Terms Of Reaction Rate Research Articles

Electron Microscopy Evidence of Interfacial Steps Controlling the Kinetics of NiSi2 Formation

ABSTRACTThe kinetics and phase formation sequence of thin-film solid-state reactions, including silicide forming reactions, have frequently been considered to be controlled by interfacial kinetic reaction barriers. These are purely phenomenological quantities which describe the finite rate of the interfacial reaction in terms of limited particle fluxes crossing the respective interfaces. No atomic mechanisms that might be responsible for the action of such barriers have so far been indicated, with the exception of Schmalzried's formulation. The latter says that the interfacial barrier is due to the limited relaxation time needed by the particles to rearrange into the proper sublattice after having crossed the interface. We present correlated kinetic and structural observations during the 2Ni + Si → NiSi2 reaction on the Si(111) surface and discuss them with the help of a model involving the formation and lateral propagation of interfacial steps of different height. The model allows us to explain the kinetic observations by reaction barriers formed as a result of the crystallographic boundary conditions of the reaction.

Reaction rates for 8Li(p, α) and 8Li(p, n, α) and their effect on primordial nucleosynthesis

Differential-cross-section data are presented for the 8Li(p, α) 5He reaction at 1.5 MeV center-of-mass energy, measured with a (radioactive) 8Li beam. The data are used to calculate one of the terms of the thermonuclear reaction rate for destruction of 8Li. In addition, other data are used to estimate the total reaction rate for p+ 8Li, which is found to be comparable to that used in previous calculations of abundances from primordial nucleosynthesis.

Some New Approaches to Semiclassical and Quantum Transition State Theory

AbstractSemiclassical and quantum mechanical transition state theory is reviewed and two new approaches described. One is a general implementation of a semiclassical rate expression [Miller, Faraday Discuss. Chem. Soc. 62, 40 (1977)] that involves the “good” action‐angle variables associated with the saddle point (i.e., transition state) of a potential energy surface. The other is an evaluation of a formally exact quantum expression for the rate [Miller, Schwartz, and Tromp, J. Chem. Phys. 79, 4889 (1983)] in terms of Siegert eigenvalues associated with the transition state. Siegert eigenvalues are usually associated with scattering resonances, so their identification with the saddle point of a potential surface, and the expression for the reaction rate in terms of them, is quite an unexpected and novel development.

An integral method for mixing, chemical reactions, and extinction in unsteady strained diffusion layers

An integral method is presented for determining the evolution of molecular mixing, finite rate chemical reactions, and local extinction in diffusion layers under the effect of an unsteady strain rate. The partial differential equations governing the reactant, product, and temperature profiles are used to derive ordinary differential equations governing the evolution of moments for the product and temperature profiles and for the reactant gradient profiles. The actual profiles enter these equations only through integral moments resulting from the reaction rate terms (referred to as “reaction integrals”). As a consequence, it is possible to accurately track the evolution of the profile moments, and thereby determine global properties of the layer such as burning rates and extinction conditions, using remarkably simple representations for the actual profiles to evaluate the reaction integrals. Here these profile shapes are specified as self-similar families of curves parameterized by just a few degrees of freedom, which then evolve from the moment equations. Results for combustion in isolated strained diffusion layers, as well as for consumption of a burning fuel strip, are generally within a few percent of the results from finite difference solutions of the full equations.

Boundary integral solution method for axial-dispersion-with-reaction problems

This paper describes a new method for the solution of one-dimensional diffusion-convection-reaction type of problems with a non-linear reaction rate term. The procedure is based on the boundary element concepts using suitably defined weighthing functions such that the differential operator is eliminated. Thus in the present method the numerical errors associated with the process of discretization of the differential operators are avoided. The rate non-linearities are handled by the process of quasi-linearization over subintervals in the main domain. The numerical procedure developed here is shown to be simple to use and robust even for the cases of large Peclet numbers and fast reactions.

On singular or dual positional specificity of lipoxygenases. The number of chiral products varies with alignment of methylene groups at the active site of the enzyme.

We tested a simple model which explains the singular or dual specificity of lipoxygenases. The dual specificity considered here is typified by the oxygenation of arachidonic acid by the reticulocyte lipoxygenase: two chiral products are formed (12S- and 15S-hydroperoxides, ratio approximately 1:9) via hydrogen abstraction from two separate methylene groups (C-10 and C-13). The rate-limiting step is known to involve this hydrogen abstraction, and we assumed that alignment of the methylenes with the hydrogen acceptor on the enzyme is critical in terms of reaction rate and positional specificity. Optimal alignment will be associated with a fast rate of reaction and formation of a single chiral product. A shift in position of the double bonds (and hence of the methylene groups) should be associated with a slower rate of reaction and formation of two chiral products; two methylenes are now able to react, although neither has perfect alignment. We tested this idea using two lipoxygenases and polyenoic fatty acids differing in the number and position of the double bonds. Optimal substrates for the soybean lipoxygenase had a doubly allylic methylene in the n-8 position, while the reticulocyte enzyme preferred substrates with a n-9 methylene. These substrates were converted to a single chiral product. With both enzymes, the other series of substrates reacted more slowly and were converted to two chiral products. We conclude that alignment of methylene groups of the substrate at the active site is a major determinant of the reaction rate and the singular or dual specificity of lipoxygenases.

Catalytic hydroliquefaction of Bergueda lignite with fuel oil and anthracene oil under various reactive atmospheres

The hydrogenation of Berguedà lignite was studied using hydrogen donor and non-hydrogen donor solvents, such as anthracene and fuel oil, respectively, under a hydrogen atmosphere, in the temperature range 400–450 °C. The presence of a reactive hydrogen atmosphere increased coal conversion. Maximum yields were reached at 425 °C for the anthracene oil. The product distribution quality improved with the use of a commercial hydrodesulphurization catalyst, which also increased the production of hydrogen sulphide. Under these conditions, a pseudo-first order kinetic model appropriately described the hydroliquefaction reaction rate in terms of pseudocomponents.

Viscosities of Binary Liquid Mixtures of Tetrachloroethylene with Some Aliphatic, Alicyclic and Substituted Aromatic Hydrocarbons

Abstract Viscosities of binary mixtures of tetrachloroethylene with hexane, heptane, cyclohexane, methylcyclohexane, toluene, chlorobenzene, bromobenzene and nitrobenzene were measured at 303.15 K. The values of ηE are negative in all the systems except in tetrachloroethylene + bromobenzene system. An inversion in sign from positive to negative is observed in the system, tetrachloroethylene + bromobenzene. The viscosity data were analysed in terms of absolute reaction rate and free volume theories of liquid viscosity. Further, Grunberg and Nissan Parameter has also been evaluated.

An investigation of CaO sulfation mechanisms in boiler sorbent injection

AbstractA laboratory differential reactor study was conducted at temperatures of 975–1,275 K to investigate CaO sulfation mechanisms with calcine (CaO) samples derived from pulverized limestones and dolomites, and hydrated limes. The limestones had 8–115 μm mean particle sizes, a range applicable for boiler sorbent injection for SO2 control. Under conditions applicable for boiler sorbent injection, the sorbent performance, in terms of initial reaction rate and saturation utilization, was limited by the SO2 pore diffusion rate and by premature pore mouth plugging by CaSO4 product and, thus, depended on CaO particle size and pore size. Hydrated lime and Na2CO3‐promoted limestones showed performance superior to that of limestones, because they produced calcines with larger pores. Intrinsic reaction rate as reported by Borgwardt and Bruce (1986) was observed at temperatures below 1, 100 K.

Adsorption and Tribochemical Reactions

ABSTRACTA model for chemisorption based on the Lewis theory is proposed and developed. The model reproduces the observed relationship between aqueous solubility and band gap for covalent oxides. A relationship is shown between the calculated free-energy of the surface reactions and the observed effect of water on the wear of these materials. This relationship is explained in terms of surface reaction rate and equilibrium. Similar calculations for the adsorption of -CH3functional groups are presented. The results are compared to water adsorption calculations and related to experimental wear test results for model fluids on covalent oxides. The effect of water on lubricant adsorption is discussed.

Isothermal decomposition of almora magnesite

Thermoanalytical techniques are used extensively for the evaluation of kinetic parameters, viz. rate constant K, energy of activation E, order of reaction and reaction mechanism. The general approach in kinetic analysis is to express rate of reaction in terms of the degree of transformation (Y and a temperature dependent function K(T). If the reaction is isokinetic over the range of temperature, the latter term becomes the rate constant governed by the Arrhenius law. Solid state thermal decomposition can be studied under isothermal and non-isothermal conditions. Under isothermal conditions, the temperature of the test sample is to be raised to a desired temperature without any appreciable decomposition taking place. Generally carbonates undergo stepwise endothermic decomposition due to the losses of water of hydration, hydroxyl ions and carbon dioxide and resulting in the formation of stable metal oxides.

Kinetics of enzymic hydrolysis of malto-oligosaccharides: A comparison with acid hydrolysis

The hydrolysis of malto-oligosaccharides G3-G6 catalysed by porcine pancreatic alpha-amylase was investigated kinetically at 25°. Kinetic parameters corresponding to different positions of enzymic attack were determined and product inhibition was evaluated. The enzymic hydrolysis was compared in terms of reaction rate and pattern of action with hydrolysis in 0.1m H2SO4 at 70°. Mathematical models for the mechanism of hydrolysis were developed and a good rationalisation of the experimental results was achieved.

Response of an enzyme-modified pH-sensitive ion-selective device; analytical solution for the response in the presence of pH buffer

An analytical solution for the steady-state response of an enzyme-modified pH-sensitive ion-selective device in the presence of pH buffer is derived. The model assumes effective equilibrium is maintained between the buffer and protons and describes the enzyme reaction rate in terms of conventional Michaelis-Menten kinetics, though no account is made for any pH dependence in the kinetics. Theoretical response curves, consistent with those observed experimentally, are derived, illustrating their dependence upon buffer capacity, solution pH and various kinetic parameters. Limitations to the analytical application of the device are considered.

A second-order closure prediction of premixed turbulent combustion in jets

In this paper, a numerical prediction is reported involving second-order closure of a turbulent flow of a vertically burning, lean mixture of premixed combustible gases discharging from a pipe and developing into a turbulent combusting roundjet. Classical closures are used where available. Expressions for the chemical reaction rate term and other unclosed terms related to variable density flow in the Favre-averaged turbulent transport equations are based on the Bray–Moss–Libby aerothermochemistry for premixed turbulent combustion, extended to variable enthalpy systems. Mixing of hot burned and cool ambient gases and the attendant buoyancy effects are found to be significant physical phenomena in the behavior of such lean premixed combusting jets. Results of the simulation are compared with experimental data of Yoshida [Proceedings of the Eighteenth International Symposium on Combustion (The Combustion Institute, Pittsburgh, 1981), p. 931] with which reasonable numerical agreement is obtained. Reasons for discrepancies and possible lines for future research are discussed.

Comparison of immobilized enzyme reactors for flow-injection systems

Straight, coiled, beaded, and packed-bed reactors containing immobilized glucose oxidase or l-(+)-lactate dehydrogenase were compared in terms of reaction rate, sensitivity, sample throughput, and sample dispersion. The enzymes were covalently attached to the inside walls of 5.0-cm long, 1.12-mm i.d. nylon tubes, and the resulting reactors were tested in a flow-injection system. The beaded enzymatically-active reactors (BEARs) were filled with solid glass beads of 0.5-mm or 1.0-mm diameter. Reactors with the larger beads had 2–4 times the activity, twice the sensitivity, and better throughput than the open reactors; they also minimized the physical and chemical contributions to dispersion. Packed-bed reactors were superior in the lactate determination, but the beaded reactors were better for the determination of glucose. With BEARs containing 1.0-mm beads, glucose was determined in the range 10–800 μM with a conversion efficiency of 0.056 mol of product per mole of substrate; for lactate, the range was 8–64 μM with a conversion efficiency of 0.13 mol mol −1.

Analytic Representations of Thermonuclear Reaction Rates

The basic physical principles common to all nonresonant and resonant thermonuclear reaction rates are taken as a basis to derive the standard forms of both types of reaction rates. Results are given for the closed‐form representations of the standard nonresonant and resonant thermonuclear reaction rates in terms of a certain type of Meijer's G‐function. Then, for physical reasons the nonresonant reaction rate with modified Maxwell‐Boltzmann distribution is considered. The mathematical method for the derivation of closed‐form representations for the modified nonresonant reaction‐rate integral, which is also suitable for numerical computations, is given in detail. Some new techniques developed by the authors are used for deriving the results. The reaction‐rate systematics thus obtained in terms of Meijer's G‐function is a contribution to Fowler's methods for the numerical computation of thermonuclear reaction rates.

Disappearance of criticality in thermal ignition for a simple reactive viscous flow

Conditions for the disappearance of criticality for reactive plane-Poiseuille flow are obtained by means of a perturbation series in the viscous heating parameter; an approximation to the Arrhenius reaction-rate term proposed in this journal by Bazley and Wake is used.

Modelling of spent shale gasification

A model describing the gasification of a spent shale particle in an atmosphere of steam, hydrogen, carbon monoxide and methane is developed. Both diffusion and reaction rate terms are included. Simulations on spent Australian oil shale show that the external mass transfer coefficient has no effect on the overall gasification rate. Increasing the temperature above 1500 K does not increase the gasification rate significantly. No methane was produced under the conditions of the study. The simulations show that neither the homogeneous reaction model nor the shrinking-core model applies for the expected range of operating conditions. Effects of temperature, pressure, particle size and the ratio of carbon dioxide to steam on the conversion and yield are discussed in this paper.

Practical implications of fatty acid topography modification by unsaturation

AbstractUnsaturated fatty acids are markedly different from their saturated counterparts in a number of areas that relate only to the physical properties given them by their topography. In systems where free single bond rotation is restricted, such as monolayers or biomembranes, the effectiveness of chain packing and the fluidity of the fatty chain region is critical and here may best be studied. Topography plays a role in many chemical reactions in terms of reaction rate and product stereochemistry, but is less critical here. Topography hindrances to reaction rate often may be overcome by raising the reaction temperature. When stereochemistry is important, however, the topography of the reactants must be considered. In tightly packed crystalline bulk or condensed monolayer states, the conformation assumed by the chain is its most stable one, excluding polymorphism. This conformation usually will approximate the one of greatest stability for the free molecule in a nonrestricting environment. However, because differences in energy between rotational conformers is generally small, departures from the preferred conformation will be abundant in the absence of constraints imposed by neighboring molecules. Therefore, in the absence of a restricting environment, topographical differences will be minimized. The counterpoint to this is that a large percentage of the fatty acids in use are in such restricting environments in biomembranes, soaps, wetting and foaming agents, fabric afteners, etc., or are purified through a process maiing use of differences in chain packing efficiency, crystallization, where their topography is the primary source of their utility.

Theory of diffusion-controlled reactions: Formulation of the bulk reaction rate in terms of the pair probability

We have formulated the bulk reaction rate rigorously, namely, in terms of the pair probability. Although Shlesinger criticized the usual method of calculating the bulk reaction rate, our result shows that the usual method is correct, at least with respect to the point raised by him. By use of the new formalism we have also analyzed the Wilemski-Fixman model for diffusion-controlled reactions. The present result is in contrast with their approximate one, indicating that the approximation used by them is inadequate.