Shock-wave observations of rate constants for atomic hydrogen recombination from 2500 to 7000 °K: collisional stabilization by exchange of hydrogen atoms

1969 ◽  
Vol 310 (1501) ◽  
pp. 253-276 ◽  
Keyword(s):  
Shock Waves ◽  
Rate Constant ◽  
Rate Constants ◽  
Atomic Hydrogen ◽  
Functional Form ◽  
Kcal Mole ◽  
Hydrogen Atoms ◽  
Hydrogen Molecules ◽  
Hydrogen Recombination ◽  
Equilibrium Dissociation

Rate constants for the recombination of atomic hydrogen with hydrogen molecules, hydrogen atoms, and argon atoms as the third bodies are presented in functional form for the range of temperatures from about 2500 to 7000 °K and are critically compared with the results of other workers. The rate constants are evaluated from detailed analyses of spectrum-line reversal measurements of the fall in temperature accompanying dissociation behind shock waves in gas mixtures containing 20, 40, 50 and 60% of hydrogen in argon. The rate constants for recombination with hydrogen molecules ( k -1 ) and argon atoms ( k -3 ) fit the equations log 10 k -1 = 15.243 - 1.95 x 10 -4 T cm 6 mole -2 s -1 , log 10 k -3 = 15.787 - 2.75 x 10 -4 T cm 6 mole -2 s -1 , with a standard deviation of 0.193 in log 10 k -1 . The rate constant for recombination with hydrogen atoms is about ten times larger than these at 3000 °K and shows a steep inverse dependence on temperature ( ~ T -6 ) above 4000 °K. Below this temperature the power of this dependence decreases rapidly and there is strong evidence that the value of this rate constant has a maximum around 3000 °K. This behaviour is interpreted on the basis of a process of collisional stabilization by atom exchange, requiring an activation energy around 8 kcal mole -1 and taking place under conditions of vibrational adiabaticity. The over-all results indicate that the assumption of equality between the equilibrium constant and the ratio of the rate constants for dissociation and recombination is valid throughout the region of non-equilibrium dissociation and at all temperatures in the shock waves examined.

Curve-crossing collisions of excited lead atoms in flames

1981 ◽  
Vol 376 (1767) ◽  
pp. 545-564 ◽  
Keyword(s):  
Rate Constant ◽  
Atomic Hydrogen ◽  
Negative Temperature Coefficient ◽  
Negative Temperature ◽  
Model Systems ◽  
Attractive Potential ◽  
Hydrogen Atoms ◽  
Lennard Jones ◽  
Increasing Temperature ◽  
Curve Crossing

Lead atoms, present as a trace additive in a series of premixed H 2 –N 2 –O 2 flames, were excited to the 7 3 P o 1 state by 405.8 nm radiation from a nitrogen-pumped dye laser. Rate constants for spin-orbit relaxation to the 7 3 P o 0 state were obtained separately for collisions with atomic hydrogen and for collisions with the bulk flame gas, by measuring the relative intensities of fluorescence at 364.0 and 368.3 nm as a function of distance from the reaction zone in each flame. For hydrogen atoms the rate constant is typically 1 x 10 -9 cm 3 molecule -1 s -1 , decreasing with increasing temperature; for the bulk flame gas the rate constant is typically 1 x 10 -11 cm 3 molecule -1 s -1 , increasing with increasing temperature. Numerical calculations for model systems, with the use of Morse and Lennard-Jones potentials to describe the interaction of the colliding species, show that the negative temperature coefficient found for atomic hydrogen can be attributed to the crossing of attractive potential curves, corresponding to bound excited states of PbH.

Kinetics of the reaction H + C2H6 → H2 + C2H5 in the temperature region of 1000 K

1984 ◽  
Vol 62 (1) ◽  
pp. 86-91 ◽  
Author(s):  
J.-R. Cao ◽  
M. H. Back
Keyword(s):  
Equilibrium Constant ◽  
Rate Constant ◽  
Temperature Region ◽  
Recent Measurement ◽  
Hydrogen Atoms ◽  
Elementary Reactions ◽  
Temperature Range ◽  
Hydrogen Molecules ◽  
Kinetics Of ◽  
Rate Controlling Step

A system for the measurement of rate constants for elementary reactions of hydrogen atoms in the temperature region of 1000 K is described. The concentration of hydrogen atoms is controlled by the equilibrium constant for dissociation of hydrogen molecules. The kinetics of the rate of conversion of ethane to ethylene in this system has been studied over the temperature range 876–1016 K. The results show that the rate-controlling step is[Formula: see text]and the value obtained for the rate constant is[Formula: see text](R = 1.987 cal mol−1 deg−1). This value is compared with values obtained from other methods over the temperature range 300–1400 K. Combination with a recent measurement of the rate constant for the reverse reaction yields an experimental value for the equilibrium constant for the reaction.

BrHgO• + C2H4 and BrHgO• + HCHO in Atmospheric Oxidation of Mercury: Determining Rate Constants of Reactions with Pre-Reactive Complexes and a Bifurcation

2019 ◽  
Author(s):  
Khoa T. Lam ◽  
Curtis J. Wilhelmsen ◽  
Theodore Dibble
Keyword(s):  
Hydrogen Atom ◽  
Quantum Chemistry ◽  
Transition State ◽  
Rate Constant ◽  
Rate Constants ◽  
Minimum Energy ◽  
Atmospheric Oxidation ◽  
Minimum Energy Path ◽  
Oh Radical ◽  
Hydrogen Atoms

Models suggest BrHgONO to be the major Hg(II) species formed in the global oxidation of Hg(0), and BrHgONO undergoes rapid photolysis to produce the thermally stable radical BrHgO•. We previously used quantum chemistry to demonstrate that BrHgO• can, like OH radical, readily can abstract hydrogen atoms from sp<sup>3</sup>-hybridized carbon atoms as well as add to NO and NO<sub>2</sub>. In the present work, we reveal that BrHgO• can also add to C<sub>2</sub>H<sub>4</sub> to form BrHgOCH<sub>2</sub>CH<sub>2</sub>•, although this addition appears to proceed with a lower rate constant than the analogous addition of •OH to C<sub>2</sub>H<sub>4</sub>. Additionally, BrHgO• can readily react with HCHO in two different ways: either by addition to the carbon or by abstraction of a hydrogen atom. The minimum energy path for the BrHgO• + HCHO reaction bifurcates, forming two pre-reactive complexes, each of which passes over a separate transition state to form a different product.

The mechanism of the pyrolysis of 2, 2, 3, 3-tetramethylbutane

1978 ◽  
Vol 363 (1715) ◽  
pp. 503-523 ◽  
Keyword(s):  
Hydrogen Atom ◽  
Rate Constant ◽  
Rate Constants ◽  
Volume Ratio ◽  
Reaction Scheme ◽  
Secondary Source ◽  
Kinetic Characteristics ◽  
Hydrogen Atoms ◽  
Reaction Conditions ◽  
Total Product

The pyrolysis of 2, 2, 3, 3-tetramethylbutane (TMB) was investigated in the ranges 699-735 K and 3-19 Torr (0.4-2.5 kPa) at up to 4% decomposition. The reaction is strongly self-inhibited and sensitive to the surface/volume ratio of the reaction vessel. A simple Rice-Herzfeld chain terminated by the heterogeneous removal of hydrogen atoms is proposed for the initial, uninhibited reaction generating isobutene and hydrogen in a 2:1 ratio. Self-inhibition is due to abstraction by hydrogen atoms of hydrogen atoms from product isobutene giving resonance-stabilized 2-methylallyl radicals which participate in homogeneous termination reactions. The kinetic characteristics of the major primary products (> 95% on a mole basis), isobutene and hydrogen, are accounted for when reasonable values are assumed for the rate constants for hydrogen atom abstraction by hydrogen atoms from TMB and from isobutene and for initiation and heterogeneous termination of the chain reaction. The kinetic characteristics of the formation of methane and propene (2-4% of total product) are accounted for by the secondary reaction scheme H + i-C 4 H 8 → i-C 4 H 9 , i-C 4 H 9 → CH 3 + C 3 H 6 , CH 3 + TMB → CH 4 + C 8 H 17 , when a reasonable value for the rate constant for the hydrogen atom addition to isobutene is assumed. The kinetic characteristics of the formation of ethene ( ca . 0.1% of total product) are accounted for by the tertiary reaction scheme H + C 3 H 6 → n -C 3 H 7 n -C 3 H 7 → CH 3 + C 2 H 4 , when a reasonable value for the rate constant for the hydrogen atom addition to propene is assumed. The kinetic characteristics of the formation of isobutane ( ca . 1% of total product) are much less affected by an increase in surface/volume ratio of the reactor than are those of the other products. A heterogeneous, secondary source is suggested, viz. 1/2H 2 ( g ) ⇌ H (wall), H (wall) + t-C 4 H 9 ( g ) ⇌ i-C 4 H 10 ( g ), which can generate the observed dependence of the isobutane yield on the reaction conditions but the reasonableness or otherwise of the values of the equilibrium and rate constants it is necessary to postulate is impossible to assess without further work designed specifically to investigate this problem.

REACTIONS OF THE ETHYL RADICAL: VI. ADDITION TO CONJUGATED DIENES

1965 ◽  
Vol 43 (5) ◽  
pp. 1102-1109 ◽  
Author(s):  
A. C. R. Brown ◽  
D. G. L. James
Keyword(s):  
Rate Constants ◽  
Chain Transfer ◽  
Conjugated Dienes ◽  
Energy Of Activation ◽  
Conjugated Diene ◽  
Kcal Mole ◽  
Hydrogen Atoms ◽  
Ethyl Radical ◽  
Diene System ◽  
Rate Constants For Addition

Arrhenius parameters have been measured for the addition of the ethyl radical to the conjugated diene system in three representative molecular environments. Significant differences are found among the values of the energy of activation for addition, which are: 4.5 ± 0.2 kcal/mole for 2,3-dimethylbutadiene-1,3, 5.2 ± 0.3 kcal/mole for cyclohexadiene-1,3, and 6.6 ± 0.3 kcal/mole for 2,5-dimethylhexadiene-2,4. The increase in the energy of activation in this series is paralleled by an increase in the degree of shielding of the terminal carbon atoms of the conjugated system by substituent groups. The energy of activation for metathesis is significantly lower for cyclohexadiene-1,3 (5.4 ± 0.5 kcal/mole) than for 2,5-dimethylhexadiene-2,4 (7.6 ± 0.4 kcal/mole); the activated hydrogen atoms of the former are all secondary, whereas those of the latter are all primary. The ratio of the rate constants for addition and metathesis at 60° indicate that the radical homopolymerization of cyclohexadiene-1,3 and 2,5-dimethylhexadiene-2,4 should be subject to extensive degradative chain transfer.

The reaction of hydrogen atoms with propylene

1971 ◽  
Vol 324 (1559) ◽  
pp. 433-446 ◽  
Keyword(s):  
Rate Constant ◽  
Rate Constants ◽  
Flow System ◽  
Primary Process ◽  
Absolute Rate ◽  
Hydrogen Atoms ◽  
Vibrationally Excited ◽  
Flow Rates ◽  
Detailed Mechanism ◽  
Excited Species

A detailed study has been made of the products of the reaction of hydrogen atoms with propylene. A discharge-flow system at 290±3 K was used. Total pressures in the range 4 to 16 Torr (550 to 2200 N m -2 ) of argon were used and the flow rates of hydrogen atoms and propylene ranged individually up to about 12 μ mol s -1 . As found by others the main products are methane, ethane, ethylene, propane and isobutane. Trivial amounts of 2,3-dimethylbutane, but no n-butane, were detected. A detailed mechanism accounting adequately for the reaction is proposed. It is confirmed that formation of the vibrationally excited species, i-C 3 H 7 *, is the predominant primary process. Novel processes which are shown to be important are H+i-C 3 H 7 * → CH 3 +C 2 H 5 and, C 3 H 8 * → CH 4 +C 2 H 4 . A number of rate constant ratios have been evaluated from the data and these allow calculation of absolute rate constants of some individual reactions. The agreement with previously reported values is, in most instances, good.

THE PYROLYSIS OF TOLUENE

1962 ◽  
Vol 40 (7) ◽  
pp. 1310-1317 ◽  
Author(s):  
S. J. Price
Keyword(s):  
Activation Energy ◽  
Rate Constant ◽  
Rate Constants ◽  
Contact Time ◽  
Flow System ◽  
Kcal Mole ◽  
Homogeneous Process ◽  
First Order ◽  
Order Rate ◽  
Toluene Concentration

The pyrolysis of toluene has been studied in a flow system from 913 to 1143 °K. First-order rate constants are independent of the toluene concentration but decrease approximately 9% when the contact time is reduced from 1.0 to 0.41 second. Increasing the contact time from 1.0 second to 2.07 seconds does not affect the rate constant. The overall rate has been resolved into homogeneous and heterogeneous components. It is suggested that the activation energy of the homogeneous process, 85 kcal/mole, may be associated with D(C6H5CH2—H).

THE KINETICS OF THE REACTION OF ATOMIC HYDROGEN WITH METHANE

1964 ◽  
Vol 42 (7) ◽  
pp. 1638-1644 ◽  
Author(s):  
J. W. S. Jamieson ◽  
G. R. Brown
Keyword(s):  
Activation Energy ◽  
Electric Discharge ◽  
Atomic Hydrogen ◽  
Flow System ◽  
Steric Factor ◽  
Primary Reaction ◽  
Kcal Mole ◽  
Hydrogen Atoms ◽  
Fast Flow ◽  
Kinetics Of

Reinvestigation of the reaction of hydrogen atoms, produced by electric discharge, with methane in a fast flow system has given an activation energy of 7.4 ± 1.1 kcal/mole and a steric factor of about 10−3 for the primary reaction, H + CH4 → H2 + CH3.

scholarly journals Modeling the OH-Initiated Oxidation of Mercury in the Global Atmosphere Without Violating Physical Laws

2019 ◽  
Author(s):  
Theodore Dibble ◽  
Hanna Tetu ◽  
yuge jiao ◽  
Colin Thackray ◽  
Daniel J. Jacob
Keyword(s):  
Quantum Chemistry ◽  
Rate Constant ◽  
Atmospheric Chemistry ◽  
Rate Constants ◽  
Bond Energy ◽  
3D Model ◽  
Kcal Mole ◽  
Present Rate ◽  
Physical Laws ◽  
Atmospheric Radicals

We present a way for modelers to include the OH + Hg reaction while accounting quantitatively for the dissociation of HOHg•. We use high levels of quantum chemistry to establish the HO-Hg bond energy as 11.0 kcal/mole, and calculate the equilibrium constant for OH + Hg = HOHg•. Using the measured rate constant for association of OH with Hg, we determine the rate constant for HOHg• dissociation. Theory is also used to demonstrate that HOHg• forms stable compounds, HOHgY, with atmospheric radicals (Y = NO2, HOO•, CH3OO•, and BrO). We then present rate constants for use in in modeling OH-initiated oxidation of Hg(0). We use this mechanism to model the global oxidation of Hg(0) in the period 2013-2015 using the GEOS-Chem 3D model of atmospheric chemistry. Because of the rapid dissociation of HOHg•, OH accounts for <1% of the global oxidation of Hg(0) to Hg(II), while Br atoms account for 97%.

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