Showing papers on "Reaction rate published in 1977"

A model for unimolecular reaction of sulfur hexafluoride

Abstract: The thermal dissociation data for SF6 has been re‐evaluated using RRKM unimolecular reaction rate theory and was found to be consistent with a higher value for the sulfur–fluorine bond strength (92 kcal/mole). A model based on a rate equation formalism has been developed that includes unimolecular dissociation, collisional energy transfer (V–T processes), and absorption and emission of laser radiation. The model was designed to test theories of absorption of laser radiation for laser‐induced chemical reaction in SF6 that are amenable to a rate equation formalism. It was found that the thermal reaction and vibrational relaxation data could be adequately reproduced. Sample calculations of laser‐induced chemical reactions are performed.

A numerical model of chemical kinetics of combustion in a turbulent flow reactor

Abstract: Calculations with a numerical model incorporating detailed chemical kinetics, hydrodynamic motion, and energy transport in a turbulent flow reactor have been compared with experimental results of Dryer and Glassman. A reaction mechanism, including 19 chemical species and 56 reactions, for the reaction of dilute moist carbon monoxide in air and of dilute methane in air was established for the temperature range 1000-1350 K. HO2 and H2O2 were found to be important in the mechanism for both carbon monoxide and methane oxidation, and CH2O, CH3O, C2H6, and C2H4 were found to be important in methane oxidation. Important steps in the reaction mechanisms have been identified, and optimal values for some key reaction rates have been determined. The branching ratio between reaction 3, H + O2 = OH + O, and reaction 17, H + O2 + M = HO2 + M, was found to be important in determining the length of the induction period in each experiment. At 1100 K the value determined for k17 was 2.6 X 10/sup 15/ cm/sup 6//(mol/sup 2/ s). Decomposition reaction 7 for HCO, HCO + M = H + CO + M, was found to play a key role in methane oxidation, providing the majormore » path for production of carbon monoxide. At 1100 K, k7 was found to be 2.4 X 10/sup 10/ cm/sup 3//(mol s). Even though the reaction studied was extremely oxygen rich, recombination of methyl radicals and subsequent oxidation of the ethane thus formed was found to provide a major route for methyl radical destruction. The assumption that plug flow conditions prevail in the turbulent flow reactor was examined and found to be valid under most practical conditions.« less

A Flow‐Through Porous Electrode Model: Application to Metal‐Ion Removal from Dilute Streams

Abstract: A one-dimensional model for flow-through porous electrodes operating above and below the limiting current of a metal deposition reaction has been developed. The model assumes that there is one primary reactant species in an excess of supporting electrolyte, and that a simultaneous side reaction may occur. The model predicts nonuniform reaction rates due to ohmic, mass-transfer, and heterogeneous kinetic limitations; the effects of axial diffusion and dispersion are included. Results are compared with the experimental data observed by various authors for the deposition of copper from sulfate solutions with the simultaneous generation of dissolved hydrogen. Satisfactory agreement between model predictions and experimental data on over-all reactor performance and deposit distributions has been accomplished. For an upstream counterelectrode, distributions of reaction rate (for both single and multiple reactions), concentration, and potential describe the detailed system behavior.

Statistical–mechanical theory of gaseous ion–molecule reactions in an electrostatic field

Abstract: A theory is developed for gas‐phase swarm measurements of ion–molecule reactions in electrostatic fields of arbitrary strength. The theory allows measurements of reaction rate coefficients made at low temperatures and strong electric fields to be converted directly to equivalent thermal rate coefficients at elevated temperatures inaccessible by direct methods. It is not necessary to calculate the ion velocity distribution function explicitly, or to unfold the reaction cross section from the rate data. In first approximation the measured rate coefficient is equal to the thermal rate coefficient at an effective temperature calculated directly from the measured ion drift velocity. Higher approximations are obtained from more detailed analysis of the dependence of the rate coefficient and drift velocity on electric field strength. Comparison is made with experimental data reported in an accompanying paper by Albritton et al. In another accompanying paper, Lin and Bardsley compare the present theory with their...

The reaction of the precursor of the hydrated electron with electron scavengers

Abstract: We show that there are several compounds whose ability to decrease the initial 50-ps yield of the hydrated electron is not correlated with the reactivity of that compound. Several mechanisms have been proposed. Explicit corrections have been included for time-dependent reaction rates. It is shown that other corrections require assumptions about the mechanism of the presolvation reaction.

Active Sites in Heterogeneous Catalysis

Abstract: Publisher Summary This chapter focuses on the active sites in heterogeneous catalysis. The chapter reviews a surface structure and chemisorption characteristics of crystals cut along different crystallographic orientations. A chemical reaction is studied at low pressure to establish correlations between reactivity and surface structure and composition. The same catalytic reaction is studied at high pressures and the pressure dependence of the reaction rate is determined using the same sample over the nine orders of magnitude range. The rates and product distributions that were determined at high pressures on single-crystal surfaces are compared with the reactivity of polydispersed small-particle catalyst systems. At low pressures, a quadrupole mass spectrometer is used as a detector of both the chemical reaction rates and the product distributions. At high pressures, a gas chromatograph is employed that is as sensitive as a mass spectrometer that is used at low pressures. Small-surface-area single-crystal catalyst samples can readily be used in studies as long as the reaction rate is greater than 10−6 product molecules/surface atom/sec. The rate defined is called “turnover number” in the field of catalysis.

Ammonia, oxidation leaching of chalcopyrite —reaction kinetics

Abstract: The reaction for the ammonia, oxidation leaching of chalcopyrite, CuFeS2 + 4NH3 + 17/4 O2 + 2 OH- ⇌ Cu(NH3)+2 42 + l/2Fe2O3 + 2 SO4 + H2O was studied using monosize particles in an intensely stirred reactor under moderate pressures to determine the important chemical factors which govern the kinetic response of the system. The reaction kinetics were studied at dilute solid phase concentration so that oxygen transport at the gas/liquid interface would not limit the rate. A catalytic electrochemical surface reaction was shown to control the reaction kinetics with the reaction rate determined by the following equation derived from electrochemical considerations: dα/dl=127 f/do (OH- 1/2 (k1PO1/1+k2PO1 (k1+k2(Cu+2)o+k’2α)(1-α)2/3 (K P 1/2 Excellent agreement between theory and experiment was obtained both with regard to apparent reaction orders for oxygen, cupric, and hydroxyl, and with regard to geometric factors that influence the reaction rate. Further support for the reaction mechanism included an activation energy of approximately 10 kcal/mole obtained under a variety of experimental conditions and the fact that the initial reaction rate constant was several orders of magnitude less than predicted mass transfer coefficients.

Design criteria for uniform reaction rates in an oxygen plasma

Abstract: The physical conditions necessary to obtain uniform and reproducible chemical reaction rates in an oxygen plasma are investigated by stripping photoresist from silicon wafers. An oxygen plasma was selected due to its simplicity and the availability of a simple technique for measuring the concentration of atomic oxygen which is the reagent of importance. However, many of the considerations will apply to the design of other plasmas for other chemical reactions. Due to the electrical Skin effect at 13.56 MHz, the atomic oxygen is created near the Walls in a cylindrical reactor. However, at flow rates corresponding to residence times in the range of 1-10 s, diffusion makes the concentration uniform within a few percent except near the walls. Under conditions such that there is no by-product inhibition, the chemical reaction rate is determined bythe atomic oxygen concentration and the temperature of the silicon substrate and the reactor walls. The rate is independent of the conbination of pressure and power used to produce the atomic oxygen concentration.

Chapter 1 The Oxidation of Hydrogen and Carbon Monoxide

Abstract: Publisher Summary The chapter discusses the oxidation of hydrogen and carbon monoxide, and discusses the oxidation of hydrogen–carbon monoxide mixtures The kinetic investigation of the hydrogen- oxygen reaction involves measurement of the effects of alterations in parameters such as temperature, pressure, mixture composition, and vessel size and surface on the slow reaction rate and on the positions of the explosion limits The effect of variation of the gas composition on the explosion limits at constant temperature may be shown by plotting the partial pressures of hydrogen at the limit against the partial pressures of oxygen The kinetic investigations are carried out under essentially isothermal conditions, often with rather low concentrations of chain centres With such low concentrations of chain centres it is usually necessary only to consider reactions of free radicals with stable molecules, even when defining critical conditions leading to limit explosions Such conditions apply particularly when the reaction is carried out in vessels having moderate or high surface chain breaking efficiencies

NOx catalytic ozone destruction: Sensitivity to rate coefficients

Abstract: Model sensitivity experiments were conducted by using correlated ensemble variations of reaction rate coefficients. For all the simulation experiments tested, the theoretically derived concentration profiles of trace species were in comparable agreement with stratospheric observations. Thus current stratospheric measurements seem to be inadequate to assist in reducing the current uncertainties in kinetic rate data. Given this current range of uncertainties in rate coefficients, it is found that by varying as few as three rate coefficients within the estimated (Hampson and Garvin, 1975) uncertainty range the effect on ozone of lower stratospheric injection of NOx can be reversed. Consequently, it is clear that as a first step toward further progress more precise laboratory measurements of certain reaction rate coefficients are needed.

Studies on the cooking rate equations of rice

Abstract: The soaking and the cooking mechanisms of rice were investigated in terms of the mathematical rate equations in this study. The soaking and the cooking rate were measured by using the weighing method, and the soaking rate was examined at temperatures from 8 −50°C, and the cooking rate from 70 – 98.5°C. The rate equation involved two rate parameters: the reaction rate parameter of the rice component with water and the diffusion rate parameter of water, was assumed. The results showed that the cooking rate was mainly limited by the reaction rate of rice components with water at temperatures from 70 – 98.5°C, and the equivalent value of the activation energy of the reaction rate was nearly equal to 20 kcal/mol, though the cooking rate was relatively influenced by the diffusion rate of water in the cooked rice layer at 98.5°C. In the soaking of rice, the values of the diffusion rate parameter at from 8.0 –50°C were smaller than the values at from 110 – 150°C.

Catalyst mediated competitive protein binding assay

Abstract: Method and compositions are provided for competitive protein binding assays wherein a non-enzymic catalytically mediated reaction is sterically inhibited by receptors binding to a ligand to which the reaction intermediary is bound. The method employs first and second reactants, which only react with each other either very slowly or not at all. The reaction intermediary or non-enzymic catalyst provides for a two step reaction having as its final result the transformation of the first and second reactants. By conjugating the reaction intermediary to a ligand, whereby the presence of receptor bound to the ligand inhibits the approach of one or both of the reactants, ligand and antiligand can be determined. Alternatively, anticatalyst can be employed which is inhibited from binding to the catalyst when antiligand is bound to ligand. The binding of anticatalyst to catalyst inhibits the catalytically mediated reaction. By comparing the rate of reaction with an unknown amount of analyte present to rates with known amounts of analyte, the concentration of the analyte in an unknown may be determined.

Studies on nucleic acid reassociation kinetics: rate of hybridization of excess RNA with DNA, compared to the rate of DNA renaturation.

Abstract: The rate of reaction of double-stranded replicative form (RF) [3H]DNA of bacteriophage phiX174 with excess (+)strand DNA and (+)strand RNA was measured by standard methods of hydroxyapatite chromatography. The reactions follow pseudo-first-order kinetics and the observed rate constant for the RNA-DNA reaction differs less than 25% from that of the DNA-DNA reaction. The pseudo-first-order rate constants are close to the value predicted on the basis of the second-order rate constant measured in the renaturation of the double-stranded phiX RF [3H]DNA.

Combustion and flame propagation in coal systems: A review

Abstract: Coal particles in combustion can pyrolyse, swell, graphitise, and react internally or externally or both. Mechanisms determining or controlling the reaction rate can include: boundary layer diffusion (generally controlling for particles greater than 100 microns), pore diffusion, or surface reaction with alternatives of adsorption control or desorption control, or some intermediate. Present evidence on particles in flames is that swelling in oxidizing conditions is likely to be relatively mild, about 10% in diameter. Particles are sufficiently porous for reaction to be in either Zone II (parital penetration) or Zone I (full penetration) with reaction in the latter case at effectively constant diameter. Reaction orders, which provide information on the relative dominance of the adsorption and desorption processes, are uncertain. Numerical values favor 1/2 to unity; very low temperature chemisorption of oxygen on carbon, however, (at−70°C) implying a very low adsorption activation energy, indicates otherwise. There is limited evidence that the reaction order is zero to 1/2, with activation energies of 20 to 40 kcal identified with desorption. Coal flames propagate by several mechanisms. Rates of heating in all cases (including the fixed bed) are in the range 10 to 106 or 107 deg C/sec. In explosion flames, propagation is by turbulent exchange, probably with heterogeneous ignition; in full flame furnaces, propagation is by radiation if there is no backmix flow, with either heterogeneous ignition or prior pyrolysis; in small flame systems, propagation is by downstream pyrolysis and upstream diffusion. Effective research is still in the data-gathering and mechanism-determining stage. Qualitative mechanisms are being elucidated by quantitative investigations. Accurate predictive quantitative theories, however, do not yet exist; neither is it yet possible to predict with any confidence the combustion behavior of one coal from another. Work is moving steadily in that direction, however.

Studies of reactions of importance in the stratosphere. II. Reactions involving chlorine nitrate and chlorine dioxide

Abstract: The rate of formation of chlorine nitrate, ClO+NO2+M→ClONO2+M, has been studied over the temperature range 250–356 °K and the pressure range 1–5 torr using the discharge flow technique with mass spectrometry for detection. The resulting low pressure third order reaction rate constant in N2 is given by the Arrhenius expression kN2=4.40±0.66 ×10−33 exp(1087±70/T)cm6 molecule−2 sec−1. Helium was found to be one‐half as effective as N2 as a third body for this reaction. No reaction of ClONO2 with NO, NO2, O3, or HCl could be observed. Upper limits of 4.×10−17, 2.×10−17, 7.×10−17, and 1.2×10−15 cm3 molecule−1 sec−1 for the respective bimolecular reaction rate constants preclude the importance of these reactions as sinks for ClONO2 in the stratosphere. A heterogeneous reaction, ClONO2+HClsurface→HONO2+Cl2, was observed, and may be useful as a selective reaction for the measurement of chlorine nitrate in the stratosphere. An upper limit of 10−16 cm3 molecule−1 sec−1 was found for the reaction ClO+O3→OClO+O2 and ...

Induced Polarization: a Preliminary Study of its Chemical Basis

Abstract: An investigation of the effect that the in‐situ solution chemistry would have on the induced‐polarization phenomena was carried out by means of laboratory studies of the electrode impedances of metallic and sulfide minerals. Reaction rate theory shows that this effect is largely due to the impedance associated with the diffusion of the ions involved in the charge transfer reaction to and from the reaction sites. This impedance is inversely proportional to the concentration of the reacting ions and inversely proportional to the square root of the frequency. Impedance measurements were made with different concentrations of a variety of ions in order to identify the active ion species. The ion concentrations were controlled by doping and by excursions in Eh and pH. These measurements showed that cupric and sulfide ions are active in the charge transfer reaction and control the electrode impedances whenever their concentrations exceed 10-4N. None of the other common anions and cations tested appeared active. ...

Exothermic gas absorption with chemical reaction

Abstract: The film theory for gas absorption with chemical reaction has been extended to take into account the release of heat due to solution and reaction. The analysis is applied to the industrially significant sulfur trioxide-dodecylbenzene system, and large increases in interface temperature are predicted during absorption. The enhancement factor behavior is correspondingly complex owing to the interaction of solubility reduction and rate constant enlargement. Some initial experimental measurements with the laminar jet technique confirm the need for and the basic soundness of the theory.

The Koenigs-Knorr Reaction

Abstract: Publisher Summary This chapter focuses on Koenigs–Knorr reaction used for the synthesis of glycosides. The Koenigs–Knorr reaction is applicable to the preparation of both aryl and alkyl glycosides, and is widely used for the synthesis of glycosides having complex groups attached to the anomeric carbon atom—particularly oligosaccharides. The procedure involves the treatment of a per- O acylated glycosyl halide with an alcohol in the presence of a heavy-metal salt or an organic base as the acid acceptor; the latter enhances the rate of reaction and also prevents side reactions. It is noted that under the usual reaction-conditions the more-stable anomer is obtained, and the less stable anomer can be prepared only by a kinetically controlled reaction. Per- O -acylated glycosyl bromides react faster than the corresponding chlorides, and are therefore preferred for most reactions. Silver carbonate or silver oxide is commonly used as the acid acceptor in the Koenigs–Knorr reaction. Mercury (II) acetate instead of a silver salt has been also used.