Errors in Determining the Rate Constant of a First-order Gaseous Reaction by the Flow Method

Abstract

The rate constant of a gaseous reaction calculated from data obtained by the conventional flow method is subject to errors arising from departures from piston flow and thermal equilibrium in the reaction tube. An approximate theoretical analysis of the errors is given for the first-order pyrolysis or isomerization of an organic vapour. In the case of a reaction occurring near 1000�K in a tube 2 cm in diameter, it is shown that for the measured value of the rate constant to be accurate within about 10% the experimental conditions should be such that z > tc/p > 0.5, where p cmHg is the total pressure and tc sec the average contact time. The upper limit (z) of tc/p increases from 3 under conditions of 50% conversion to 10 at 25% and ∞ at 0%. The analysis is applied to measurements of the rate of pyrolysis of toluene. Lack of thermal equilibrium could be at least partly responsible for the observed effect of contact time on the rate constant but does not account for the effect of pressure. The error incurred by assuming piston flow in an isothermal reactor when in fact viscous flow is occurring is discussed in the Appendix.

Determination of D[(CH3)3Pb—CH3] by the Toluene Carrier Method

“…They may be discussed in terms of the following mechanism [I] Pb(CH,),+Pb(CH,), For personal use only. Table 1, reactions 5-7 are all major processes [7] CH3 + C6H5CH2 +C6H5C2H, under the experimental conditions used (toluene/ alkyl molar ratios 50 to 300 with the bulk plus the formation of dibenzyl and minor ring of these between 65 and 100). hydrogen abstraction by CH, that leads to the In Table 2 For personal use only.…”

“…1 gives k1 = 5.0 x 1014 exp (-49 400/RT) s-' with an estimated uncertainty of , l 000 cal mol-l in El. This result has been obtained at the high pressure limit and using flow conditions selected to minimize errors due to thermal equilibrium or diffusion problems (6,7). (5) it is obvious that values of E,, E,, and E, can be assigned which satisfy the relationship k, >> k,, k,, k,.…”

On the mechanism of iodine oxide particle formation

“…The contact time is determined by assuming diffusion-enabled plug flow regime (R e B 0.5, P e B 7, D a B 0.001). 32,33 The PI-TOF-MS system used in this study has been described in detail elsewhere. 30 Species entering the mass spectrometer are ionized B5 cm from the pinhole by 118 nm VUV light, produced by frequency-tripling in Xe the 355 nm output from a YAG laser.…”

The thermal isomerisation of methyl isocyanide in the temperature range 120–320 °C

“…The static experiments serve only as a consistency check on the flow results, and provide a 'handle' which is useful in extrapolating the flow results to even higher temperature. Given the constraints that we could not use a non-condensable carrier gas because that would change the rate constant in the fall-off region, that the measured pressure drop along the reaction vessel (8,9) should not exceed 10% of the mean pressure, so that the pressure itself would be fairly well-defined, that the collection time has to be sufficiently long to be measured accurately, etc., and that AT should not exceed 0.1 "C, we chose to use a 1 m length of 2 mrn-bore glass tubing at the higher end of the temperature range (28C 320 "C) and a. 10 m length of the same tubing at the lower end of the range (240-300 "C).…”