Showing papers on "Reaction rate published in 1980"

The Water-Gas Shift Reaction

Abstract: The water-gas shift is a reversible, exothermic chemical reaction, usually assisted by a catalyst, and is the' reaction of steam with carbon monoxide to produce carbon dioxide and hydrogen gas

Influence of Chemical Kinetics and Unmixedness on Burning in Supersonic Hydrogen Flames

Abstract: Good agreement has been obtained between published profiles of composition and pitot pressure with the calculated results from a computer program in which finite rate chemistry was used. Significant differences are noted between results calculated using 7 species and 8 reactions and those calculated using 12 species and 25 reactions. Differences are also found between results in which the effect of unmixedness on reaction in turbulent flow is applied or is not applied. ULTI-REACTION finite-rate chemistry has been used for many years in computer simulation of complex flowfields, and results have been good in laminar flows. Mixing of fuel and air is faster in turbulent flows than in laminar flows, but in turbulent flows the folding together of large volumes of fluid alternately rich in either fuel or oxygen produces the phenomenon of "unmixedness" in which the time-averaged temperature and composition at a point do not represent correctly the degree to which fuel and air are mixed on a molecular scale. Thus, the use of time-averaged values of temperatures and concentrations in the finite-rate chemistry equations is incorrect and can lead to serious errors in calculated results. This by no means rules out the use of time- averaged values, since the effects of unmixedness may be small for many turbulent, reacting flows. One purpose of this paper is to demonstrate that this is so by calculating some results with and without the effects of unmixedness. Another purpose of the paper is to report improvement in the ability of a computer program to simulate burning of H 2 in a super- sonic air stream when an eddy breakup chemistry model is replaced with one in which finite reaction rates, corrected for unmixedness, are used. In a prior investigation,1 the usefulness of a parabolic marching computer program was evaluated by comparing computed results with data from five experimental test cases. Mixing of fuel and oxidant was computed for parallel in- jection of H2 using a two-equation turbulence model, and the extent of chemical reaction was deduced by comparing the data with results obtained from three different assumptions: 1) no reaction, corresponding to zero combustion efficiency; 2) complete burning of all fuel mixed with oxygen, corresponding to combustion efficiency = 1; and 3) finite-rate burning based on the rate of decay of large turbulent eddies into small ones. The last of these assumptions, the eddy- breakup (EBU) model,2 provided a means for obtaining combustion efficiency values intermediate between 0 and 1 and is believed to be useful as a tool to account for the effects of unmixedness on chemical reaction in turbulent flows if chemical reaction rates are large enough so that the production of combustion products is limited by the mixing rate. In this paper three of the experimental test cases used in the previous computer program evaluation are reanalyzed using the same program but with a finite-rate chemistry system reported by Spiegler. 3 In this chemistry system the effect of unmixedness on individual reactions is modeled by decreasing any rate for which one or more of the species involved goes negative during fluctuation of its concentration about the average value. (Temperature fluctuations are not considered.) In addition to calculations using Spiegler's system of 7 species and 8 reactions, calculations were also made using 12 species and 25 reactions. The latter system required the solution of twice as many differential equations for chemical species, but this was judged to be necessary in order to examine the effect of the added equations on the generation rates of radicals such as H, O, and OH. The elemental reactions by means of which H2 and O2 are transformed into H2O provide multiple paths between the reactants and the product, most of which depend on the presence of high concentrations of radicals. The relative importance of the paths changes as conditions in the flow change, and it is important not to neglect any path which might be a large source or sink for one or more of the radicals, since such a path might be critical for prediction of ignition.

Compositional dependence of the formation of calcium phosphate films on bioglass

Abstract: Bioglass, which has a composition of sodium carbonate, calcium carbonate, phosphorous pentoxide and silica, has been shown to bond to living bone. This ability is dependent on controlled surface reactions. Investigators with 45S5 bioglass have demonstrated that the formation of a SiO2-rich layer and a calcium phosphate film on its surface in an aqueous environment is associated with the film bonding the bioglass to bone. The objects of this research were: 1. To study SiO2 dependence on the formation of a silica-rich layer and calcium phosphate films on a bioglass surface in a simulated physiological solution, and 2. To establish a correlation between in vitro surface reactions and in vivo bonding ability. It was discovered that three types of reactions occur in a simulated physiological solution depending on bioglass composition: 1. A calcium phosphate film and SiO2-rich layer form simultaneously and the reaction rate is fast for bioglasses which have a lower content of SiO2 (approximately 46 mol% SiO2). 2. A SiO2-rich layer forms first and a calcium phosphate film develops later between the aqueous environment and the SiO2-rich layer for bioglasses whose SiO2 content is between 46--55 mol %. 3. A calcium phosphate film does not form for glasses whose SiO2 content is more than 60 mol %.

The stable states picture of chemical reactions. I. Formulation for rate constants and initial condition effects

Abstract: The stable states picture (SSP) of chemical reactions is used to derive flux time correlation function (tcf) formulas for reaction rate constants. These formulas, which apply to both gas phase and condensed phase reactions, are interpreted in terms of the flux out of an internally equilibrated stable reactant and the ensuing irreversible flux into a stable product. The determination of the rate constants by dynamics in an intermediate region lying between these stable states is illustrated for a simple model of barrier crossing in liquids. Generalized rate constant expressions which hold when internal nonequilibrium in the stable states is important are derived and discussed. The SSP approach is also used to derive tcf expressions for short time initial condition effects which carry information on reactive dynamics beyond that contained in rate constants. As an illustration, it is shown how the SSP formulation provides a starting point for the resolution of primary and secondary recombination in liquid state photodissociation reactions.

Kinetics of conformational transitions in chain molecules

Abstract: A theory is developed for the rate of conformational transitions (trans→gauche) of bonds in chain molecules, such as alkanes and polymers. This is a multidimensional extension of Kramers’ reaction rate theory. Central to the understanding of how changes in the chain’s geometry affect transition rate is the determination and examination of the reaction coordinate. The reaction coordinate is a localized mode; i.e., the rotational motion of the transforming bond is accompanied by motion in neighboring bonds, but this motion diminishes with distance. Comparison is made between the calculated rates and those determined by Brownian molecular dynamics simulations.

Kinetic and dynamic aspects of rechargeable metal hydrides

Abstract: A brief review of the dynamic response of hydrogen storage alloys is presented. This includes the determination of dynamic reaction kinetics and the comparison of dynamic and conventional pressure-composition-temperature relations. Alloys of both AB5 and AB types were tested at pressures and temperatures of practical interest. Examples are shown of differences in reaction rate due to effects of intrinsic chemical kinetics, alloy hydrogen capacity and plateau pressure, and heat transfer limitations. Plateau pressures and reaction enthalpies for continuous-dynamic tests are shown to be different from those for conventional step-by-step tests. Also an apparent cyclic instability in the FeTiH2 phase is described.

Photoassisted water-gas shift reaction over platinized titanium dioxide catalysts

Abstract: The reaction of gas-phase water with CO (water-gas shift reaction) over platinized, powdered TiO/sub 2/ is found to be catalytic under uv illumination. The reaction kinetics have been studied at temperatures from 0 to 60/sup 0/C. At 25/sup 0/C, the reaction is zero order both in CO and H/sub 2/O when p/sub CO/ greater than or equal to 0.3 torr and p/sub H/sub 2/O/ greater than or equal to 5 torr and the activation energy is 7.5 kcal/mol. The wavelength dependence of the reaction rate shows a cut-off near, but slightly below, the band gap of TiO/sub 2/. The quantum efficiency of the reaction is about 0.5% at 25/sup 0/C. A mechanism is proposed which involves the photodecomposition of H/sub 2/O over platinized TiO/sub 2/.

Reaction of hydrogen with hydroxyl‐free vitreous silica

Abstract: The reacton of hydrogen with hydroxyl‐free vitreous silica has been measured for temperatures between 500 and 850 °C, pressures between 0.132 and 5.78 atm, and sample thicknesses between 0.057 and 1.06 cm. The results can be explained by a simple model often used to explain tarnishing reactions. This model assumes that the reaction rate is very fast compared to diffusion, and that a fixed concentration of trapping sites exists in the specimen.

Reaction kinetics of metal hydrides and their mixtures

Abstract: Reaction rates in the hydriding and the dehydriding processes were studied experimentally for hydriding alloys such as LaNi5, MmNi5(Mm  mischmetall), aluminium-substituted mischmetall nickels, TiMn10.5 and Ti0.8Zr0.2Cr0.8Mn1.2. We also studied binary mixtures of these hydriding alloys to examine the effects of mixing on the kinetic properties. A term N was introduced for the evaluation of the pressure and temperature dependences of the reaction rate.

Rates of reaction of butyl radicals with molecular oxygen

Abstract: Rate constants for the reaction of four different butyl radicals with molecular oxygen have been measured at room temperature The radicals were generated by flash photolysis and their time decay was followed with a photoionization mass spectrometer The radical concentrations were kept low to avoid complications from radical–radical reactions Radical lifetimes were long, up to 50 msec, thus assuring that thermalized radicals were being studied The rate constants, in units of 10−11 cm3 molecule−1 sec−1, are: n‐butyl (075±014); s‐butyl (166±022); t‐butyl (234±039); 3‐hydroxy s‐butyl (28±18) No pressure dependence of the rate constants was observed over the range 1 to 4 Torr In the absence of O2, the butyl radicals decay mainly by loss on the quartz surface of the reaction cell, with sticking coefficients in the range of 10−2 to 10−3 The Adiabatic Channel Model can predict the approximate absolute values of these rate constants using reasonable molecular parameters, but it fails to reproduce th

Impingement mixing in reaction injection molding

Abstract: Impingement mixing is the unique feature of the reaction injection molding (RIM) process but mixheads are largely designed by trial and error To visualize the impingement process we have taken high speed photographs To characterize mixing quality we have followed adiabatic temperature rise of mixtures, varying Reynolds number, mixhead geometry and the reaction rate These results are examined in terms of a simplified model which includes both fluid mechanical and polymerization aspects of the problem

Role of dimensionality and spatial extent in influencing intramicellar kinetic processes

Abstract: Reference LPI-ARTICLE-1980-006doi:10.1021/j100449a017View record in Web of Science Record created on 2006-02-21, modified on 2017-05-12

Mechanism of formation of Grignard reagents. Kinetics of reaction of alkyl halides in diethyl ether with magnesium

Abstract: The rate of reaction of organic halides with magnesium is proportional to organic halide concentration and magne- sium surface area. All organic iodides examined and many secondary alkyl bromides appear to react with magnesium at mass- transport or diffusion-controlled rates in diethyl ether. The rates of less reactive organic bromides (neopentyl, phenyl, cyclo- propyl) contain contributions both from mass transport and from chemical reaction. Most organic chlorides react at rates which are not mass transport limited. Analysis of rate-structure profiles indicates that free carbanions and carbonium ions are not intermediates, and that the transition-state structure does not resemble an Sy2 reaction. Insertion of magnesium into the carbon-halogen bond also does not seem to occur, Soluble Mg(1) can be excluded as the only reactive species responsible for disappearance of alkyl halide. The two most plausible limiting rate-determining steps are halogen atom abstraction from the organic halide by the metal and electron transfer to the organic halide from the metal.

Heterogeneous photocatalysis. Oxidation of halide ions by oxygen in ultraviolet irradiated aqueous suspension of titanium dioxide

Abstract: In uv irradiated aqueous suspension of anatase, I– ion are oxidised in the presence of O2 at a conversion ≈ 80 times greater than that for Br– ions, whereas Cl– ions withstand oxidation The quantum yields (≈ 2 % for I–) have been determined and discussed The nature of the final products depends on the pH, ie, halogens or hypohalite ions in acidic or basic medium, respectively However, the kinetics of the oxidation is not greatly influenced by the pH varying from 0 to 14 The reaction rate is proportional to the surface coverage in halide ions which is of the Langmuir type We tentatively suggest that the reaction proceeds via the neutralization of adsorbed O– ions by photogenerated holes producing activated oxygen species which oxidize adsorbed halide ions into hypohalite ions

Laboratory measurements of the O+(²D) + N2 and O+(²D) + O2 reaction rate coefficients and their ionospheric implications

Abstract: Rate coefficients which have been measured at thermal energies for the charge transfer reactions of metastable O+/2D/ ions with N2 and O2 are reported. It is found that at an effective temperature of about 550 K, k(n2) = (8 + or - 2) x 10 to the -10 cu cm/sec and k(O2) = (7 + or - 2) x 10 to the -10 cu cm/sec. Drift tube-mass spectrometer measurements employ the reaction He(+) + O2 as the source of metastable O+ ions, showing that the ions produced in this manner are in the 2D state rather than the 2P state, a possible alternative identification. Finally, consideration is given to the ionospheric implications of the laboratory measurements.

Kinetic matrix effects (response and density distribution functions): ring closure. Reaction of indolinobenzospiropyrans in glassy poly(alkyl methacrylates)

Abstract: The ring closure (bleaching) reaction of photochromic indolinobenzospirans is a fast unimolecular process with a finite activation volume. When the reagent is immobilised in a rigid matrix, the isomerization is controlled by the amount of free volume in the neighbourhood of the reacting molecule and so the distribution of reaction rates reflects the density distribution in the matrix. The theory of such kinetic matrix effects is presented and methods of evaluating the reagent response and matrix density distribution functions are discussed. Ring closure reactions of three such compounds in four polymeric matrices are reported and the complete functions evaluated for 8-bromo-2′,2′-dimethyl-6-nitro-1′-phenyl-(2H-[1]benzospiropyran-2,2′indoline) in poly(methyl methacrylate).

Rate of reaction of OH with COS

Abstract: The laser/flash photolysis-resonance fluorescence technique was employed to study the kinetics of the reaction OH + COS →k1 products. Complicating secondary reactions involving COS photofragments were eliminated by using 193 nm laser photolysis of HNO3 as a source of OH. The rate constant was found to be much slower than previous measurements indicated. Based on our experiments, the upper limit for k1 at 298K is 8.8 × 10−15 cm³molecule−1s−1. Our results suggest that the title reaction is only a minor COS degradation pathway in the troposphere.