The Rate Coefficient for CH₃ + H₂ → CH₄ + H Measured in Reflected Shock Waves; A Non-Arrhenius Reaction

Abstract

Dynamic sampling of reacting gas mixtures behind reflected shock waves has been used to determine the concentration vs. time profiles of the products of the recombination chain reactions of methyl radicals in the presence of molecular H2. The radicals were produced by the rapid pyrolysis of azomethane. By analysis of profiles from experiments in the temperature range 1272–1370°K we have deduced a value of the rate coefficient of reaction 5: CH3 + H2 → CH4 + H.[Formula: see text]It is shown that although this value of k5 does not lie on the Arrhenius line drawn through lower temperature data, it does lie close to the upward curved Arrhenius plot of values of k5 calculated by the B.E.B.O. transition state method. Taking a mean activation energy calculated by the B.E.B.O. method, our results suggest the following description for k5 over the temperature range 1200 to 2000°K:[Formula: see text]

Semi-empirical extrapolation and estimation of rate constants for abstraction of H from methane by H, O, HO, and O2

“…Clark and Dove's BEBO calculation [28] kl = I04,35T s exp ( -4404/T) cm 3 / (mol s) (25) shown in figure 2, gives excellent agreement with eq (24) up to about 700 K and then is significantly higher. Figure 2 shows that eq (24) and (25) are essentially linear below 700 K, hut then start to curve upward, with eq (25) curving more steeply.…”

“…Comparison of measured rate constants for the reaction H+CH4=H2+CH:; with au evaluation based on the reverse reaction [29], JANAF thermochemical data [4], and JANAF thermochemical data adjusted to give I~.H:~" =0. The data points are in Benson [25,26] has pointed out that in general rate constants in the middle of an Arrhenius plot are more accurate than at the extremes because experimental problems are less severe at intermediate temperatures. From the experimental data in figure 1, kl = 10 l1 • l1 cm;!/ (mol s) at 667 K has been selected as the best single rate constant for reaction (1).…”

“…Clark and Dove[25] have measured k_l = 10 10 . 66 cm 3 /{mol s) at 1340 K. The JANAF[4] value for Kl at this temperature is 101.36, giving kl = 1012.o2cm3/ (mol s) at 1340 K.…”

Rate constants for H + CH4, CH3 + H2, and CH4 dissociation at high temperature

“…These [15][16][17][18][19] were mostly designed to measure isotope effects that were subsequently rationalized using semiempirical theory. Absolute rate constant determinations have also been reported [20][21][22][23][24].…”

Combustion Chemistry

“…A review of this reaction is given by Clark and Dove (1973b). A review of this reaction is given by Clark and Dove (1973b).…”