Kinetics of the Reactions of Atomic Hydrogen Effect of Diffusion on the Determination of H Atom Concentration

Schulz, W. R.; Roy, D. J. Le

doi:10.1139/v62-367

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“…Whether, in view of the uncertainty in AH0,,, and its temperature coefficient, the discrepancy of 0.7 kcal mole-' relative to the value of Berlie and Le Roy (12) is significant or not is doubtful. However, they did not correct their results for diffusion effects (27) and we believe that this would lead to a value of E l l that would be slightly too low.…”

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confidence: 88%

The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H

Reid¹,

Roy²

1968

Can. J. Chem.

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A quantitative study has been made of the reaction of ethyl radicals with molecular hydrogen in the gas phase in the temperature range 240 to 320 "C. The mercury (63P1) photosensitized decomposition of hydrogen in the presence of ethylene was used to generate ethyl radicals. Extinction coefficients for the absorption of 2537 &, by mercury vapor were measured and Beer's law was shown to be obeyed under the experimental conditions used. T h e corrections required to allow for the nonuniformity of radical concentrations in the cell were small. After delineating the experimental conditions necessary to minimize secondary reactions, the rate constant (cn13 mole-' s-') for the reaction CzHS + Hz = CzH6 + H was found to be given by log,, k = 12.57 -13.718. Experiments in the presence of added carbon dioxide showed the absence of hot radical effects at the working pressure of 92 Torr of hydrogen.Canadian Jorlrnal of Chemistry, 46, 3275 (1968) Relatively few atom transfer reactions of the I type R + X, = R X + X, where X = H or D, have been studied (1-3), and in the even fewer cases where the reaction of a given radical with both H, and D, has been investigated there is some doubt about the magnitude of the isotope effect on the activation energy. For n-C,F, radicals, values of 14.0 (4) and 13.8 (5) kcal mole-' have been found for the reaction with D,, 1.9 (4) and 1.5 (5) kcal mole-' greater than for the reaction with Hz. For the reaction of C2F5 with D, and Hz (4) the activation energies were almost the same: 12.6 and 12.4 kcal mole-', respectively. An intermediate isotope effect was found in the case of CF,; values of 9.5 (6) and 10.2 (7) kcal mole-' were found for the reaction with D,, 0.9 (6) and 0.7 (7) kcal mole-' greater than for the reaction with Hz, although the probable errors quoted by the authors in these cases were such that a negligible isotope effect would also be compatible with their results. In the case of C2H5 radicals the value found for the reaction with D, was 13.3 kcal mole-' (8) while the value for the reaction with H, (9), as subsequently corrected (lo), was only 11.3 kcal mole-'.The apparent isotope effect for C2H5 radicals (8, lo), 2.0 kcal mole-', is very close to the difference between the zero point energies of Hz and D,. However, there is no theoretical justification for equivalence with zero point energy differences, and recent measurements in this laboratory (1 1) show no such correlation in the 'Present address: Research Department, Imperial Oil Enterprises Ltd., Sarnia, Ontario.case of the three-center exchange reactions of atomic and molecular hydrogen and deuterium.If we accept a value of 6.8 kcal mole-' (12) or greater (13) for the activation energy of the reaction H + CzH6 = CzHs + Hz and a value of about 6.2 kcal mole-' (14, 15) for -AH, then the activation energy for the reaction CzHs + Hz = CzH6 + H should be > 13.0 kcal mole-'. In view of the uncertainty in this value we felt it necessary to reinvestigate the reaction.Aside from possible errors in the interpretation of the ea...

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The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H

Reid¹,

Roy²

1968

Can. J. Chem.

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“…At sufficiently high pressures and flow rates, the convective term predominates. We then plot atom concentration [proportional to signal/ V f under conditions of convective flow (14)] against time a t varying flow rates. For any given value of atom concentration, the decay rate should be independent of V,.…”

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Diffusive and convective flow in a hydrogen-atom flow system

Rudnick¹,

Toby²

1968

Can. J. Chem.

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Diffusive and convective flow was studied in a hydrogen-atom flow system using an isothermal calorimetric detector. By varying detector area, total pressure, and linear flow rate, the importance of axial diffusion in the system was investigated. Under conditions of predominantly diffusive flow, signal-distance plots were independent of flow rate. When convective flow predominated, concentration-time plots were independent of flow rate. The efficiency of a detector was clearly dependent o n its surface area within wide limits, but diffusion gradients produced by the presence of even large detectors did not appear to be important.

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Importance of taking diffusion into account for flow kinetics

Кондратьев

1964

Russ Chem Bull

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Kinetics of the Reactions of Atomic Hydrogen Effect of Diffusion on the Determination of H Atom Concentration

Cited by 18 publications

References 1 publication

The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H

The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H

Diffusive and convective flow in a hydrogen-atom flow system

Importance of taking diffusion into account for flow kinetics

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Kinetics of the Reactions of Atomic Hydrogen Effect of Diffusion on the Determination of H Atom Concentration

Cited by 18 publications

References 1 publication

The mercury (63P1) photosensitized hydrogenation of ethylene. Kinetics of the reaction C2H5 + H2 = C2H6 + H

The mercury (63P1) photosensitized hydrogenation of ethylene. Kinetics of the reaction C2H5 + H2 = C2H6 + H

Diffusive and convective flow in a hydrogen-atom flow system

Importance of taking diffusion into account for flow kinetics

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Product

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The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H

The mercury (6³P₁) photosensitized hydrogenation of ethylene. Kinetics of the reaction C₂H₅ + H₂ = C₂H₆ + H