1982
J.-T. Cheng and C. Yeh
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Russak and Co., Inc., New York, N.Y., 1973, Chapter 2. J. Grundnes, M. Tamres, and S.N. Bhat, J. Phys. Chem., 75, 3682 (1971). C.N. R. Rao, G. C. Chaturvedi, and S.N. Bhat, J. Mol. Spectrosc., 33, 554 (1970). (a) L. M. Julien, Ph.D. Dissertation, The Universlty of Iowa, 1966; (b) L. M. Julien, W. E. Bennett, and W. B. Person, J. Am. Chem. Soc., 91, 6915 (1969). W. Duerksen and M. Tamres, J. Am. Chem. Soc., 90, 1379 (1968). M. Kroll, J. Am. Chem. Soc., 90, 1097 (1968). Inorg. Syn., 5, 172 (1957). M. Brandon, M. Tamres, and S.Searles, J. Am. Chem. Soc., 82, 2129 (1960). G. Herzberg, "Molecular Spectra and Molecular Structure. I. Spectra of Diatomlc Molecules", 2nd ed, Van Nostrand-Reinhold, New York, N.Y., 1950.
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Pressure Dependence of the Rate Constant of the Reaction H 4- CH3
-
CH4
Jung-Teang Cheng and Chuln-tih Yeh" Institute of Chemistry, National Tsing Hua University, Hsinchu, Taiwan 300, Republic of China (Received March 4, 1977)
-
Ethane gas of 30-2300 Torr has been decomposed by the mercury photosensitization technique at 35 "C to investigate the variation of the rate constant &Sex) of the reaction H + CH3 CH4 upon system pressure. A correlation of the obtained and the published kgex has been made. This rate constant is seen increasing with system pressure, following third-order kinetics at a pressure of 50 Torr or less, and falling off around 150 Torr. Through an extrapolation of the data in the falloff region, the second-order ksex at the high pressure limit is assessed to be (2.0 f 0.9) X 1014 cm3 mol-I sbl. Introduction The reactions of hydrogen atoms with hydrocarbon molecules in the gas phase have been of continuous int e r e ~ t . ' - ~Rate constants for these reactions have been extensively studied under different system conditions. However the reactions between hydrogen atoms and alkyl radicals have not attracted much attention. Although some low pressure rate constants have already been reported for these important radical-atom termination reactions,4-*data in the high pressure region are yet to be measured. Recently we have studied ethane decomposition at a pressure of 300 Torr.g In that study, the mercury photosensitization technique is used. The significant primary processes of this decomposition reaction a t high light intensity have been confirmed as follows: Hg(6'S0) t hv(2537 A ) -,Hg(6jPI) (1,) Hg(63P,) i C,H, H + C,H, + Hg(6'So) H
-,
+ C,HG
2C,H,
H, t C,H,
n-C4Hlo
ZC,H, C,H, t C,H, H t C,H, C,H,* +
+
C,H,* -+ ZCH, C,H,* t M - C,H,
+M
(1) (2) (3)
(4) t 5a) (5b)
H t CH, CH, 2CH, -+ C,H, CH, t- C,H, * C,H, -+
CH, .t C,H, CH, + C,H, H t C,H, -+ C,H, -+
-
The rate constant for the termination reaction H + CH3 CH4 was calculated, based on the above reaction mechanism and the obtained experimental data, to be 1.5 The Journal of Physical Chemistry, Vol. 8 1, No. 2 1, 1977
X 1014 cm3 mol-l s-l. This rate constant a t 300 Torr is considerably larger than those measured constants (around 3 X 10l2cm3 mol-I s-l) reported for this same reaction over the system pressure of a few T ~ r r . ~This - ~ discrepancy is expected from the following unimolecular decomposition mechanism: kGex
I
k6b h6'"
=
-
k,c[MI
k,a
H t CH, + CH,* --
4 CH,
+
k6a~6c[Ml/(~6c[M]
+
h6b)
(1) According to eq I, the value of kgex,the experimental measured rate constant, should be proportional to the system pressure as long as k6,[M]
