein, et a1.,2 on H2-D2C0 mixtures, with the help of isotopic rate constant ratio theory, to derive the ratio k(D H2)/kl = 3 exp( - 3400/RT). Using our recent experimental result6 Ic(D Hz) = 4.4 X 1013exp(-7600/RT), which encompassed the same temperature range, the photolysis experiments predict Icl = 1.5 X 1013exp(-4200/RT). This is good agreement with our preexponential factor but a somewhat higher activation energy. At G5O0K the above expression gives kl = 6 X lo", and at room temperature (far outside the photolysis range) the predicted value is 1.4 X 1O'O. The corresponding results from our own Arrhenius expression are lc1(650°) = 7.3 X 10" and h(300") = 2.6 X 10'0, so that the agreement is fair at temperatures within the actual range of the photolysis work. The other high-temperature work on Icl is that of Baldwin and Cowe4on the inhibition of Hz-02 explosion limits by H2C0 at 813"K, from which they arrived a t a 02)= 320. If the recommended value ratio kl/k(H from the Leeds revicws (which, to be sure, relies somewhat on the result in ref 4) is used for Ic(H OZ), ie., 6.8 X 109 a t 813"K, the inhibition work yields kl = 2.2 X 1012. Our own Arrhenius relation yields kl = 1.3 X 10l2at this temperature, We are unaware of any other reliable data in the literature for comparison. et
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+
+
sumed in this system has been demonstrated earlier.' H2C0 Thus the overall reaction was taken to be 2H -+ 2Hz CO. The results are summarized in Table I and the mean values at each temperature are plotted in Figure 1. The Arrhenius expression is kl = 1.35 X 1013 exp(-3760/RT) om3 mol-' sec-'. Our room temperature value Icl = 2.6 X 1O'O is in
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The Journal of Physical Chemistry, Vol. 7 6 , N o . 16,1078
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(8) "High Temperature Reaction Rate Data," University of Leeds, Report No. 3, April 1969.
