Showing papers on "Reaction rate published in 1968"

Flame structure and flame reaction kinetics II. Transport phenomena in multicomponent systems

Abstract: The transport fluxes in multicomponent flame systems due to diffusion and thermal conduc­tion, and to thermal diffusion and its reciprocal effect, are considered from the standpoint of the extension of the Chapman–Enskog kinetic theory to polyatomic gases by Wang Chang, Uhlenbeck & de Boer (1951, 1964) and the subsequent development by Mason, Monchick and coworkers (1961-66). Equations are given for the various diffusional, thermal diffusional and thermal fluxes which it is necessary to derive in order to obtain reaction rates from experimental temperature and composition profiles in flames; and the organization of com­puter programs for calculation of the multicomponent diffusion and thermal diffusion coefficients and the thermal conductivity is described. The use of matrix partitioning tech­niques in suitable circumstances to reduce the amount of computation is also discussed. The expressions for the transport fluxes are next used to derive equations for the mole fraction and temperature gradients in flowing reaction systems such as flames where trans­port processes and reaction occur side by side. From the mole fraction and temperature at one point in the system it is then possible by a numerical integration method such as the Runge–Kutta procedure to compute the complete composition and temperature profiles. Two methods of obtaining the mole fraction and temperature gradients are described, one of which, the Stefan–Maxwell formulation, leads to the more economical computation. A hydrogen–oxygen–nitrogen–steam mixture was chosen under conditions which simu­lated the pre-reaction region of a hydrogen–oxygen–nitrogen flame that had been studied experimentally, and the detailed composition profiles due to diffusion were computed. The experimental method of measurement involved continuous sampling from the flame and mass spectrometric analysis, a technique which had not previously been checked on a flame system itself. Good agreement between theory and experiment was found when thermal diffusion was considered in the calculation, although the computed hydrogen profile was slightly displaced with respect to the experimental one. This last observation is possibly due to diffusion effects in the pressure gradient at the probe tip. Otherwise the experimental technique seemed to be satisfactory. The computed profiles also showed a number of interesting features such as a maximum in the nitrogen concentration profile caused by thermal diffusion effects.

Thermochemistry of Reacting Materials

Abstract: This paper is concerned with the formulation of a thermomechanical theory of a mixture of chemically reacting materials without diffusion. The independent variables that influence the response of the mixture are assumed to be the temperature, the temperature gradient, the deformation gradient, the time rate of change of the deformation gradient, and the extent of reaction. The Clausius–Duhem inequality is used to derive a set of necessary and sufficient conditions that the constitutive functions must obey. Among these conditions are ones that determine the role of De Donder's chemical affinity in determining whether or not chemical reactions are present. In particular, a criterion is deduced that determines whether or not false equilibrium is possible. It is also shown under what circumstances vanishing of the chemical affinity is equivalent to vanishing of the reaction rate. The additional restrictions on the response of the mixture by the axiom of material frame—indifference and special types of material symmetry are also presented. It is also shown that if the free energy of the mixture satisfies a certain inequality, then any equilibrium state that has both a zero chemical affinity and a zero reaction rate is necessarily stable for chemical reactions taking place at constant temperature and deformation.

The oxidation of titanium diboride and zirconium diboride at high temperatures

Abstract: Powdered and compacted TiB2 have been heated in oxygen at pressures of 760–200 mm Hg. The TiB2-O2 reaction rate is parabolic except near 950 °C where it is cubic. The activation energy below 950 °C is 45 kcal/mole and above 950 °C is 31 kcal/mole. The ZrB2-O2 reaction rate is parabolic throughout the range 700 °–1050 °C and the activation energy is 28 kcal/mole. With both TiB2 and ZrB2 there is a minimum rate of reaction at about 600 mm pressure. The variation of oxidation rate with temperature and pressure is discussed in terms of a protective but volatile layer of B2O3.

The Cool Flames of Hydrocarbons

Abstract: This review describes the properties of the cool-flame and two-stage ignition processes that characterize the gaseous oxidation of hydrocarbons and discusses the chemical reactions that are responsible for these phenomena. Cool flames result from a chainthermal acceleration of reaction rate. It is probable that the free-radical chain involved is propagated by the reaction of an alkyl radical with oxygen to give an alkyperoxy radical which isomerizes to a hydroperoxyalkyl radical. The decomposition of this radical produces a hydroxyl radical, which attacks the hydrocarbon rapidly and unselectively to regenerate an alkyl radical. Branching probably results from the pyrolysis of mono- and dihydroperoxides, from the oxidation of aldehydes, and from radical-molecule reactions. This reaction scheme explains the existence of a low-temperature ignition peninsula and the relation of the extent and shape of this peninsula to the molecular structure of the hydrocarbon. The chemical relevance of cool flames to abnormal combustion phenomena, such as knock, in gasoline engines is discussed.

Ion chemistry governing mesospheric electron concentrations

Abstract: A theory and methodology for investigating the ion chemistry governing mesospheric electron concentrations is presented. In this theory, macroscopic reaction coefficients are defined in terms of the microscopic chemical reactions that take place between positive ions, electrons, and negative ions. The role of minor constituents of the atmosphere, O, O3, CO2, NO, and NO2, is discussed using reaction rates measured in the laboratory. One reaction, the associative detachment of O2− by atomic oxygen, is so fast that mechanisms for inhibiting or suppressing this reaction are required. The need for such mechanisms is dictated by field data on riometer absorption induced by the nuclear explosions of the 1962 Christmas Island air drops. The importance of terminating negative ions, i.e., negative ions that withstand the attack of atomic oxygen, is emphasized. Also emphasized is the influence of the ion-ion mutual neutralization coefficient of terminating negative ions. Four illustrative models of the ion chemistry are presented ranging from the simplest O2− model, which is certainly unrealistic, to a more complex model involving six species of negative ions. This model, which is suggested by recent measurements of reaction rates in the laboratory, does not include the effect of water vapor in the reaction scheme since little laboratory data exist on the reaction of negative ions with water.

Chemical reaction cross sections and rate constants

Abstract: Examines bimolecular collisions in a gas and the collision cross section; reaction rates and cross sections for chemical reactions; calculations of reaction cross sections; and experimental methods for the determination of reaction cross sections.

Thermal energy ion--molecule reactions.

Abstract: Publisher Summary This chapter describes various aspects of thermal energy ion-molecule reactions. Some new experimental techniques have led to a great deal of quantitative information on ion-molecule reactions, especially for reactions involving small molecules at low energies. The majority of ion-molecule reaction rates has been measured in experiments utilizing the ion source of a mass spectrometer. A product ion may be produced from several reactions and the separation into the component reactions is often uncertain. The ability to vary the ion energy can be used to advantage in mass spectrometer ion sources, as the energy dependence of reactions can be investigated in this way. A few endothermic reactions have been studied in mass spectrometer ion sources, with the accelerating field supplying sufficient kinetic energy to make the reaction observable. The atomic positive ions are produced in weak plasma and largely deexcited to the ground state by superelastic collisions with electrons before reaching the neutral port. The ion-atom interchange reaction is also elaborated in this chapter.

Structure and reactivity of transition-metal complexes with polyatomic ligands. Part V. Electron spin resonance spectra of intermediates and some kinetic results for the acid hydrolysis of [Cr(CN)5NO]3–

Abstract: Three intermediates in the acid hydrolysis of [Cr(CN)5NO]3– are detected in solution by e.s.r. at room temperature, the final product being [Cr(NO)(H2O)5]2+. Each species is characterised by different A(14N), A(53Cr), or g values. Kinetics of the last two stages of the reaction have been studied to determine the dependence of reaction rates on acid concentration and to evaluate the activation parameters.

Oxidation of Graphite catalysed by Palladium

Abstract: PALLADIUM significantly increases the reaction rate of graphite with oxygen in the 600°–700° C temperature range1. The activation energy for the catalysed reaction in this range is 124.5 kcalories/mole, however, compared with 48.8 kcalories/mole for the uncatalysed reaction, so the pre-exponential term A in the Arrhenius equation k = Ae−E/RT is correspondingly large. (In the equation k is the rate of reaction, E the activation energy and R and T have their usual identity.) This implies that the catalysed reaction takes place on surface sites which differ in both quantity and nature from those of the uncatalysed reaction.

Mechanism of catalytic decomposition of formic acid on a nickel surface

Abstract: The decomposition of formic acid on a nickel surface was studied by volumetric measurement of the adsorption during the reaction and also by the infra-red technique. The chemisorbed species during the decomposition of formic acid were only formate ion and proton. The rate of the decomposition was proportional to the formic acid pressure and decreased markedly by the increasing amounts of formate ion on the surface. Accordingly, the decomposition proceeded on the surface not covered by the formate ion. The rate of the decomposition of the formate surface layers was expressed by the Zeldovich-Roginsky equation as a function of coverage. A part of the overall reaction proceeded via the formate ion, and the rest between the formic acid molecule and the surface not covered by the formate ion. The overall reaction rate was given by rate =kPHCOOHf(1–θ)+k′ exp [a(x–xo)] where (1–θ) is the fractional surface not covered by formate ion; the second term gave the rate via the formate ion in the surface. The exchange reaction of formate ion and proton in the surface with those in formic acid molecules in the ambient gas is a process independent of the decomposition reaction.

Pyrolysis of ethylene

Abstract: The initial stages of the pyrolysis of ethylene have been studied at temperatures in the range 798–924°K and with reactant pressures between 10 and 270 torr. The reaction is a degenerately branched chain process in which the “branching” or secondary initiation step is 1–C4H8→ C3H˙5+ CH˙3 A lower limit to the velocity constant of this reaction has been determined and values have been established for the rate parameters of the reaction, C2H˙5+ C2H4→C2H6+ C2H˙3. The formation of methane and propylene, important products of the reaction, is largely of a secondary nature.

On the structure of a hydrogen-oxygen diffusion flame

Abstract: A simplified scheme of four chemical reactions is chosen to represent the kinetics of the hydrogen-oxygen system; m particular, this scheme includes the influence of the hydroxyl radical. The diffusion flame supported by this set of reactions is assumed to form behind a (planar, two-dimensional) body of parabolic meridian profile with downstream-pointing vertex. The body initially separates the oxygen and hydrogen streams, which are assumed to have equal speeds and pressures far upstream. (The pressure is subsequently assumed to be constant everywhere.) For pressures of about one atmosphere it is found that nett reaction rates can be treated as infinitely fast, the four reactions then yield four chemical equilibrium equations whose behaviour is dominated by the largeness of the equilibrium constant for the (thermal) dissociation-recombination reaction of hydrogen. The flame-sheet model emerges as the limiting solution when the reciprocal of this large quantity is allowed to vanish. The method of matched asymptotic expansions is used to investigate the structure of the flame which results from a relaxation of this limit. The results bear a satisfactory resemblance to some experimental measurements which, although made in other gas mixtures, exemplify the behaviour of the type of diffusion flames considered.