Showing papers on "Elementary reaction published in 1981"

Chemical kinetics and reaction mechanisms

Abstract: Reactions and reaction rates reactions with a simple kinetic form reversible and concurrent reactions consecutive reactions - the steady state and other approximations consecutive mechanisms - intermediates and numerical solutions deduction of reaction mechanisms transition state theory and microscopic reversibility chain reactions and oscillating reactions reactions in solution extrakinetic probes of mechanism reactions at extreme rates.

Transition state theory

Abstract: The introduction of the activated complex permits a simple model of the crossing of potential barriers. With this model, the rate constant of any elementary reaction can be calculated, if the structure of the activated complex is known. The general results of transition state theory lead to an understanding of some important predictions of geochemical interest. For example, the effect of ionic strength on rates or the compensation law can be analyzed using transition state theory. The theory can also be applied to transport in condensed media as well as to heterogeneous reactions. Although the potential energy surfaces needed to calculate the rates of geochemical reactions can only be estimated in a rough way, transition state theory provides an important framework, which allows a molecular approach to kinetics. It is this unifying characteristic of the theory that makes it worthwhile for use in geochemical kinetics. (JMT)

The structure of laminar alkane-, alkene-, and acetylene flames

Abstract: The detailed knowledge of combustion mechanisms is important for example for the control of (kinetically determined) pollutant formation (e.g., NO, hydrocarbons, soot), or for the extrapolation to technologically important but experimentally inaccessible conditions. By suitable separation and elimination of unimportant reactions, a mechanism is developed with the aid of the present kinetic data for the elementary reactions involved. This mechanism explains, without fitting, the currently available experimental results for laminar premixed flames of alkanes, alkenes, and acetylene (flame velocity and structure of free flames, concentration and temperature profiles in burner-stabilized flames). These experimental results are simulated by the solution of the corresponding conservation equations with suitable models describing diffusion and heat conduction in the multicomponent mixture considered. In lean and moderately rich flames the hydrocarbon is attacked by O, H, and OH, in the first step. These radicals are produced by the chain-branching steps of the oxyhydrogen reaction. The alkyl radicals formed in this way always decompose to smaller alkyl radicals by fast thermal elimination of alkenes. Only the relatively slow thermal decomposition of the smallest alkyl radicals (CH 3 and C 2 H 5 ) competes with recombination and with oxidation reactions by O atoms and O 2 . This part of the mechanism is rate-controlling in the combustion of alkanes and alkenes, and is therefore the reason for the similarity of all alkane and alkene flames.

Chemical kinetics and modeling of combustion processes

Abstract: Chemical kinetic modeling is an important tool in the analysis of many combustion systems. The use of detailed kinetic models in the interpretation of fundamental kinetics experiments in shock tubes and plug flow reactors is widespread. Recently these models, coupled with fluid mechanical models, have become valuable in helping to understand complex phenomena in practical combustion devices. This paper reviews the mechanisms used for the combustion of hydrocarbon fuels and some of the practical problems to which they have been applied. Chemical kinetic reaction mechanisms are strongly hierarchical in that mechanisms for the combustion of more complex fuels contain within them submechanisms for simpler fuel molecules. With this basic structure in mind, mechanisms for H2−O2, CO, CH4, and CH3OH are discussed, followed by C2 species including ethane, ethylene, and acetylene, and finally by a single C3 species, propane. Validation of the elementary reactions and rates in a reaction mechanism is strongly influenced by the combustion environment being studied. For this reason, we emphasize comprehensive reaction mechanisms developed using data from a variety of experimental systems. We review some of the principles and techniques involved in the development and application of these kinetics models for hydrocarbon fuels. Applications of reaction mechanisms to combustion systems include purely kinetic problems and others requiring a treatment of transport effect. The former class includes shock tubes, plug flow reactors, and stirred reactors, while the latter includes laminar flame propagation, flame quenching, and flame inhibition. Each of these combustion systems is discussed, emphasizing the role of chemical kinetics.

Saddle point model for atom transfer reactions in solution

Abstract: A simple classical model for a collinear atom transfer (exchange) reaction in solution is investigated. The reaction is analyzed in terms of reactive and nonreactive modes in a saddle point region. Dynamical solvent effects are treated at the generalized Langevin equation level. The reaction rate constant k is determined and compared to the transition state theory prediction kTST. It is found that, for typical reactions governed by sharp potential barriers, the solvent is not very effective in the saddle region in reducing k much below kTST. In addition, solvent‐induced coupling of the reactive mode to nonreactive modes of motion typically has only a small influence on k. Some limitations of the model are discussed.

A theoretical study of unimolecular reactions of vinyl fluoride. Potential surface characteristics and their mechanistic implications

Abstract: Ab inito MO calculations have been performed for potential energy surfaces of unimolecular reactions of vinyl fluoride. The geometries and energies of the reactant, transition states, reaction intermediates, and product have been determined for ten elementary reactions both on the triplet and the singlet surfaces. The mechanism of the unimolecular dissociation reaction has been discussed in terms of the energy diagram obtained by the present calculation and compared with the interpretation derived from photosensitizer and IR‐multiphoton experiments. We have also calculated the intrinsic reaction coordinate (IRC) for the αα‐ and αβ‐HF direct elimination reactions and discussed the mechanism of energy partitioning in the reaction product. We propose an experiment that would determine which of the αα and αβ processes is dominant in the HF elimination reaction.

An overall and detailed kinetic study of the pyrolysis of propane

Abstract: Overall and detailed kinetic descriptions of the pyrolysis of C3H8 have been proposed as a result of a turbulent flow reactor investigation in the temperature range of 1110–1235 K and at atmospheric pressure. The overall reaction was described by a first-order rate expression with an activation energy of 58.65 kcal/mol and a preexponential factor of 3.2 × 1012 sec−1. This expression agrees with previously reported rate data. In addition, a kinetic mechanism involving 13 chemical species and 32 elementary reactions has been postulated to describe the kinetics. Experimental data from the present flow reactor experiments and from static vessel and shock tube experiments reported in the literature were used to verify the mechanism. Agreement over the temperature range of 800–1400 K and over the pressure range of 0.1–8.5 atm was obtained by adjusting three rate constants. Previously reported values for these rate constants appear to require reexamination. The reactions in question are the following: The sum of the rate constants for reactions (2a) and (2b) and the rate constant for reaction (23) are best represented by and which differ with the expressions in the literature.

A Computational Study of the Chemical Kinetics of Hydrogen Combustion.

Abstract: : A set of elementary reactions and their corresponding rate coefficients has been assembled to describe the homogeneous H2-O2 reaction system over the temperature range 300-3000 K The reaction mechanism was drawn together assuming that H2-O2 reactive mixtures could be adequately described in terms of self-consistent, thermal distributions of electronically neutral, ground-state reactants, intermediates and products The resulting time-dependent ordinary differential equations describing the system were integrated assuming various initial pressures, temperatures and initial concentrations of reactants and diluents The computed results have been compared with experimentally observed induction times, second explosion limits, the rate of reaction above the second explosion limit and the temporal behavior of reaction species The good agreement between the computational and experimental results attests to the accuracy of the assembled mechanism in its description of the homogeneous reaction system and supports the validity of the set of associated rate coefficients for the elementary reactions of the mechanism over a broad range of reaction conditions (Author)

Basic questions in mass spectrometry

Abstract: The unimolecular dissociation of ionized molecules seldom consists in a direct bond cleavage where the reaction coordinate can be adequately represented by a simple bond stretch. The coordinate which controls the progress of the reaction is not always the reaction coordinate (defined as the degree of freedom whose extension leads to spatial separation of the molecular fragments). The specificity of a dissociation mechanism in a polyatomic species is due to its inherently multidimensional character, i.e. it requires participation of several degrees of freedom. It often consists of a sequence of elementary steps and is therefore controlled by several bottlenecks. The complicated and multistep nature of the reaction mechanism results in a natural tendency towards energy randomization. Radiationless transition from the initial upper electronic states to the ground state of the ion is not always very fast with respect to dissociation. A unimolecular reaction should be seen as a flux in phase space through a critical surface. A transition state corresponds to a surface of least flux, i.e. to a bottleneck of the reaction. For a given elementary step, several may exist, whose positions may vary with energy. The nature of these transition states is not immediately obvious. For instance, they do not necessarily coincide with saddle points or with rotational barriers.

Elementary reactions in the oxidation of alkenes

Abstract: Elementary steps in the oxidation of alkenes have been investigated by the addition of C 2 −C 5 alkenes to slowly reacting mixtures of H 2 +O 2 at 480°C, and rate constants for some of the reactions have been obtained. Oxirans are formed by an addition reaction with HO 2 radicals, lower aldehydes and ketones are formed by OH addition. The competition between decomposition and stabilisation of the vibratioally-excited alkyl radicals formed by H-addition favours stabilisation as the carbon chain length increase. The predominant reaction of alkenyl radicals for C 4 and C 5 alkenes appears to be with O 2 to form conjugated dienes, but where this reaction is not possible, O 2 addition to the radical occurs to give unsaturated aldehydes, or formaldehyde and CO.

Diffusion controlled vinyl polymerization

Abstract: It is well known that the polymerization rate and the molecular weight distribution of vinyl polymers can change markedly during the course of polymerization and that these changes are due to the influence of self-diffusion upon the termination reaction. This phenomenon is commonly referred to as the gel effect and in order to explain the polymerization behavior after the onset of the gel effect, the chain length dependence of the termination reaction should be considered. A new method of handling polymerization kinetics with the chain length dependence termination reaction is proposed, which is largely independent of the form of the chain length dependency and is capable of dealing with both disproportionation and recombination modes of termination with chain transfer reaction to monomer. The vinyl polymerization kinetics is modelled for each of the four distinct phases which show different polymerization kinetics-physical property interactions. During Phase I, no interaction is significant and the polymerization kinetics conforms to the conventional kinetic and the molecular weight distribution to the Schulz-Flory most probable distribution. During the Phase II, the termination reaction is controlled by the translational diffusion of the macroradicals. The polymerization kinetics begin to deviate from the conventional kinetics and the termination reaction rate constant shows chain length dependence and conversion dependence. The chain length dependence is modelled with the chain entanglement concept and the conversion dependence with the free volume theory. During the Phase III, the gel effect disappears due to the change of the controlling mechanism of termination from translation diffusion to the excess chain mobility of the chain ends coupled with the propagation reaction. The resulting termination rate constant lacks chain length dependency and is named as the residual termination. During the Phase IV, the propagation reaction and other elementary reactions become diffusion controlled, further slowing down the polymerization rate. A method of estimating the diffusion controlled propagation reaction is proposed. These models, with the aid of general method of polymerization kinetics, were integrated to simulate the vinyl polymerization systems over the whole range of conversion. Methyl methacrylate, ethyl acrylate, n-propyl acrylate, vinyl acetate, ethyl methacrylate, and styrene polymerization data are analyzed with the integrated model which has only one adjustable parameter and excellent agreements are observed.

Kinetic aspects of the syntheses using short-lived radionuclides

Abstract: In syntheses using short-lived radionuclides, such as11C, the reaction conditions are usually such that the concentrations of the reactants, except for the labelled reactant, can be considered constant during the reaction. Two kinetic models have been investigated-irreversible and reversible bimolecular elementary reactions. The influence of the rate constants, of the equilibrium constants, and of the ratio between the starting reactants on the yield of the labelled product has been studied. The results show that, even in cases with unfavourable equilibrium constants, high yields of the labelled products can be obtained if the rate constant for the forward reaction is large. In addition, the specific activity of the labelled product as a function of time has been studied for the irreversible bimolecular case.

The Thermal Decomposition of n-Hexane: Kinetics, Mechanism, and Simulation

Abstract: The kinetics and the mechanism of the thermal decomposition on n-hexane have been investigated in a static apparatus in the temperature range from 650 to 840 K. Based on the kinetic results and the experimental product distributions, a reaction mechanism is proposed which fits the experimental results reasonably well up to medium extents of the reaction. Problems arising in simulation are briefly discussed.

A study of the kinetics of the NH3−NO−O2−H2O2 reaction

Abstract: The addition of H2O2 to NH3−NO mixtures was found to promote the NO reduction reaction and to initiate it around 500°C. A model of the NH3−NO−H2O2−O2 reaction was investigated to establish the characteristics of the reaction. We evaluated the relative importance of individual elementary reactions based on the experimental results and by comparison with previously reported rate constants. The final model consisted of 21 reactions of 14 species. The results obtained by numerical integration of the differential equations were in good agreement with the experimental ones.

Kinetic studies of free radical reactions by mass spectrometry. II. The reactions of SO + ClO, SO + OClO and SO + BrO

Abstract: The rates of several novel elementary reactions involving ClO, BrO and SO free radicals in their ground states were studied in a discharge-flow system at 295 K, using mass spectrometry. The rate constant k2 was determined from the decay of SO radicals in the presence of excess ClO radicals: The SO + OClO overall reaction has a complex mechanism, with the primary step having a rate constant k5 equal to (1.9 ± 0.7) × 10−12 cm3 sec−1: A lower limit for the rate constant of the rapid reaction of SO radicals with BrO radicals was determined:

Reaction of hydrogen atoms with diethyldisulfide and ethylmethyldisulfide

Abstract: H atoms react with C2H5SSC2H5 to give C2H5SH as the sole retrievable product with ϕ = 2.32 at 25°C and 2.84 at 145°C. The primary reaction is postulated to be H + C2H5SSC2H5 C2H5SH + C2H5S with k1 = (4.73 ± 0.64) × 1013 exp [−(1710 ± 69)/RT] cm3/mol·s relative to the rate constant of the H + C2H4 C2H5 reaction. The high value of the entropy of activation suggests the presence of partial hydrogen bonding in diethyldisulfide which is broken in the transition state. Ethylmethyldisulfide reacts similarly: H + C2H5SSCH3 C2H5SH + CH3S or CH3SH + C2H5S. The thiyl radicals propagate a chain of radical exchange reactions forming the symmetrical disulfides with exposure-time-dependent quantum yields. The overall kinetics conform to a 16-step mechanism from which the rate constants of the elementary reactions could be established by computer modeling. Thiyl radicals react considerably more slowly with disulfides than H atoms.

Chemical kinetics of the gas-phase reaction between uranium hexafluoride and hydrogen

Abstract: A study was made of the chemical kinetics of the homogeneous gas-phase reaction between uranium hexafluoride and hydrogen by measuring the rate of disappearance of UF/sub 6/. It has been concluded that the rate-limiting step for which the kinetics have been measured is UF/sub 6/ + HF + H (2). The reaction has been studied in a steady-state flow system over a temperature range of approx.625 to 825 K. Various surface-to-volume ratios were employed to aid in distinguishing gas-phase reactions from surface reactions. The steady-state concentration of the UF/sub 6/ after reaction with H/sub 2/ was monitored in a special multipass infrared spectrophotometer at the 626-cm/sup -1/ absorption band of UF/sub 6/. The principal problems were corrosion, plugging, surface intrusion, and the deleterious effects of minute traces of water; these problems have greatly slowed progress in this field. Several series of measurements involving different initial species concentrations and residence times, with each series at constant temperature, show that the rate is first order in UF/sub 6/. Our measurements yield a dependable Arrhenius curve in terms of a second-order expression for k, the bimolecular specific reaction rate constant for the disappearance of UF/sub 6/. It is believed that this overall ratemore » of the disappearance of UF/sub 6/ is somewhat less than twice that of the critical reaction step 2, indicated above, so that the specific reaction rate constant k/sub 2/ is approx.8.7 c 10/sup 14/ exp(-34550 kcal/RT) cm/sup 3/ mol/sup -1/ s/sup -1/. Conclusions have been reached concerning the relative importance of the various elementary reaction steps involved in the chemical mechanism.« less

A computational study of the chemical kinetics of hydrogen combustion. Memorandum report

Abstract: A set of elementary reactions and their corresponding rate coefficients has been assembled to describe the homogeneous H/sub 2/-O/sub 2/ reaction system over the temperature range 300-3000 K. The reaction mechanism was drawn together assuming that H/sub 2/-O/sub 2/ reactive mixtures could be adequately described in terms of self-consistent, thermal distributions of electronically neutral, ground-state reactants, intermediates and products. The resulting time-dependent ordinary differential equations describing the system were integrated assuming various initial pressures, temperatures and initial concentrations of reactants and diluents. The computed results have been compared with experimentally observed induction times, second explosion limits, the rate of reaction above the second explosion limit and the temporal behavior of reaction species. The good agreement between the computational and experimental results attests to the accuracy of the assembled mechanism in its description of the reaction system.

Modelling and Study of Derivative Signal Curves of Complex Chemical Reaction Mechanisms at Constant Heating Rate

Abstract: In recent studies of the chemistry and kinetics of organic intermediates in solution at 90–400 K, methods of Thermal Analysis, especially Differential Thermal Analysis (DTA) and nonisothermal u.v. spectroscopy, were proven to be very efficient for the elucidation of complex reaction mechanisms [14.1–5].

Resonance absorption measurements of atom concentrations in reacting gas mixtures. 9. Measurements of O atoms in oxidation of H/sub 2/ and D/sub 2/

Abstract: : Resonance absorption spectroscopy has been used to measure oxygen atom concentrations in shock heated H2-O2-Ar and D2-O2-Ar mixtures. Rich, lean and stoichiometric compositions have been studied in the temperature range 1000 - 2500 K. Under all conditions studied, the hydrogen-oxygen reaction could be adequately described by a small number of elementary reactions, and the rate constants could be deduced for several of them.