Showing papers on "Elementary reaction published in 1970"

Adsorbed Atomic Species as Intermediates in Heterogeneous Catalysis

Abstract: Publisher Summary This chapter discusses the thermodynamic activity of an atomic species on the surface of a catalyst and its determination under steady-state conditions of a catalytic reaction. If the rate of an individual step of a catalytic reaction depends on the concentration of adsorbed atoms occurring as intermediates, it is imperative to determine this dependence. The determination of the surface concentration of molecules adsorbed on a catalyst is feasible under favorable conditions. In the case of atomic species, it has been found to be convenient to use the thermodynamic activity rather than the surface concentration as the relevant variable. The primary problem of chemical kinetics is the formulation of an empirical rate law that represents the rate of a reaction as a function of the concentrations or partial pressures of reactants, products, and catalysts present in a gaseous or liquid phase. The knowledge of the mechanism of a reaction may be used to formulate a rational rate law that may represent the empirical rate data in a more logical form than an empirical rate law.

Rates of elementary reactions involving the BrO(X2Π) and IO (X2Π) radicals. Part 1.—Formation and decay of the BrO radical

Abstract: Ground state BrO (X2Π,v″= 0) radicals, produced by reaction (1), Br(2P)+O3→BrO+O2(1) were detected using time-resolved electronic absorption spectrophotometry in a discharge-flow system. Removal of BrO redicals occurs through a rapid bimolecular disproportionation reaction producing bromine atoms(2), [graphic ommitted]. The value of k2(cm3 mol –1 s–1) at 293 K was (6.3±0.9)× 1012, and its temperature dependence from 293 to 573 K was given by k2= 10(13.5±0.2) exp [(–450±300)/T]. Reaction (2) shows an anomalously large steric factor.

The Study of Free Radicals and their Reactions at Low Temperature Using a Rotating Cryostat

Abstract: Publisher Summary This chapter describes the principle and construction of the rotating cryostat and discusses several of the systems which have been studied. The overall course of such a reaction is controlled to a great extent by the properties of the transitory species that participate in these elementary reactions. The properties are difficult to measure because of the short lives and low concentrations of these intermediates and have been inferred usually from the course of the overall reaction. A highly successful approach produced large concentrations of the intermediates by a rapid and precisely timed injection of energy into a system. The intermediates can be identified and the variations of their concentrations with time followed by spectroscopic methods. Plash photolysis, shock waves and pulse radiolysis have also been used to good effect. An approach has been to generate relatively high concentrations of the intermediates by rapid mixing in flow systems, or by continuous high energy irradiation, or photolysis, and then to make observations under steady-state conditions. The chapter endeavors to control reactions so that only one elementary step can take place, and then to preserve the products, which would normally have but a transitory existence, for examination. To restrict reaction to a series of single steps, suitable reactive intermediates must be generated but prevented from reacting with each other. Photolysis is more selective and the matrix can be selected independently of the radical precursor, but the yield of free radicals is very low because of cage effects.

Rates of elementary reactions involving the BrO (X2Π) and IO (X2Π) radicals. Part 2.—Reactions of the BrO and IO radicals

Abstract: The mechanisms and rate constants at 293 K of a number of elementary reactions of the ground state BrO (X2Π, v″= 0) radical are reported. BrO is found to react rapidly only with other free radicals (NO,O), [graphic ommitted] It is unreactive towards singlet ground-state molecules (H2, CH4, C2H4, C2H6), BrO+H2→products; k5 < 109.5; BrO+CH4, C2H4, C2H6→products; k < 1010.0. Free X atoms (X = Br, Cl)(formed by bimolecular disproportionation of 2 XO radicals) are normally present in low-pressure systems containing XO radicals. The possible effects on reaction rates of the coexistence of XO and X are considered. The ground state IO(X2Π, v″= 0,1) radical, produced by the reaction,. I(2P)+O3→IO+O2, was detected by means of the A2Π—X2Π band absorption system. This reaction leads to preferential population of the vibrational level v″= 1. Detailed quantitative study of the kinetics of reaction of IO was precluded. However, the results suggest that the decay of IO was closely analogous to that of BrO in mechanism and rate, [graphic ommitted], with k9= 1012.5±0.3 at 293 K.

Kinetic mechanisms in nitrogen + chlorine radical systems. Part 2.—Kinetics of elementary reactions of nitrogen trichloride and of the dichloramino free radical

Abstract: The use of a photometric method for the measurement of NCl2 radical concentrations is described The kinetics of the reactions, [graphic ommitted] and [graphic ommitted] were studied; over the range 259-373 K, the rate constants k2 and k3 are given by k2= 10(117 ± 01) exp [(0 ± 200)/T] cm3 mol–1 s–1, k3= 10(126 ± 04) exp [(–1550 ± 500)/T] cm3 mol–1 s–1 Kinetic studies on the decomposition at 298 K of NCl3 in the presence of H2 CH4 and C2H6, and atom and radical scavengers (ClNO, Br2) were made The results showed that one chlorine atom was produced in reaction (2) for every NCl2 radical reacted The following mechanism for this reaction is suggested; NCl2+ NCl2→ N2Cl3+ Cl, N2Cl3+ M ⇌ N2CI2+ Cl + M The energies of nitrogen-chlorine bonds are discussed

Infrared chemiluminescence from carbon monoxide in the reactions of atomic oxygen with acetylene and carbon suboxide

Abstract: Vibration-rotation transitions of CO(X 1 Ʃ + ) have been observed as infrared chemiluminescence from the reactions of O( 3 P) with C 2 H 2 , C 3 O 2 and HC 3 N. The emission spectra, observed from a spherical flow reactor at low pressures (0.1 to 2 Torr) were characteristic for each reaction system, and in some cases depended on the relative flow rates of the reactants and were affected by the addition of atomic hydrogen. These changes in spectral distribution can yield valuable information about the mechanisms of the elementary reactions involved. Thus in the O + C 2 H 2 reaction, the distribution extending to v 9 = 14 which predominates at low fuel flows is associated with the reactions O + C 2 H 2 = CO + CH 2 + 48 kcal/mol (200 kJ/mol), O + CH 2 =CO + 2H + 73 kcal/mol (306 kJ/mol). An additional distribution extending up to v 9 = 33 (163 kcal/mol, 680 kJ/mol) is observed at high acetylene flows and in the presence of added hydrogen. This is attributed to the reaction O + CH = CO + H + 176 kcal/mol (736 kJ/mol). Similar emission is also observed from the O + C 3 O 2 reaction when atomic hydrogen is added; in its absence, the dominant CO emission comes from levels v ≼ 8 and is attributed to the reaction O + C 3 O 2 = 3CO+115 kcal/mol (480 kJ/mol).

Nonequilibrium Vibrational Excitation Mechanism for the Reaction H2+D2→2HD

Abstract: A reaction mechanism for the four‐center exchange reaction H2+D2→2HD is described in detail. The principal idea is that H2 and D2 react with higher probability when the energy present originates in vibrational rather than translational energy. Detailed calculations are presented which show that the mechanism agrees with available results of shock tube experiments, when experimental rate constants are used for the vibrational relaxation. It is shown that the energy necessary for reaction to occur is about 38 kcal, in addition to the vibrational zero‐point energy, and that the main part of the experimental HD yield is formed in elementary reactions which involve either D2(3) or H2(2), where the superscripts are the vibrational quantum numbers. The concentrations of these species are depleted below the equilibrium values, and in this way the mechanism explains the surprising experimental dependence of the reaction rate on the concentration of Ar, which constitutes about 90% of the reaction mixture. It is ver...

Light Scattering from Chemically Reacting Fluids: Coupled Chemical Reactions

Abstract: The total light scattering intensity from a fluid where chemical relaxation takes place via a sequence of coupled reactions is calculated and examined. The fast reaction limiting case where the rate coefficients in one of the steps are much greater than those in the other step is studied in considerable detail. It is found that only if the reaction is fast and the effective polarizabilities of the species participating in the fast step are equal will the spectrum be simplified. In this case, the broadening of the Rayleigh peak will be due principally to the slow chemical reaction. When one of the reactions is very fast, the spectrum can be analyzed by assuming chemical equilibrium between the species undergoing rapid reaction.

Computational Study of the Kinetics of the Hydrogen-Oxygen Reaction Behind Steady State Shock Waves. Application to the Composition Limits and Transverse Stability of Gaseous Detonations,

Abstract: : The rate equations for the H2-O2 reaction have been integrated numerically under the conditions of steady flow in a Zeldovich-Doring-von Neumann detonation A reaction scheme of 28 elementary reactions was used The initial conditions covered the composition range from 16 to 92% H2 at pressures from 0001 to 01 atm, 16 to 95 per cent at 1 atm, and 30 to 90 per cent at 10 atm Comparison with the limited experimental data available shows that the computed induction times and reaction times are shorter than the experimental values The reaction kinetic behavior of the H2-O2 system under conditions close to the isothermal branched-chain explosion limits is considered The application to detonability limit calculations is discussed (Author)

Plane Steady Navier-Stokes Detonations of Oxyozone

Abstract: Structure of plane steady detonation waves propagating with Chapman-Jouguet velocity is analysed in consideration of transport phenomena. For rich mixtures of oxyozone, only O 3 + X →O 2 +O+ X and O+O 3 →O 2 +O 2 have to be considered in the analysis as associated elementary reactions, where numerical integrations can be carried out from hot boundary to cold boundary singularities since the hot boundary constitutes a nodal singularity due to the first-order character of reaction. Detonation solutions are found out for arbitrarily lean mixtures. For lean mixtures, on the other hand, the inclusion of the recombination reaction O+O 2 + X →O 3 + X becomes necessary and consequently the reaction order near the hot boundary is raised from the first to the second. The examination of such hot boundary presents non-existence of the integration curve satisfying both boundary conditions.

Combinatorial analysis of a chemical network

Abstract: This paper demonstrates the combinatorial relationships among the sets of chemical components, reactions, mechanisms catalyzations, etc. in a chemical system by solving a problem of the following nature: 28 chemical equations are given; one of them describes the overall chemical reaction of a particular chemical system, and the other 27 describe elementary reactions which possibly participate in the system; the problem is to determine mathematically every possible mechanism for the system. The formalization presented here does not depend on the particular chemical system used as an illustration, but is applicable to chemical systems with an arbitrarily large and intricate network structure.

A Theory on Determination of Elementary Reaction Rates in the Catalytic Hydrogenation of Olefins

Abstract: It is shown that reaction rates in forward and backward direction at each elementary step of the catalytic hydrogénation of olefins can be evaluated throughout the reaction from deuterium-distribution in all the reactants and products. The standard associative mechanism is adopted as the reaction scheme and no other ad hoc assumptions are introduced. The treatment may be extended to a generalized associative mechanism of hydrogénation of olefins.

Kinetics of the homogeneous water gas reaction

Abstract: The kinetics of the homogeneous water gas reaction CO/sub 2/ plus H/sub 2/ yield CO plus H/sub 2/O in the forward and reverse directions has been studied in the temperature range 900 to 1100 C, both far from, and close to, equilibrium. Measurements were carried out in a gradientless reactor at a total pressure of 1 atm. The rate of reaction is described by an equation which corresponds to a chain mechanism at an equilibrium concentration of H atoms. This equation agrees with the requirements of thermodynamics for equilibrium. By use of the results of experiments on the accelerating action of traces of O/sub 2/, it was established that the chain length lies in the range 1000 to 10,000. From the observable rate of reaction, the rate constant, k/sub -2/, for the elementary reaction OH plus CO to CO/sub 2/ plus H was determined.