Kinetics and chemiluminescence in the reaction of methane with heated ozone

Abstract

A simplified combustion system was generated by reacting methane with decomposing ozone in the temperature range °C. Under most conditions an unusual and reproducible period of induction was observed followed by a rapid depletion of 03. With added 02 the reaction slowed. These effects were simulated by a 70-step mechanism which qualitatively accounted for the period of induction. A simpler 10-step mechanism which was more algebraically tractable was used to account quantitatively for the kinetics of inhibition with added oxygen. The main ozone-destroying chain is postulated to be the following: CH3 + 03 -CH30 + 02 (4), CH30 + 02 -CH20 + H02 (6), OH + CH4 -H20 + CH3 (7), and HOz + 03 -* OH + 202 (9). This gives the overall stoichiometry for ozone destruction as CH4 + 203 -CH20 + H20 + 202.No evidence was found for a molecular reaction between 03 and CH4. The reaction was chemiluminescent and the emission was identified as due to Meinel bands, presumably from + 03 -* OH* + 02 (35), where OH* = ( 2 , <9). No emission was found when the helium carrier gas was replaced by oxygen. The activation energy for light emission was measured as 28.7 ± 1.2 kcal/mol. This contrasts with the results of experiments where the methane was replaced by ethylene and where the light emission showed no temperature dependence.

Class size and teaching evaluation: Or, the "general chemistry effect" revisited

“…The most striking feature of the results was that, when the "learning curve" of conclusion (1) was factored out, most faculty showed a consistent decrease in student evaluations as class size increased. We dubbed this the "General Chemistry Effect" (5).…”

Reactivity of the Ozone−Ethane System

“…In previous work on the ozone−methane system we measured the reaction rate in the range 75−175 °C and found a period of induction of less than a minute . Using a complex free radical mechanism, we were able to give a satisfactory computer simulation of the induction period.…”

“…The apparatus has been previously described. 5 It was clear from the work of Morrissey and Schubert 6 that attempting to measure all of the many products of this reaction would be a lengthy undertaking that would not greatly add to our knowledge of the kinetics involved. We therefore followed the progress of the reaction by measuring three parameters: ozone concentration, total pressure, and light emission.…”

“…Total light emission was measured using a cooled Hamamatsu R636 photomuliplier placed at right angles to the axis of the cylindrical reaction cell. Some experiments were done in a flow system 5 and spectra taken with a 0.25 m Jarrell-Ash monochromator.…”

“…4 In previous work on the ozone-methane system we measured the reaction rate in the range 75-175 °C and found a period of induction of less than a minute. 5 Using a complex free radical mechanism, we were able to give a satisfactory computer simulation of the induction period. Since the C-H bond in ethane is about 15 kJ/mol weaker than in methane, one would expect more reactivity in the ozone-ethane system, and we report here on this reactivity.…”