Nonequilibrium Reaction Rate Research Articles

Energy distribution function of translationally hot O( 3P) atoms in the atmosphere of Earth

Translationally hot O( 3P) atoms are produced in the atmosphere of Earth by photolysis of O 2 and O 3 and quenching of O( 1D). A rigorous kinetic theory analysis of this problem is developed and compared with the approach previously employed by Logan and McElroy [ Planet. Space Sci. 25, 117 (1977)]. It is shown that the kinetic theory employed by the previous workers is somewhat deficient. With the line-of-centers cross-section, the rates of reactions of the translationally hot O( 3P) atoms with other atmospheric gases are calculated and found to be in some instances many orders of magnitude larger than the equilibrium rates. Though the non-equilibrium reaction rates with O( 3P) are substantially increased, they are still not competitive with the corresponding reaction rates with O( 1D).

Nonequilibrium velocity distribution and reaction rate in the hot 18F+H2 reaction

The nonequilibrium velocity distribution and reaction rate in the hot 18F+H2 reaction are studied with the realistic cross sections by solving the time-dependent Boltzmann equation with the Monte Carlo simulation. The explicit time-dependent velocity distribution, temperature, rate constant, and yield from initial to steady states are obtained. The effects of the initial hot-atom velocity distribution, temperature, and the ambient temperature and the isotope effect on the rate constant are investigated. A high-temperature steady state does not exist.

Nonequilibrium velocity distribution and reaction rate in hot-atom reactions

The nonequilibrium velocity distribution and reaction rate in the hot-atom reactions are studied by solving the time-dependent Boltzmann equation with the Monte Carlo simulation. The explicit time-dependent velocity distribution, temperature, and rate constant of hot atoms from initial to steady states are obtained for a simple model system, where hot atoms are dispersed in the heat bath of surrounding molecules without internal degrees of freedom and the cross sections are chosen as a simple hard sphere model. The low- or high-temperature steady state exists in consistence with the prediction of Keizer with the Maxwell distribution approximation. The hot-atom velocity distribution for the high-temperature steady state is, however, different from the Maxwell distribution. The nonequilibrium velocity distribution yields a much smaller value of the lower limit of the ratio of reactive to elastic cross sections for the existence of the high-temperature steady state and a smaller rate constant than those of the Maxwell distribution approximation by Keizer.

Nonequilibrium velocity distributions and reaction rates in fast highly exothermic reactions

The nonequilibrium velocity distributions and reaction rates in fast highly exothermic reactions with a low activation energy ε*f/k T0 ≤slant 5, a high steric factor P ≥slant 0.1, and a large heat of reaction Q*/k T0 ∼ O (10), are investigated by solving the semiclassical Boltzmann equation with a Monte Carlo method. The explicit time-dependent velocity distribution functions and reaction rates from the initial equilibrium to the final chemical equilibrium are obtained for a model system, where the chemical species without internal degrees of freedom undergo a bimolecular single reversible reaction. The velocity distributions and the reaction rates indicate significant departures from equilibrium. The extent of departure of the reaction rate from the equilibrium rate increases with the heat of reaction and the activation energy and can be greater than 100%.

A Detailed Formulation of Kinetic Processes. II. The Role of Collision Processes

A detailed treatment is given of the activational processes occuring in both unimolecular and bimolecular processes. Such a treatment leads very naturally to the postulate of two critically energized species, one related to the products and one to reactants. In the case of unimolecular isomerizations it is shown that the properties of either one or both of these species may dominate the rate of the nonequilibrium reaction rate expression, and the form of the rate expression is given in terms of the Slater theory of unimolecular reactions. In the case of unimolecular decompositions, only the complex corresponding to reactants is important, whereas in the inverse case of association reactions only that for the products is of importance. Both of these types of reactions have the same form of pressure dependence in the steady-state concentration region. The above considerations apply even when the reactions are not reversible. That is, the critically energized complex related to products is still of importance in determining the reaction rate. In the case of bimolecular and higher order processes, a detailed investigation shows that the activational processes are always maintained at their equilibrium values, and there is no region in which the rate constants will show a significant dependence on concentration of inert gas.