Browsing by Author "Marshall, Roger M."

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Evidence for a Molecular Component in the Thermal Decomposition of Azomethane
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1978, 74 (08), 1978) Marshall, Roger M.; Page, Nicholas D.
The pyrolysis of a mixture containing 0.432 % azomethane in isobutane has been investigated in the ranges 580–657 K and 50–150 Torr. The high pressure limiting value of the rate constant for CH3N2CH3→ 2CH3+ N2(1) has been found to be given by log (k1/s–1)= 13.90 ± 0.22 – 193.6 ± 2.5 kJ mol–1/(2.3RT) in the temperature range 534–657 K from a combination of the results of the present work with previously published values. Below about 50 Torr, values of k1 diminish in accord with the predictions of unimolecular rate theory. To provide a simple, self-consistent account of the relationship between values of the rates of formation of methane and of ethane, a small (2 to 3%) molecular component in the thermal decomposition of azomethane is postulated, viz., CH3N2CH3→ C2H6+ N2. (8), The rate constant is found to be given by log (ks/s–1)= 11.53 ± 0.59–184.5 ± 7.1 kJ mol–1/(2.3RT). This hypothesis also accounts for some previously published results. The implications of the occurrence of reaction (8) are discussed.
Rate Constants for the Initiation of n-Butane Pyrolysis and for the Recombination of Ethyl Radicals
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (1-6), 1974) Hughes, David G.; Marshall, Roger M.; Purnell, J. Howard
Study of the pyrolyses of n-butane and of ethane in the temperature range 895–981 K permits the evaluation of the rate constant, k1/s–1= 1015.3 ± 0.9 exp[(–323 ± 17) kJ mol–1/RT] assuming that k1= 3k2. n-C4H10→22H5˙(1), n-C4H10→CH3˙+ n-C3H7˙(2) This leads via the use of thermochemical data to the result k–1/cm3 mol–1 s–1= 1011.6 over the whole temperature range of the present work, which is in remarkable agreement with other recent estimates made using widely differing techniques at temperature in the range 350–600 K. The present value of k-1 is much lower than previously thought and the possibility that this discrepancy stems from errors in the thermochemistry of ethyl is discussed in detail. It is concluded that this possibility can be discounted and that the current thermochemistry is reliable. Values of the rate constants for the reactions CH3˙+ n-C4H10→CH4+ C4H9˙(3) and C2H5˙+ n-C4H10→C2H6+ C4H9˙(4) are obtained: k3/cm3 mol–1 s–1= 1013.5 ± 0.9 exp[(–61.8 ± 15.0) kJ mol–1/RT] and k4/cm3 mol–1 s–1= 1013.9 ± 0.4 exp[(–92.8 ± 6.7) kJ mol–1/RT]. These results are in good agreement with our previous measurements.
Reaction of Hydrogen Atoms with Ethane
(Journal of the Chemical Society : Faraday Transaction - I. The Chemical Society, London. 1974, 70 (08), 1974) Camilleri, Patrick; Marshall, Roger M.; Purnell, J. Howard
The reaction between hydrogen atoms and ethane has been studied in a flow-discharge system over the wide temperature range 503–753 K at pressures between 8 and 16 Torr. The major products are methane and ethane, reformed by methyl recombination. Minor products are propane and ethylene with traces of n-butane. The detailed mechanism has been established and computer calculations have been used to derive the set of best-fit rate parameters which reproduce all the experimental results. The results of this work yield the result k1/cm3 mol–1 s–1= 1014.27 ± 0.13 exp (–40.9 ± 1.6 kJ mol–1/RT); H + C2H6→ H2+ C2H5. (1) A survey of all published data some of which have been revised by us to take account of wrongly assumed stoichiometry in the original work, shows that, over a range of 1000 K, the data can be represented by the Arrhenius expression, k1/cm3 mol–1 s–1= 1014.12 ± 0.09 exp (–39.2 ± 0.9 kJ mol–1RT). There is thus no reason to suppose curvature in this plot as has been suggested. Values of the rate constants for the reactions H + C2H5→ 2CH3(2), CH3+ H → CH4(5), and, H + C2H5→ H2+ C2H4(11), are found to be k2/cm3 mol–1 s–1= 1013.57, k5/cm3 mol–1 s–1= 1012.04 at 8 Torr, 1012.20 at 12 Torr and 1012.34 at 16 Torr and k11/cm3 mol–1 s–1= 1012.23. We have reassessed our earlier data on the reaction of hydrogen atoms with ethylene in the light of the recent “low” values for the rate constant of ethyl recombination. From this, we find values for k2 and k5 at 290 K which are, respectively, lower and higher than the corresponding values in the range 503–753 K. It is shown that the slight temperature dependence observed is consistent with the order of reactions (2) and (5).