Showing papers on "Reaction rate published in 1971"

Mixing and chemical reaction in steady confined turbulent flames

Abstract: A calculation procedure has been developed for solving the partial parabolic differential equation of turbulent flame spread. This procedure has been applied to the spread of flame behind a baffle in a plane-walled duct, with two distinct models for the kinetics of the reaction. In the first model, the time-mean reaction rate is related to the time-mean concentrations and temperature at the point in question by a bimolecular Arrhenius expression. In the second model, the local reaction rate is taken to depend also on the rate of break-up of the eddies by fits the experiemntal data better than the first; the eddy-break-up term appears to be essential if the dominance of hydrodynamic processes is to be correctly simulated. A third model of turbulent combustion is also described. It involves the calculation of the magnitude of the fluctuating concentrations, and correctly predicts themain features of turbulent diffusion flames. One of its implications is a finite reaction-zone thickness, even through there is no chemical-kinetic resistance.

Microscopic Reversibility for Rates of Chemical Reactions Carried Out with Partial Resolution of the Product and Reactant States

Abstract: Applications of the principle of microscopic reversibility are considered for reaction rates measured in experiments that reveal some of the dependence on the rotational, vibrational, and/or translational states of the reactants or products. Each type of measurement establishes a characteristic set of quantum states for the reactants and products, suggesting that removal of trivial statistical factors (densities of states) from the data to retrieve a purely dynamical quantity ω(E), the state‐to‐state reaction rate suitably averaged over the initial and final sets of states at fixed total energy E. This quantity is fully symmetric with respect to the direction of the reaction—either the forward or reverse rate coefficient can be obtained by multiplying ω with the proper density of final states. Thus ω reflects the intrinsic probability of the process, disregarding the statistical bias introduced by the nature of the experiment itself. Methods of accurate computation of state densities appropriate to sev...

Reaction intermediates in methyl alcohol decomposition on ZnO

Abstract: The decomposition of methyl alcohol on ZnO was studied using infra-red spectroscopy during the course of the reaction. The concentrations and reactivities of the chemisorbed and gas phase species were measured as well as the overall reaction rate under various non-stationary conditions. When CD3OD vapour was introduced over ZnO, methoxide ion and formate ion were observed, and D2, CO2 and CO were evolved into the gas phase. When the CD3OD in the ambient gas was removed by a dry ice-methyl alcohol trap during the course of the reaction, the evolution of D2 and CO2 stopped, while the evolution of CO continued unchanged. At 240°C, the decomposition rate of the surface formate ion was in reasonable agreement with the rate of production of carbon monoxide. On releasing the trapped methanol, the rates of formation of CO2 and D2 increased again, surface methoxide reappeared, and the concentration of the surface formate ion decreased correspondingly. These results lead to the conclusion that CO was produced mainly by the decomposition of formate ion, and D2 and CO2 came from the reaction between CD3OD and DCOO(a).

Kinetic modeling of the hydrolysis of sucrose by invertase

Abstract: The kinetics of the enzymatic hydrolysis of sucrose by invertase have been examined, with particular emphasis on high substrate concentration. Initial rates of reaction were determined by following the production of glucose directly as a function of time over a wide range of substrate concentrations (0.04M to 2.06M). The resulting data reveal a reaction rate that increases gradually until the sucrose concentration reaches about 0.29M, after which the reaction velocity decreases with increasing sucrose concentration. Previous workers (e.g., Nelson and Schubert1) have reported a peak reaction velocity as determined by indirect polarimetric measurements of glucose, at a sucrose concentration of about 0.17M. These measurements, however, neglect the intermediate oligosaccharides formed by the transferase action of invertase,8–10 and assume equal amounts of glucose and fructose. According to Anderson et al.,10 these oligosaccharides interfere by producing an erroneously low reaction rate. Experimental results of this work confirm Anderson's observations, and show a further reaction rate increase of nearly 20% between sucrose concentrations of 0.177M and 0.285M under the same conditions of temperature, pH, and enzyme-concentration. Effects of substrate diffusion, solution viscisity, water concentration, and substrate inhibition were experimentally studied and the results incorporated into a kinetic model that has proven satisfactory in modeling the experimental results. This model takes into account inhibition by primary substrate, with concentration of the secondary substrate water, as a rate limiting factor at sucrose concentrations greater than 0.285M. The effects of the mixing, in terms of volumetric power input, on the relation rate have been tested. Approximately 40-fold increase in volumetric power input caused on increase in the reaction rate. These experiments have shown that bulk mass transfer is not a rate limiting factor under the experimental conditions.

Reaction Kinetics in Stochastic Models

Abstract: Three distinct topics in the theory of stochastic models of chemical reaction kinetics are discussed. The first, treated in Sec. II, concerns the case of two or more simultaneous reactions, as typified by the mutual interconversion of three or more species. The formula that relates the rate constants to the microscopic transition rates is discussed. The same formula, alternatively interpreted, is then seen also to be the relation that one would use to derive experimental rate constants from the amplitudes and time constants that are the primary experimental data. The second topic is that treated in Sec. III, which is on a class of stochastic models for which the conventional steady‐state approximation is essentially exact. The origins and implications of that circumstance are discussed. The third subject is that of Sec. IV, on a fundamental difference between three‐center exchange reactions A+BC⇄AB+C and (bimolecular) isomerizations A⇄B. The difference is in certain dynamical features of the respective collision mechanisms, which in one case imply that an ``equilibrium'' approximation to the rate constant, such as that given by transition‐state theory, would be a good approximation, and in the other case not.

Kinetics of ozone decomposition and reaction with organics in water

Abstract: The decomposition of ozone in water is found to be second order at pH 2 and 4 with the rate insensitive to pH. At pH 6 the reaction order is 3/2 to 2 and at pH 8 it is first order. Above pH 6 the rate increases rapidly with pH. Ozonation can greatly reduce the organic matter in waste water. The reaction rate of ozone with organics in flocculated secondary sewage effluent is found to depend on the rate of ozone decomposition, which is independent of the subsequent reaction with the organics.

The kinetics of martensite formation in small particles

Abstract: The kinetics of martensite nucleation in “atomized” particles of Fe-24.2 Ni-3.6 Mn and Fe-22 Ni-0.49 C have been investigated as a function of particle size (10 to 140 μ) and reaction temperature. The dependence of particle fraction transformed on particle size indicates that martensite nucleates at surface or near-surface sites for the Fe−Ni−C powder and throughout the bulk for the Fe−Ni−Mn powder. It is shown that, in contrast to kinetic measurements on bulk samples where autocatalysis predominates, the present technique measures reaction rate due solely to the sites present initially. The measurements show that there is no detectable incubation time for nucleation and that the nucleation sites have a specific distribution of activation energies. A method for extracting the distribution from experimental measurements is given and the result is used to develop a revised equation for describing isothermal martensite formation. The isothermal kinetics of martensite formation in Fe-22 Ni-0.49 C are investigated despite the fact that such measurements are not possible in bulk samples because the alloy transforms by “bursting”. It is found that the apparent activation entropy for martensite nucleation is significantly higher for this alloy than for Fe-24.2 Ni-3.6 Mn. This suggests that the dislocation-dislocation interactions at the critical nucleation step are longer-range in the Fe−Ni−C alloy than in the Fe−Ni−Mn alloy.

Picosecond Pulse Radiolysis. III. Reaction Rates and Reduction in Yields of Hydrated Electrons

Abstract: Rate constants for reactions of the hydrated electron with a wide range of compounds, including simple salts and amino acids, have been measured in the time region 20–350 psec. In nearly all cases the second‐order rate constant for reaction with the hydrated electron did not change in the concentration range studied. However, the measured rate constant was in general different to that obtained in dilute solutions, and this was attributed to incomplete formation of the ionic atmosphere around the hydrated electron before reaction. Apart from the hydronium ion, Haq+, nearly all compounds decreased the initial hydrated electron yield: in all cases this decrease showed an exponential dependence on concentration. Compounds most efficient at decreasing the hydrated electron yield were cystine and cadmium salts, in both cases 0.39 mole/liter being needed to reduce the initial yield to 37%. The ability to reduce the initial yield showed no direct correlation with the corresponding hydrated electron rates. This an...

Calculation of multicomponent distillation accompanied by a chemical reaction

Abstract: Iterative method for the determination of stage temperatures, stage reaction rates and interstage flow rates in the problem of multicomponent distillation accompanied by a simultaneous chemical reaction is discussed, and the use of a modified Muller''s method for the convergence of the column temperature profile is proposed. Derivation of the equation is simplified by using matrix notation, which also has the advantage that any interstage flow pattern is allowed. For the solution of the linearized material balance equation, the tridiagonal matrix algorithm is employed. Some problems are discussed to demonstrate the feasibility of the calculations and the fact that quadratic convergence is obtained.

Absolute Rate of the Reaction H+H2S

Abstract: Flash photolysis coupled with resonance fluorescence of Lyman‐α radiation at 121.6 nm has been used to investigate the rate of reaction of H atoms with H2S over the temperature range 190–464°K. Conditions were chosen under which atom–radical and radical–radical reactions were unimportant and only the H‐atom–H2S reaction occurred. The rate constant thus obtained can be expressed as k1 = (1.29 ± 0.15) × 10−11exp[− (1709 ± 60) / 1.987T] cm3molecule−1·sec−1. Comparison of the Arrhenius A factor with that predicted by entropy considerations suggests a somewhat loose activated complex, but not as loose as expected on the basis of the exothermicity of the H+H2S reaction.

Nonequilibrium contribution to the rate of reaction. iii. isothermal multicomponent systems.

Abstract: The nonequilibrium contribution to the reaction rate of an isothermal multicomponent system is obtained by solution of the appropriate Chapman–Enskog equation; the system is composed of reactive species in contact with a heat bath of inert atoms M It is found that the perturbation of the velocity distribution functions is determined by the extent of the departure of the reactive collision frequency R(0)(cγ) for each reactive component γ from R(0)(cggr), the collision frequency that leaves the distribution function unaltered by reaction; the function R(0)(cγ) obtained for an isothermal systems is different from that introduced previously for an isolated system An illustrative application is made to a model system corresponding to the H2+Cl⇆HCl+H reaction without internal degrees of freedom; the deviation of the rate coefficient ratio (kf / kr from the equilibrium value (kf(0) / kr(0)) is a few percent

Flame stabilization by plasma jets

Abstract: The possibility of increasing flame reaction rates, stability and hence the throughput of chemical energy achievable by the addition of a small proportion of electrical power is stuided. The power is added to a subsidiary stream of different gases by a magnetically rotated plasma jet. Rates of rotation of the order 10 5 rev/min contribute to uniform heating and mixing with the very much larger main stream flow (up to blow-out) of methane + air mixtures. The products are sampled by a traversing micro-probe and analysed. Quite small additions of electrical power (e. g. 10% of the chemical energy flux—equivalent to an increase of approx. 116 °C in final temperature) produce large increases in throughput— almost 700 % with N 2 plus argon as the carrier gas. This compares with about 50 % predicted for a perfectly stirred system on the basis of measured global kinetics. Even the effect of argon alone, as the carrier gas, cannot be accounted for by such predictions. Radicals known to be important in flame propagation, such as OH, H and O were deliberately produced by including H 2 O, O 2 and CH 4 in the carrier stream . These were an improvement over argon alone but none appreciably exceeded N 2 in effectiveness. The conclusion is that a limited amount of electrical power used to stabilize a large throughput of flame reactants is most effective if employed to generate energetic and long-lived molecular fragments by imparting it in high concentration to a species of large dissociation energy which is capable of producing, subsequently, radicals important in flame propagation. The practical implications may be important, e. g. for stabilizing large throughputs in jet propulsion.

Shock tube measurements of specific reaction rates in the branched chain CH4-CO-O2 system

Abstract: Rate constants of two elementary bimolecular reactions involved in the oxidation of methane were determined by monitoring the exponential growth of CO flame band emission behind incident shocks in three suitably chosen gas mixtures.

Reaction Rate of Vibrationally Excited Hydroxyl with Ozone

Abstract: Reaction rate of vibrationally excited hydroxyl with ozone, obtaining hydroxyl emission spectra by Fourier transform spectroscopy

Studies in gas-solid reactions: Part I. A structural model for the reaction of porous oxides with a reducing gas

Abstract: A structural model is presented for describing the reaction of a porous metal oxide pellet with a reducing gas. It is suggested that the pellet is made up of a large number of grains and the overall rate of reaction is computed by summing the contributions of all these individual grains. The model thus incorporates structural parameters, such as grain size, porosity (pore size distribution) and allows a quantitative assessment of the role played by these quantities in determining the rate of progress of the reaction.

Theory of Atomic Recombination on Surfaces

Abstract: A simple kinetic theory for atomic recombination on solid surfaces employing the Rideal mechanism is presented The temperature dependence of the reaction rate is discussed in terms of ``transition temperature'' which characterizes change in the kinetic order of the recombination reaction with respect to the gas‐phase atom concentration The transition temperatures are shown to correspond to the maxima and the minima of the reaction rate The experimental data of Wood and Wise and others for hydrogen recombination on glass surfaces are satisfactorily reproduced with the assumption of two types of adsorption sites on glass surfaces having the binding energies of 42 and 21 kcal/mole, respectively Qualitative agreement is also found with the limited experimental data for hydrogen recombination on metals A comparison of the present theory with the steady state theory of De Boer and Van Steenis is given in the Appendix with a refined definition of the partition function for the activated complex in the reaction on surface