Rate Constants Of Elementary Reactions Research Articles

Absorption of carbon dioxide into diisopropanolamine solutions of polar organic solvents

The chemical absorption rate of carbon dioxide with diisopropanolamine (DIPA) was measured in such non-aqueous solvents as methanol, ethanol, n-propanol, n-butanol, ethylene glycol, propylene glycol, and propylene carbonate, and aqueous solvent at 298 K and 101.3 kPa using a semi-batch stirred tank with a planar of gas–liquid interface. The pseudo-first-order reaction rate constant obtained from the measured rate of absorption was used to obtain the elementary reaction rate constants in complicated reactions represented by zwitterion mechanism and the order of overall reaction of CO 2 with amine. The enhancement factor of carbon dioxide at a low concentration of DIPA was obtained using the reaction rate constants obtained under the condition of a pseudo-first-order, fast reaction regime. Correlation between the elementary reaction rate constant and the solubility parameter of the solvent is presented.

Ignition of acetylene-oxygen mixtures behind shock waves

The delay time of ignition of C2H2-O2-Ar mixtures of various compositions behind reflected shock waves were measured at 980–1300 K and 0.65 ± 0.05 MPa. A kinetic scheme of the ignition of acetylene based on the available rate constants of the key elementary reactions was developed. The scheme satisfactorily describes the experimental data from various works over wide temperature, pressure, and concentration ranges: 980–2400 K, 0.01–1.0 MPa, and 0.5–20.3 vol % acetylene and 1.25–20.4 vol% O2.

Reaction Kinetics of Carbon Dioxide with Diethanolamine in Polar Organic Solvents

The chemical absorption rate of carbon dioxide with diethanoamine was measured in such nonaqueous solvents as methanol, ethanol, n‐propanol, n‐butanol, ethylene glycol, propylene glycol, and propylene carbonate, and in water at 298 K and 101.3 kPa using a semibatch stirred tank with a plane gas‐liquid interface. The overall reaction rate constant obtained under the condition of fast reaction regime from the measured rate of absorption was used to get the elementary reaction rate constants in complicated reactions represented by reaction mechanism of carbamate formation and the order of overall reaction of CO2 with amine. Correlation between the elementary reaction rate constant and the solubility parameter of the solvent was presented.

A Dipyridoimidazolium Salt from Phenylchlorocarbene and 2,2'‐Bipyridyl: Synthesis, Reaction Mechanism, and Effect of Rotamerism

AbstractPhenylchlorocarbene, generated by photolysis or thermolysis of the phenylchlorodiazirine, reacts with 2,2'‐bipyridyl to give, in alkane solvents, the 6‐phenyldipyrido[1,2‐c:2',1'‐e]imidazolium chloride. We investigated the mechanism of the reaction by flash photolysis. Like the mechanism of formation of 3‐phenylindolizine from phenylchlorocarbene + 2‐vinylpyridine, the process involves generation of the carbene, formation of a pyridinium ylide, and cyclization of the syn rotamer of this ylide, with the two N atoms of the bipyridyl in a syn relationship. The cyclization product then eliminates Cl– to give a stable aromatic cation. We observed and identified all the intermediate species and measured the rate constants for the successive elementary reactions. This enabled us to define experimental conditions under which the reaction is nearly quantitative (the chemical yield of isolated product is >90 %). By contrast, use of a polar solvent such as dichloromethane yields a mixture of the products resulting from the cyclization of both the syn‐ and the anti‐bipyridylium ylide rotamers. Semiempirical calculations qualitatively account for the effect of solvent polarity on the relative rates of cyclization of the syn‐ and anti‐bipyridylium ylide rotamers and thus on the nature of the products. These calculations predict that, in the presence of o‐phenanthroline, a model for the syn bipyridyl rotamer, phenylchlorocarbene should not give the expected diazacyclopentaphenanthrene, in agreement with the experiment findings. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

A New Model for Laser-induced Thermal Damage in the Retina

We describe a new model for laser-induced retinal damage. Our treatment is prompted by the failure of the traditional approach to accurately describe the image size dependence of laser-induced retinal injuries and by a recently reported study which demonstrated that laser injuries to the retina might not appear for up to 48 h post exposure. We propose that at threshold a short-duration, laser-induced, temperature rise melts the membrane of the melanosomes found in the pigmented retinal epithelial cells. This results in the generation of free radicals which initiate a slow chain reaction. If more than a critical number of radicals are generated then cell death may occur at a time much later than the return of the retina to body temperature. We show that the equations consequent upon this mechanism result in a good fit to the recent image size data although more detailed experimental data for rate constants of elementary reactions is still required. This paper contributes to the current understanding of damage mechanisms in the retina and may facilitate the development of new treatments to mitigate laser injuries to the eye. The work will also help minimize the need for further animal experimentation to set laser eye safety standards.

Molecular, Thermodynamic, and Kinetic Parameters of the Free Radical C2H5· in the Gas Phase

The wave function, energy, equilibrium geometry, and normal vibration frequencies of the ground state of the free radical C2H5· were obtained by ab initio calculations with inclusion of electron correlation effects at the UB3LYP/6-311++G** level. The resulting molecular parameters were used to estimate the thermodynamic functions of an ideal gas of C2H5·. From the thermodynamic functions of C2H5·, I·, C2H5I, C2H4, and HI and the kinetic curves of isothermal pyrolysis of ethyl iodide, the absolute rate constants of elementary reactions of free ethyl radical and the mentioned iodine compounds were estimated. The dissociation energy ED,0(C2H5-I) and the standard formation enthalpy ΔfH0298 (C2H5·) were found.

Kinetics and mechanism for the H‐for‐X exchange process in the H + C6H5X reactions: A computational study

AbstractThe addition of H atoms to benzene and toluene and subsequent transformations were investigated using high level ab initio and density functional theory methods. Molecular structures and vibrational frequencies calculated at the B3LYP/6‐311++G(d,p) level of theory were used in combination with adjusted G2M energetic parameters for RRKM rate constant calculations. Standard heats of formation for cyclohexadienyl and cyclohexadienyl, 6‐methyl radicals calculated through isodesmic reactions amounted to 49.5 ± 2 and 42.9 ± 3 kcal/mol, respectively. Rate constants for various elementary reactions involved in the H‐for‐X exchange (X = D, CH3) were calculated and closely correlated with the available experimental kinetic data. © 2001 John Wiley & Sons, Inc. Int J Chem Kinet 33: 633–653, 2001

Understanding Homogeneous and Heterogeneous Contributions to the Platinum-Catalyzed Partial Oxidation of Ethane in a Short-Contact-Time Reactor

This paper describes a computational study of the partial oxidation of ethane to ethylene in a short-contact-time reactor, using a two-dimensional computational fluid dynamics model with full heat and mass transport. Detailed heterogeneous and homogeneous chemical kinetic mechanisms are employed to describe the chemistry. Rate constants for elementary surface reactions are determined from literature sources or by fitting model predictions to experimental data. Simulations using these mechanisms suggest that platinum-catalyzed heterogeneous chemical processes are responsible for the oxidation of surface carbon and hydrogen, resulting in localized heat release into the gas phase. This heat release drives endothermic homogeneous and heterogeneous cracking of ethane to ethylene and H2. The proportion of homogeneous and heterogeneous contributions depends strongly upon the reactor operating conditions. In addition to predictions of ethane conversion and ethylene selectivity, the model also predicts the production of all other major products: H2O, H2, CH4, CO, and CO2. A good fit is obtained between model predictions and experimental data for ethane/oxygen mixtures. The model is applied to ethane/hydrogen/oxygen mixtures and good agreement with this set of experimental data is also obtained.

Kinetics and mechanism of cyclohexane oxidation with molecular oxygen in the presence of propionic aldehyde

The kinetics and mechanism of the liquid-phase oxidation of cyclohexane with molecular oxygen in the presence of the additives of propionic aldehyde are studied at 303.0, 322.5, and 341.5 K by measuring the rates of oxygen and propionic aldehyde consumption and the yields of the main reaction products (cyclohexanol (COL), cyclohexanone (CON), cyclohexyl hydroperoxide, and propionic acid and peracid). A kinetic scheme is proposed and rate constants of elementary reactions are estimated based on the analysis of their rates and the yields of the main cyclohexane products. The key reactions of the main steps (including chain initiation, propagation, and termination) are determined. An increase in the rate of cyclohexane oxidation and the yield of the target products (cyclohexanol, cyclohexanone, and cyclohexyl hydroperoxide) in the presence of propionic aldehyde suggests that highly active acylperoxy radicals participate in chain propagation. The [CON]/[COL] ratio indicates that these products are mainly formed in chain propagation. The strong effect of the Baeyer-Villiger rearrangement on both the rate of oxygen consumption and the yield of the target products at the initial stages of the process and at high propionic aldehyde concentrations is explained.

Mechanisms and kinetics models for hydrocarbon pyrolysis

Kinetics models based on the governing mechanism for a multi-step chemical reaction provide insight into the underlying chemistry and they can be extrapolated more confidently than can empirical kinetics models. The field of thermal hydrocarbon chemistry is sufficiently mature that the types of free-radical reactions that are important have been reasonably well elucidated. Moreover, rate constants for many elementary reactions have been measured experimentally, and rate constants for other reactions can be estimated from thermochemical kinetics considerations. These circumstances facilitate the development of quantitative mechanism-based mathematical models for hydrocarbon pyrolysis. This paper provides an overview of the current approaches for mechanistic modeling of hydrocarbon pyrolysis. The steps involved in developing both analytical and numerical models are described and illustrated with examples from the literature.

Kinetics of complex formation of 2-methoxyphenol with ferric ion and hexacyanoferric ion

Complexes containing one molecule of organic ligand have been formed in the reaction of 2 methoxyphenol with ferric ion or hexacyanoferric ion. Kinetic investigations showed that the reaction order with respect to ferric ion or hexacyanoferric ion is close to one. The reaction order with respect to 2-methoxyphenol is 0.8 in the reaction with ferric ion and close to 0 in the reaction with hexacyanoferric ion. The reaction mechanism, based on kinetic investigations, involves the decomposition of the initial complex, formed with the pentacoordinated complex and water or cyanide ion. In a second stage of the reaction, the pentacoordinated complex adds 2-methoxyphenol; and one molecule of initial ligand, water, or cyanide ion, comes off. Some ratios of rate constants of elementary reactions have been calculated. © 2000 John Wiley & Sons, Inc. Int J Chem Kinet 32: 184–191, 2000

98/02535 Heterogeneous combustion of porous graphite particles in microgravity : Chelliali, H. K. and Miller, F. J. NASA Conf. Publ., 1997, 10194, 511–516

Recent theoretical investigations on graphite particle combustion have employed several levels of heterogeneous reaction models, ranging from global to elementary models, to describe the oxidation of carbon to gaseous products. Unlike the counterpart homogeneous reaction models, these heterogeneous reaction models are not well developed because of the difficulties associated with decoupling the physical characteristics of the solid (e.g. surface area taking part in combustion) from the chemical kinetic data. This is certainly true for porous graphite particle combustion, where heterogeneous and homogeneous reactions occur within the pores and play an important role in the overall oxidation process. As a result, there are considerable uncertainties of physical phenomena predicted using different heterogeneous kinetic models available in the literature. A good example, discussed later in this paper, is the predicted critical particle size below which the mass burning rate becomes exponentially small. The main goal of this study is to understand the basic mechanism controlling such rapid changes in burning rates, by developing a model where physical contributions are decoupled from chemical rate constants in a consistent manner. Another important goal of the proposed study is to develop a truly intrinsic, detailed heterogeneous reaction model for porous graphite combustion at high-temperatures, and to derive a systematically reduced heterogeneous reaction model in terms of the elementary reaction rate constants of the detailed model. The validation of chemical kinetic models describing the heterogeneous and homogeneous combustion in and around a spherically symmetric porous graphite particle can be considerably simplified by experimental measurements obtained under microgravity conditions. A vital component of this study is to conduct such supporting experiments on particle burning rate and surface temperature using NASA microgravity facilities, in close coordination with the theoretical effort. The basic understanding obtained and models developed as part of this project will be useful for optimal design of coal combustion devices. These models can also be extended to investigate the role of heterogeneous chemistry on pollutant formation pathways in combustion devices. The theoretical approach developed here, with pore diffusion effects decoupled from the chemical effects, can also be extended to understand the heterogeneous combustion of other porous fuels, for example, combustion of magnesium in a CO2 environment for propulsion in the Martian atmosphere.

Use of sensitivity analysis methods in the modelling of electrochemical transients.

Abstract We propose a systematic and mathematically rigorous procedure of performing the expansion or reduction of kinetic models arising in the area of electrochemical transient modelling. The procedure is based on the use of sensitivity analysis and involves calculation of first-order sensitivity coefficients of the transient curves, with respect to the parameters of various candidate or inherent model components (e.g. rate constants of elementary reactions in the reaction mechanisms). By confronting the sensitivity coefficients with the deviations of simulated transients from experimental curves, decisions can be taken as to whether a particular model component should be added to or removed from the model. The utility of this procedure is verified by its application to selected cyclic voltammetric data for two example electrochemical problems: anodic oxidation of 2, 2′, 3, 3′, 4, 4′, 5, 5′-octamethyl-1,1′-bipyrrole in dichloromethane at a Pt stationary disk electrode (model expansion from E rev to E rev C irr and cathodic reduction of bis([2 2 ] paracyclophane) Ru(II) bis(tetrafluoroborate) in propylene carbonate at a glassy carbon stationary disk electrode (model reduction from E rev E rev -DISP to E rev E rev ). The procedure may become part of a wider, largely automated computer-aided strategy of developing kinetic models in electrochemistry.

REACTKIN—a program for modelling the chemical reactions in electrolytes solutions

In this paper, REACTKIN, the new computer program for modelling the kinetics of complex homogeneous chemical reactions in electrolyte and non-electrolyte solutions has been presented. This program makes possible the creation of a model of a reaction in the convenient form of a graph as well as simulating the course of a reaction on the basis of the model created. REACTKIN allows the determination of the rate constants of elementary reactions using experimental kinetic data concerning the influence of the ionic strength of the solution. The program REACTKIN can be of importance in the studies of multi-step chemical reactions in electrolyte solutions as well as in chemistry education.

Modelling of ring-free crosslinking chain (co)polymerization

Modelling of copolymerization of a monounsaturated monomer and a bisunsaturated monomer with equal and independent reactivities of double bonds is described. No rings are assumed to be formed before the gel point, and uncorrelated circuit closing beyond the gel point is allowed. These assumptions are approximately met in the case of copolymerization of α, ω-unsaturated telechelic polymers. Modelling is based on a combination of kinetic and statistical polymer formation and crosslinking theories. The bisunsaturated monomer is split into two monounsaturated fragments and these fragments are copolymerized with the monounsaturated monomer to form ‘primary’ chains. The elementary reactions of initiator dissociation, initiation, propagation, chain transfer, and termination by recombination and disproportionation are considered. The primary chains are then recombined by joining fragments of the bisunsaturated monomers into branched molecules (sol) and gel. This step is performed by the formalism of the theory of branching processes (cascade theory) employing the generating functions transform. Examples of variations of molecular-weight averages of primary chains and branched molecules, gel fraction and concentration of elastically active network chains as a function of polymerization conversion for several sets of rate constants of elementary reactions are given. ©1997 SCI

Hydrogen bond network in the distal site of peroxidases: spectroscopic properties of Asn70 --> Asp horseradish peroxidase mutant.

The distal His in peroxidases forms a hydrogen bond with the adjacent Asn, which is highly conserved among many plant and fungal peroxidases. Our previous work [Nagano, S., Tanaka, M., Ishimori, K., Watanabe, Y., & Morishima, I. (1996) Biochemistry 35, 14251-14258] has revealed that the replacement of Asn70 in horseradish peroxidase C (HRP) by Val (N70V) and Asp (N70D) discourages the oxidation activity for guaiacol, and the elementary reaction rate constants for the mutants was decreased by 10-15-fold. In order to delineate the structure-function relationship of the His-Asn couple in peroxidase activity, heme environmental structures of the HRP mutant, N70D, were investigated by CD, 1H NMR, and IR spectroscopies as well as Fe2+/Fe3+ redox potential measurements. While N70D mutant exhibited quite similar CD spectra and redox potential to those of native enzyme, the paramagnetic NMR spectrum clearly showed that the hydrogen bond between the distal His and Asp70 is not formed in the mutant. The disappearance of the splitting in the 1H NMR signal of heme peripheral 8-methyl group observed in 50% H2O/50% D2O solution of N70D-CN suggests that the hydrogen bond between the distal His and heme-bound cyanide is also disrupted by the mutation, which was supported by the low C-N vibration frequency and large dissociation constant of the heme-bound cyanide in the mutant. Together with the results from various spectroscopies and redox potentials, we can conclude that the improper positioning of the distal His induced the cleavages of the hydrogen bonds around the distal His, resulting in the substantial decrease of the catalytic activity without large structural alterations of the enzyme. The His-Asn hydrogen bond in the distal site of peroxidases, therefore, is essential for the catalytic activity by controlling the precise location of the distal His.

Kinetics of Supercritical-Phase Thermal Decomposition of C10−C14 Normal Alkanes and Their Mixtures

Kinetics of thermal decomposition of C10−C14 n-alkanes and their mixtures was studied under near-critical and supercritical conditions. Supercritical-phase thermal decomposition of n-alkanes can be represented well by an apparent first-order kinetics, even though the decomposition was not a true first-order process. A generalized expression was developed to predict the apparent first-order rate constants for the decomposition of C8−C16 n-alkanes at 425 °C. Pressure had a significant effect on the apparent first-order rate constant in the near-critical region. This large pressure effect can be attributed to the significant changes in density and possibly to the changes in the rate constants of elementary reactions with pressure in this region. Individual compounds interacted with each other in the thermal reaction of n-alkane mixtures. The overall first-order rate constants for n-alkane mixtures can be predicted satisfactorily from the rate constants for pure compounds.

Transient Isotopic Studies and Microkinetic Modeling of Methane Reforming over Nickel Catalysts

The kinetics of elementary surface reactions involved in the reforming of methane to synthesis gas over supported nickel were studied using transient isotopic methods. To investigate methane adsorption and dehydrogenation, the reaction between CD4and H2was studied. To investigate water adsorption and dissociation, the reaction between H2O and D2was studied. To investigate the formation and cleavage of C–O bonds on the nickel surface, transient CO methanation experiments were performed. Rate constants of surface elementary reactions were extracted from the data by fitting the measured response curves to microkinetic models. An overall model that describes the reactions of methane with steam and CO2in microkinetic terms was constructed based on these rate constants and on previously published steam reforming and CO2methanation data. The model suggests that there is no single rate-determining step in methane reforming with either steam or CO2, and that under some conditions the availability of surface oxygen may play a key role in determining the rate.

Bases of the Kinetics of Chemical Processes

The fundamental approach to the description of the kinetics of chemical reactions has been considered, and principle of chemical reaction independence and active mass law have been taken into account. Models for rate constants of elementary reaction have been discussed on the basis of the collision theory, the theory of activated complex, and the theory which takes into account the movement of intermediate particles to the reaction barrier. The collision theory is suggested for use at high temperatures, the theory of activated complex is recommended for application at moderate temperatures, and the theory which takes into account the movement of intermediate particle to the reaction barrier is relevant to low temperatures. It is emphasized that the principle of maximal change of free energy, which was suggested earlier, may be applied for analysis of the mechanism of chemical processes.

Kinetics of Radical Copolymerization. XV. Absolute Rate Constants in the Copolymerization System Styrene-Ethyl Acrylate-Benzene

Abstract With the use of rate data of the system St-EA-AIBN-Bz-50°C measured earlier, the parameters of the rate equation derived in terms of the hot radical theory were determined. The reason for the reevaluation of experimental data was that the absolute values of propagation and termination rate constants could be determined in the homopolymerization of both monomers with the improved rotating sector method elaborated for nonsteady-state kinetic investigations. This enabled us to determine the rate constants of the elementary reactions and deactivation parameters (γ12 and γ21) of the copolymerization. The rate data calculated with the new parameter set are in good agreement with the measured ones, proving the applicability of the hot radical theory. The results are represented by a steric coordination system in order to call attention to the importance of the simultaneous study of composition-and dilution-dependence in copolymerization.