Showing papers on "Reaction rate published in 1990"

Chemical kinetics: The study of reaction rates in solution

Abstract: Introduction to chemical kinetics simple rate equations complicated rete equations fast reactions theory of chemical kinetics phenomena for study structure-reactivity relationships medium effects appendices.

Chemical Kinetics and Combustion Modeling

Abstract: ion of H atoms from fuel or other species by R02 produces alkyl hydroperoxides ROOH that then decompose to produce RO and OH radicals. However, for hydrocarbon fuels more complicated than n-butane, a more rapid process for R02 is isomerization via internal abstraction of H atoms ( 1 94). The general features of this R02 isomerization theory provide the kinetic basis for global reaction schemes for engine knock in internal combustion engines ( 1 96-1 98). The major steps consist schematically of R02 +=t QOOH (internal H atom abstraction) QOOH � QO + OH (0-0 homolysis). At sufficiently low temperatures, molecular oxygen can add further to the QOOH radicals, leading eventually to an overall reaction QOOH + 02 = products + OH + OH. Both alternatives are important since they produce OH radicals through reaction sequences with relatively low energy barriers. ROz isomerization rates are determined primarily by the size of the ringlike intermediate transition state, by the bond energy of the H atom being abstracted internally, and by the equilibrium constant of the R02 addition reaction. For fuels of larger hydrocarbons many isomerizations are possible, and 0-0 homolysis of the QOOH product of the iso­ merization reaction yields a different stable oxygenated species for each isomerization reaction. Thus for n-alkanes, a 1 ,4-H -atom abstraction, followed by 0-0 bond fission, leads to a 3-membered oxygenated ring, an oxiran. Similarly, 1 ,5-processes lead to oxetans, 1 ,6-abstractions produce tetrahydrofurans, and l ,7-abstractions produce tetrahydropyrans. Cur­ rent models generally use activation energies tabulated by Baldwin et al ( 199), but with A factors slightly lower than the 1 0 1 2. 1 S 1 recommended by Baldwin et aI, closer to the value of 1 0 1 1 . 5 S 1 recommended by Benson ( 1 89) for unimolecular reactions involving a cyclic transition state. The isomerization reactions are reversible, and activation energies for the reverse isomerizations are easily computed from the activation energy of the forward (endothermic) reaction and the AH of the reactions ( 1 94). ROz isomerization through internal abstraction of an H atom from a site adjacent to the C-O bond, followed by breakage of the C-O bond, will lead to a conjugate olefin and H02_ Direct abstraction paths leading to the same products have been discussed by Gutman and co-workers (200, 20 1 ), favoring a path proceeding through R02 isomerization. The current work of Wagner et al (20 I) provides some insight into the diffi382 MILLER, KEE & WESTBROOK culties of this reaction, but this is one of the simplest of the R02 iso­ merizations, and there are many more such reactions for which complex analyses are needed to understand fully the detailed reaction rates and mechanisms. Reactions of the product epoxide and other oxygenated species must be included in kinetic models, but very few quantitative studies of H atom abstraction or other reactions for these species have been reported. Current models must estimate both the rates and products for reactions of the epoxides, primarily attributed to H atom abstraction by OH or H02•

Stable isotope analysis of multiple carbonate samples using selective acid extraction

Abstract: Selective chemical separation techniques for isotopic analysis of coexisting calcite, dolomite, siderite and magnesite have been investigated in this study. Comparison of reaction rates of all pure carbonate minerals with 100% phosphoric acid at 25° and 50°C demonstrated that a uniformly fine grain size (<200 mesh) is required. Tests with fine-grained (<200 mesh) pure calcite, dolomite, siderite and magnesite reacted at 25°C (calcite) and 50°C (others) show that δ18O of evolved CO2 increases during the course of reaction. This is probably due to a slight kinetic isotope effect associated with rapid dissolution of sub-micrometer carbonate crystallites adhered to the outer surfaces of the very fine-grained crystals. Isotopic analysis of carbonates based on CO2 from incomplete acid reactions is accurate only after minimum reaction times. Conversely, much longer reaction times than the minima are not required. Experiments conducted on variable mixtures of calcite-dolomite, dolomite-siderite and calcite-dolomite-siderite demonstrated the magnitudes of cross-contamination on the inferred isotopic compositions of the individual minerals. The isotopic compositions and magnitudes of their uncertainty can be evaluated for individual carbonates selectively extracted from natural mixtures by variable time-temperature protocol.

The glass transition temperature (Tg) as an index of chemical conversion for a high‐Tg amine/epoxy system: Chemical and diffusion‐controlled reaction kinetics

Abstract: The glass transition temperature (Tg) is used as a parameter to monitor the isothermal cure of a tetrafunctional aromatic diamine and a difunctional aromatic epoxy system. There is a one-to-one relationship between Tg and conversion that is independent of cure temperature, Tcure. Prior to vitrification (Tg = Tcure), the reaction is only kinetically controlled; after vitrification, the reaction becomes diffusion-controlled. Time-temperature shifts of Tg vs. In (time) data at different cure temperatures to form a master curve for the kinetically controlled reaction at an arbitrary reference temperature yield a single Arrhenius activation energy (15.2 kcal/mole). The master curve and the reaction activation energy are used in calculating iso-Tg contours prior to vitrification and also the vitrification contour in the time-temperature-transformation (TTT) isothermal cure diagram for the system. The chemical kinetics of the reaction is satisfactorily described by an autocatalyzed reaction mechanism. The overall rate constant of the reaction in both kinetically and diffusion-controlled regimes is modeled by a combination, in parallel, of the chemical rate constant and the diffusion rate constant. The chemical rate constant has the usual Arrhenius form, whereas the diffusion rate constant is assumed to be given by a modified form of the WLF equation. Results suggest that both primary and secondary amino hydrogens are equally reactive. A theoretical model for calculating Tg as a function of conversion is presented for a network-forming system with one bond-forming reaction.

Ab initio transition state theory calculations of the reaction rate for OH+CH4→H2O+CH3

Abstract: We have carried out ab initio calculations using Mo/ller–Plesset perturbation theory, scaling all correlation energy in second order (MP‐SAC2) with several large basis sets, for the reaction OH+CH4→H2O+CH3. We found that correlation has a large effect on the geometry, barrier height, and vibrational frequencies of the transition state. The final calculated values, obtained with a correlation‐balanced basis set, for the forward and reverse classical barrier heights are 7.9 and 21.2 kcal/mol, respectively. We have used these with transition state theory and an Eckart model for semiclassical tunneling calculations of the rate constants for the above reaction in the temperature range from 200 to 2000 K. We found that the present model, which requires information only at the reactants, transition state, and products, predicts rate constants of the same order of magnitude as the experimental data for this wide temperature range.

Kinetics of Carbon Dioxide with tertiary Amines in aqueous solution

Abstract: The reaction of CO2 with TEA, DMMEA, and DEMEA has been studied at 293, 303, 318 and 333 K. All the kinetic experiments were carried out in a stirred cell reactor operated with a flat, smooth and horizontal gas-liquid interface. A numerical method, which describes mass transfer accompanied by reversible chemical reactions, has been applied to infer rate constants from the experimental data. It is argued that the contribution of the CO2 reaction with OH- to the observed reaction rate may have been overstimated in most literature on tertiary amine kinetics as serious depletion of OH- toward the gas-liquid interface usually occurs. For all the amines studied, the reaction order in amine was found to be about one for each temperature investigated. This is in good agreement with the base catalysis mechanism proposed by Donaldson and Nguyen (1980). All kinetic data could be summarized reasonably well in one Bronsted relationship.

Degradation of Aluminum Nitride Powder in an Aqueous Environmet

Abstract: The degradation of A1N powder in excess H2O at room temperature for up to 24 h was investigated. Samples were characterized by various techniques (IR; XRD; SEM; XPS; C, H, N analysis; surface area, particle size, and weight change measurements). The reaction rate was found to be significant, with 80% of the A1N being consumed in 24 h. The initial reaction product was found to be a porous, amorphous, hydrated alumina with stoichiometry near AlOOH. After ∽16 h a crystalline phase, bayerite Al(OH)3, was detected which became the predominant phase after 24-h contact. The kinetics of the A1N consumption were found to be first order and the reaction rate linear. The kinetic data fitted an unreacted core model with a porous product layer where the surface chemical reaction controlled the overall kinetics.

Dynamics of ion pair interconversion in a polar solvent

Abstract: The reaction rate and mechanism of the interconversion between a contact ion pair and solvent separated ion pair in model polar solvents is investigated by molecular dynamics (MD) simulation. The full rate constant for the model reaction is estimated from the product of the transition state theory (TST) rate constant, determined from the potential of mean force between the ions in an equilibrium solvent, and the transmission coefficient, which depends on the details of the dynamics. The collective character of the solvent motion in the reaction barrier crossing is examined in some detail, and the important role of solvent dynamics in causing the reaction rate to markedly deviate from the TST rate is discussed. The MD results are compared with the predictions of Kramers and Grote–Hynes theories.

Temperature and pressure dependence of ozone formation rates in the range 1-1000 bar and 90-370 K

Abstract: The recombination O+O2+M→O3+M in the bath gases M=He, Ar, and N2 was studied over the temperature range 90–370 K and the pressure range 1–1000 bar. The temperature and pressure dependences of the reaction rates show an anomalous behavior which is attributed to superpositions of mechanisms involving energy transfer, complex formation and participation of weakly bound electronically excited O3 states. The results also show an analogy to oxygen isotope enhancements observed in ozone recombination and dissociation. Experiments in compressed liquid N2 were also made showing a transition to diffusion control.

Detailed kinetic modeling of autocatalysis in methane pyrolysis

Abstract: A detailed kinetic model is developed to explain the observed acceleration in rates of product formation during the pyrolysis of methane at 0.58 atm and 1038 K. This model, obtained by a systematic simplification of a much larger one, consists of 44 reactions involving 25 species. The model includes a number of reactions involving chemically activated complexes formed by radical addition and recombination reactions, with rates for unimolecular reaction and bimolecular stabilization of these energized complexes evaluated with a QRRK formalism. A combination of reaction rate analysis as well as sensitivity analysis is used to show that these chemically activated reactions lead to surprisingly rapid production of cyclopentadiene at very low extents of conversion ({approx} 0.1%) and that dissociation of cyclopentadiene accounts for the acceleration in rate. An optimization procedure was used to obtain an accurate description of the observed kinetics without compromising the theoretical plausibility of the rate constants.

The role of organic acid in the abiogenic reduction of sulfate and the sulfur isotope effect

Abstract: Abiogenic reduction of sulfate by acetic acid has been conducted at temperatures ranging from 241 to 340°C. The sulfides produced and the remaining sulfates have been chemically and isotopically quantified. The reaction of the aqueous sulfate reduction obey first-order kinetics. The rate constants are strongly dependent on temperature. The temperature dependence of the reaction rate (min-1) is given by the following rate law: log k=-10.3×103/T+15.1. The activation energy for the reaction of sulfate reduction is 208 kJ mol-1. The remaining sulfates display distinct sulfur isotopic variations under a closed system due to the kinetic isotope effect. The kinetic isotope fractionation factor can be expressed as: 103(α-1)=3.32×106/T2-4.19. The kinetic isotope effects during abiogenic reduction of the sulfate may depend on the atomic hydrogen produced, independent of the reducing agents.

Kinetics of the gas-solid heterogeneous photocatalytic oxidation of trichloroethylene by near UV illuminated titanium dioxide

Abstract: Kinetics of the gas/solid heterogeneous photocatalytic oxidation of dilute trichloroethylene (TCE) vapors by ultraviolet-illuminated titanium dioxide have been determined using a fixed-bed dynamic photoreactor. Reaction rate dependences on inlet TCE, oxygen and water vapor concentrations were found to consist of both reactant sensitive and insensitive regions. In the reactant sensitive regions, measured limiting apparent reaction rate orders for TCE, oxygen and water vapor are 0.8, 1.7 and — 3, respectively. Water vapor in the reactant stream lowersinitial reaction rates relative to corresponding water free conditions, but is required to sustain photocatalytic activity for extended periods of time.

Kinetics of Titanium(IV) Chloride Oxidation

Abstract: The oxidation rate of TiCl4 vapor was measured by FTIR spectroscopy between 700° and 1000°C in a furnace aerosol reactor. The reaction rate was found to be first order with respect to TiCl4 and to follow the Arrhenius form. The apparent activation energy for the reaction was 88.8 ± 3.2 kJ/mol and the preexponential factor 8.26 × 104 s−1. A kinetic mechanism for the oxidation of TiCl4 was proposed that was consistent with the observed dependence of oxygen concentration on the apparent rate constant.

Mathematical modeling of kinetic oscillations in the catalytic CO oxidation on Pd(110): The subsurface oxygen model

Abstract: Experimental investigations of the catalytic CO oxidation on a Pd(110) surface revealed that the temporal oscillations in the reaction rate (measured in a range 10−3 Torr

High temperature reaction rate coefficients derived from N-atom ARAS measurements and excimer photolysis of NO

Abstract: Mixtures of NO and NO/H 2 in Ar were shock-heated and photolyzed with an ArF excimer laser. Measurements in these experiments of N-atom profiles using atomic resonance absorption spectrophotometry (ARAS) permitted the determination of two rate coefficients. The rate coefficient for the reaction (R1) N+NO→O+N 2 was found to be 4.29×10 13 exp(−787/T) cm 3 mol −1 sec −1 (±20% at 1400 K to ±10% at 3500 K). This is the first direct high temperature measurement of this rate coefficient in the exothermic direction. The rate coefficient for the reaction (R2) N+H 2 →H+NH was found to be 1.60×10 14 exp(−12650/T)(±35% from 1950 to 2850 K). To our knowledge, this is the first direct measurment of this rate coefficient. A study of the N-atom ARAS absorption behavior revealed a noticeable pressure dependence, as well as a weak temperature dependence, in the Beer-Lambert law absoprtion coefficient. Proper consideration of these effects is important when the N-atom ARAS diagnostic is used for absolute concentration measurements

Effect of pressure on an enzymatic reaction in a supercritical fluid

Abstract: Three different authors have reported on the use of four different enzymes in supercritical fluids. Lipase carries out transesterification reactions in the presence of supercritical carbon dioxide. Polyphenyl oxidase is active in supercritical CO{sub 2} and fluoroform. It has been shown that alkaline phosphatase and cholesterol oxidase are active in supercritical CO{sub 2}. More recently, an examination of the effect of aggregation of cholesterol on cholesterol oxidase activity in CO{sub 2} using electron paramagnetic resonance (EPR) was done. They found that when cosolvents which promoted aggregation were added, the reaction rate increased in proportion to the amount of aggregation. To date, no data on the effect of pressure on reaction rate have been presented. The objective of this work is to determine whether pressure-induced changes in the physical properties of a supercritical fluid solvent affect the rate of an enzymatic reaction and if so, which properties are responsible for the change.

Silica to Silicon: Key Carbothermic Reactions and Kinetics

Abstract: The primary carbothermic reactions for the reduction of silica to produce silicon were defined and the reaction kinetics were determined. Most possible reactions between silicon oxide and carbon or carbon compounds were studied by a series of thermogravimetric analyses at temperatures up to 2000°C. Four key sequential reactions occur with SiC and SiO as intermediate reactants; two reactions involve SiO2 and two involve SiO. Reaction rate versus temperature, activation energy, and preexponential factors were determined for each of six reactions involving SiO2 or SiO. These kinetic studies show that SiO, when combined with either carbon or Sic, reacts in the gaseous state, and the sublimation of SiO is not the rate-limiting reaction for forming silicon.

A non-linear diffusion-reaction model of electrical conduction in semiconductor gas sensors

Abstract: A non-linear diffusion-reaction model of electrical conduction in homogeneous semiconductor gas sensors has been developed. It is shown that the transient response time of a device may be classified according to the relative importance of diffusion and reaction rates. Experimental results on tin oxide thick films show that the response time is predominantly diffusion limited, suggesting that an improvement in performance may be achieved through a modification in physical properties, such as film porosity, instead of the underlying chemical kinetics.

Lignin in wheat internodes. Part 1: The reactivities of lignin units during alkaline nitrobenzene oxidation

Abstract: The alkaline nitrobenzene procedure for determining lignin building units has been applied to wheat internodes. The effect of reaction time and temperature on the rate of reaction has been measured and the nature of the aldehyde and acid products determined by gas-liquid chromatography. Survival factors for individual phenolic compounds were determined and found to be in the range 0.85–4.99, and were constant for each compound under the reaction conditions (c 6 % nitrobenzene in 2 M NaOH, 170°C, 2 h in a stainless steel pressure vessel), optimised for maximum recovery of products using wood and wheat internode meals. The reaction products fiom 24 monomeric and dimeric lignin model compounds were quantitativety determined to provide information on the susceptibility of various lignin structures to alkaline nitrobenzene oxidation. The results of these and earlier studies by other authors suggest that at the higher temperature a heterolytic reaction mechanism is operating on nitrobenzene oxidation conditions. On nitrobenzene oxidation, ferulic and p-coumaric acids survived in amounts which allowed the procedure to be used, with appropriate correction factors, to determine the nature and amount of these cinnamic acids associated with walls of grasses.

The rate and temperature of reaction of CIF3 with silicate minerals, and their relevance to oxygen isotope analysis

Abstract: A variety of silicate-CIF3 reaction experiments have been performed in order to determine the rate of reaction and minimum temperatures for which there is complete liberation of oxygen. Fast reaction rates ( ⩽ 2 hr.) and low minimum reaction temperatures ( ∼ 450°C) are shown by minerals such as quartz, feldspar, biotite and pyroxene while garnet reacts much slower and requires higher temperatures (4–8 hr. and 600°C, respectively). It is not possible to completely liberate oxygen from Mg-rich olivine using CIF3. A comparison of δ18O-values obtained from incomplete oxygen extractions to those for complete extractions indicates that a minimum oxygen yield of 95% is required for accurate δ18Odeterminations. These results indicate that CIF3 should not be used as a reagent when attempting to liberate oxygen preferentially from one mineral in a multi-mineral sample.