Showing papers on "Reaction rate published in 1979"

Reaction mechanism for the acid ferric sulfate leaching of chalcopyrite

Abstract: The acid ferric sulfate leaching of chalcopyrite, CuFeS2 + 4Fe+3 = Cu+2 + 5Fe+2 + 2S0 was studied using monosize particles in a well stirred reactor at ambient pressure and dilute solid phase concentration in order to obtain fundamental details of the reaction kinetics. The principal rate limiting step for this electrochemical reaction appears to be a transport process through the elemental sulfur reaction product. This conclusion has been reached in other investigations and is supported by data from this investigation in which the reaction rate was found to have an inverse second order dependence on the initial particle diameter. Furthermore, the reaction kinetics were found to be independent of Fe+3, Fe+2, Cu+2 and H2SO4 in the range of additions studied. The unique aspect of this particular research effort is that data analysis, using the Wagner theory of oxidation, suggests that the rate limiting process may be the transport of electrons through the elemental sulfur layer. Predicted reaction rates calculated from first principles using the physicochemical properties of the system (conductivity of elemental sulfur and the free energy change for the reaction) agree satisfactorily with experimentally determined rates. Further evidence which supports this analysis includes an experimental activation energy of 20 kcal/mol (83.7 kJ/mol) which is approximately the same as the apparent activation energy for the transfer of electrons through elemental sulfur, 23 kcal/ mol (96.3 kJ/mol) calculated from both conductivity and electron mobility measurements reported in the literature.

Mechanistic Pathways in the Catalytic Carbonylation of Methanol by Rhodium and Iridium Complexes

Abstract: Publisher Summary The chapter focuses on the mechanistic pathways in the catalytic carbonylation of methanol by rodium and iridium complexes. Kinetic studies of the rhodium-catalyzed methanol carbonylation reaction show a remarkably simple behavior. The iodide promoter can be charged to the reaction in several different forms without marked differences in the reaction rate being noted. Many different types of rhodium compounds can be charged to the reaction, and at typical reaction temperatures of 1500–2000C, they function as effective catalysts. The generation of the initial metal–carbon bond in the catalytic cycle by reaction of methyl iodide with a metal carbonyl, containing species has been proposed as a key step in both the cobalt and rhodium catalyzed systems. Iridium is also an excellent homogeneous catalyst for the carbonylation of methanol under relatively mild reaction conditions. There are apparently complex interactions among solvent, water, iodide form, and carbon monoxide pressure, and this complicates interpretation. Some general kinetic observation can be studied as (1) the reaction rate is strongly dependent on water concentration, with a decrease in rate being observed at higher water levels; (2) in reaction media containing appreciable concentrations of iodide ion, the reaction rate increases with increasing carbon monoxide pressure; (3) the reaction rate is not first order with respect to methyl iodide concentration as in the rhodium system, but shows an optimum level; and (4) at low iodide levels using methyl acetate as the substrate with low levels of water present, the reaction rate is inversely dependent on carbon monoxide pressure.

Applications of a stretch model to mixing, diffusion, and reaction in laminar and turbulent flows

Abstract: In a Lagrangian frame of reference based on lamina (fluid filament) thickness and in a warped time scale based on a single, flow dependent quantity, mixing, diffusion, and reaction can be described in a relatively simple way. Applications are presented for stretch and fold in taffy pull, egg beater and static mixer, shear stretch, stretch of laminae in a vortex, mixing with diffusion, reaction rate controlled by diffusion of reactant through a product layer, and very fast reactions in a turbulent flow.

Ozonation of Water: Selectivity and Rate of Oxidation of Solutes

Abstract: The reactions of ozone with dissolved organics in aqueous media are considered in terms of two reaction pathways: direct reactions of ozone and free radical reactions involving a hydroxyl free radical intermediate. The selectivity and reaction rate are correlated with the molecular structure of the organic compound. The effect of free radical quenchers are examined.

Kinetics of silica polymerization and deposition from dilute solutions between 5 and 180°C

Abstract: Between 5 and 90°C the maximum rate of silica polymerization in dilute neutral solutions at constant supersaturation, increases only slowly (formal Energy of Activation = 3 kcal/mole). From 90 to 180°C the maximum rate decreases to below the rate at 5°C. The reactions show induction periods, whose lengths increase with temperature and decrease with supersaturation. The maximum reaction rates after the period of induction obey approximate 4th order kinetics. As the reaction temperature increases from 5 to 180°C, the average molecular weight of the polymers formed increases from approximately 105 to 109. A model is proposed, which is consistent with these kinetics. It assumes a slow bimolecular reaction between monomers; as polymers are formed, active sites on them react rapidly with further monomer. Because fewer, but larger, polymers are formed at higher temperatures, the reaction rate is little affected by temperature. Between 5 and 90°C the total silica concentration remains constant throughout the polymerization reaction. Above 90°C, the total silica concentration progressively decreases with rising temperature, as formation of larger polymers leads to silica deposition. Deposition is accelerated by rocking the reaction vessel. At 180°C, in an Incoloy-lined vessel, the total silica concentration falls to below the quartz solubility; possibly some iron or nickel silicates are formed. At 180°C in a gold reaction vessel, the total silica concentration falls to the quartz solubility; the deposit consists of amorphous silica, cristobalite, and a trace of quartz. It is suggested that at elevated temperatures, especially in iron pipes, aerated geothermal waters could deposit silica scales, even though the silica concentration of the water is near, or even below, the amorphous silica solubility at that temperature.

Method and apparatus for effecting subsurface, controlled, accelerated chemical reactions.

Abstract: The problem of conducting chemical reactions at elevated temperatures and pressures, such as in the wet oxidation of a waste stream, without excessive expenditures of energy is solved by the method and apparatus for effecting accelerated chemical reactions utilizing a reactor (15) extending into a vertical hole (16) in the earth and having an outer flow passage (21) receiving influent fluid from a supply (38) and supply lines (31 and 33) pumped with air by a pressure pump (29). The fluid undergoes an accelerated oxidation in the hole giving off the products of reaction, heat, and an effluent fluid which flows up an inner flow passage (22) to a settling tank (41) and/or other discharge lines (44, 42). Apparatus for control of temperature, pressure and flow rate are also provided to maximize reaction rates and minimize power requirements.

Concentration forcing of catalytic surface rate processes: Part I. Isothermal carbon monoxide oxidation over supported platinum

Abstract: Periodic feed switching between carbon monoxide/argon and oxygen/argon mixtures to a gradientless reactor has been found to significantly increase the average reaction rate as compared to the steady state rate, when the time averaged feed concentrations in both cases are stoichiometric. The rate enhancement achieved during periodic reactor operation is attributed to the attainment of more desirable surface concentrations on the platinum catalyst which dramatically increase the surface rate processes leading to carbon dioxide production.

Kinetic mechanism, structure and properties of premixed flames in hydrogen—oxygen—nitrogen mixtures

Abstract: The composite flux method described by Dixon-Lewis, Goldsworthy & Greenberg (1.975 a )for the computation of detailed temperature and composition profiles in suitable flames has been applied to the simulation of the properties of a number of fuel-rich and fuel-lean hydrogen-oxygen-nitrogen flame systems. The reaction mechanism proposed by Day, Dixon-Lewis & Thompson (1972), extended to include all the reverse reactions, has been used in the simulation, together with assumed sets of reaction rate and transport parameters. The computed profiles have then been compared with published measurements in flames, covering a wide range of experimental conditions, in order to arrive iteratively at an optimum, self-consistent set of rate parameters which also takes full account of the available elementary reaction rate data from sources other than flames. The flame properties considered in this part of the investigation were ( a ) radical recombination profiles in both fuel-rich and fuel-lean flames, and ( b ) the burning velocities and properties of the main reaction zones of several low temperature, slow burning, fuel-rich flames. Three sets of rate parameters which satisfy all the constraints, and which differ only in detail, are given as sets 1, 2 and 3 in table 4 of the paper. Measurements by Kaskan (1958 b ) of radical recombination in the hydrogen-lean systems have used the (0, 0) band ultraviolet absorption of the hydroxyl radical in order to measure its concentration. The interpretation of the measurements so as also to be consistent with the remaining flame measurements by other methods additionally allows a determination of the oscillator strength associated with the transition. A band oscillator strengths f 00 — 9.5 x 10 -4 was found. Following the establishment of the reaction rate parameters, one set of these (table 9) was used to calculate the expected properties of the whole composition range of hydrogen-air premixed flames. In these cases, as well as in the calculations already summarized, either partial equilibrium or kinetic quasi-steady state assumptions must be used in conjunction with the composite flux method. Partial equilibrium assumptions on the reactions OH + H 2 ⇌ H 2 O + H , ( i ) H + O 2 ⇌ OH + O , ( ii ) O + H 2 ⇌ OH + H , ( iii ) may be employed to relate the concentrations of H, OH, O and O 2 in calculations where only the concentration profiles in the recombination regions of the flames are required. In the calculation of complete flame properties, quasi-steady state assumptions must be used to relate the concentrations either of O, OH and HO 2 with that of H (rich flame formulation), or of H, O and HO 2 with that of OH (lean flame formulation). Subsequent investigation showed that the quasi-steady state assumptions were not completely valid for oxygen atoms everywhere in the flames. Nevertheless, further calculations on several flames by the completely different approach of implicit finite difference solution of the time-dependent flame equations, which does not involve any quasi-steady state assumptions, led to results essentially identical with the original computations. The departures from the quasi-steady state do not therefore significantly affect the flame properties computed by the composite flux method. The general pattern of flame structure which emerges from the complete flame calculations is one in which radicals are produced by chain branching reactions in the hotter regions of the flames, while the major heat releasing reactions occur at lower temperatures. Ahead of the heat release zone there is only a very small preheat zone where heating occurs purely by thermal conduction. This behaviour is different from that of flame models which assume a large preheat zone coupled with a single global exothermic reaction of high activation energy. Comparison of the results of calculations which employed respectively the partial equilibrium and quasi-steady state assumptions showed that the former were valid in the ‘recombination zones’ of the flames for predicting the concentrations of those species which are present in significant amounts. Except in lower temperature flames, for example the 15% hydrogen-air flame and to some extent the 70% hydrogen-air flame, the ‘recombination zones’ extend almost back from the hot boundaries of the flames to the maxima in the hydrogen atom mole fraction profiles. On continuing the flame integrations back from the recombination zones into the main reaction zones, the quasi-steady state overall radical concentrations, represented by X H + 2 X O + X OH , where X is mole fraction, fall below those calculated with the partial equilibrium assumptions. On the other hand, the distribution of the radical pool between H, O and OH is such that in fuel-rich flames the comparatively small quasi-steady state oxygen atom concentration, and to a lesser extent also the hydroxyl radical concentration, appreciably overshoot their partial equilibrium values. This is referred to as kinetic overshoot . It is observable only in sufficiently fuel-rich flames, and for example, there is no observable hydroxyl radical kinetic overshoot in hydrogen-air flames containing less than about 50 % hydrogen, and no similar oxygen atom overshoot in those with less than 30 % hydrogen. A fundamental feature of the flame model used is that it assumes a state of thermal equilibrium to exist at each point in the flames, so that the properties of the gas at each point can be represented by a single temperature. This assumption may not be valid in faster flames, because of the finite velocities of relaxation of thermal disequilibrium between the various degrees of freedom in the system. Properly carried out, a comparison of the computed burning velocities of the hydrogen—air flames with experimental observation should throw light on the possible effect of such slow relaxation on the flame properties. However, an attempt at such a comparison initially raised several questions about the interpretation of burning velocity measurements. These are fully discussed. The hydrogen—air flame having the maximum burning velocity is that containing 41 % hydrogen. At this composition it is concluded that the true burning velocity lies in the range (285 ± 10) cms -1 , and hence is not more than 4 or 5 % above the computed value. Finally, the effect on the computed flame properties of ( a ) changes in the diffusion coefficient and ( b ) neglect of thermal diffusion of hydrogen atoms was investigated. Other conditions being equal in fuel-rich hydrogen-air flames near stoichiometric, the neglect of thermal diffusion caused an increase of 5-6% in the computed burning velocity.

Predicting gas phase organic molecule reaction rates using linear free-energy correlations. I. O(3P) and OH addition and abstraction reactions

Abstract: Linear free-energy (LFE) correlations for gas phase O(3P) and OH addition and abstraction reactions with a number of organic compounds have been established using existing room-temperature rate constants evaluated from the literature. Addition reaction rate constant correlations with ionization potential and abstraction reaction rate constant correlations with bond dissociation energies are examined and compared to the LFE approach. Using multiple regression analysis, empirical linear equations are derived and used to predict rate constants for reactions of O(3P) and OH with a number of organic molecules. The use of LFE room-temperature rate predictions permits chemical modeling efforts to be extended to compounds where experimental determinations of rate coefficients are lacking and also serves as a useful tool in evaluation of experimental rate measurements.

Facts and hypotheses of molecular chemical tunnelling

Abstract: The similarities and differences between electron–nuclear tunnelling (discovered in 1966) and molecular tunnelling (discovered in 1973) are described here in terms of radiationless electron transitions. Examples of the low-temperature limit on a chemical reaction rate, observed since 1973, include: growth of chains of polymerisation of formaldehyde, Fe–CO rebinding in haem-containing proteins, isomerisation of radical pairs, hydrobromination of ethene, and photoinduced conversion of rhodopsin into prelumirhodopsin. The general significance of molecular tunnelling for chemistry and related subjects is discussed. The possible manifestations of molecular tunnelling in the formation of complex molecules at the surfaces and in the bulk of ‘dirty ice’ mantles of dust grains in interstellar dense clouds are also considered.

Reliable ab initio calculation of a chemical reaction rate and a kinetic isotope effect: H + H2 and 2H + 2H2

Abstract: We calculate equilibrium rate constants for ortho-para conversion in hydrogen and deuterium by an atomic mechanism. The calculations are based on an accurate ab initio potential surface, transition state theory, and an adiabatic transmission coefficient. The calculated rate constants are demonstrated to be reliable within 40-50%, and they agree with experiment within this margin.

A kinetic study of ozone‐phenol reaction in aqueous solutions

Abstract: The present research concerns mechanism and rate of reaction between dissolved ozone and phenol in homogeneous solutions. The stopped-flow technique was employed to obtain absorbances during reactions; the kinetic experiments were conducted at temperatures varying from 5° to 35°C in aqueous solutions with pH values ranging from 1.5 to 5.2. The kinetic data indicated that the absorbance of a mixed solution increased rapidly in the very early portion of the reaction and then declined slowly in the remaining period. The rate of reaction in the early period was first order with respect to both phenol and ozone concentrations. The rate constant increased with pH value and temperature, and an activation energy of 5.74 K cal/mole was obtained. Further tests showed that the dissolved ozone was consumed completely in the first period and that in the second period the intermediate products were decomposed without depletion of ozone. Catechol, o-quinone, hydroquinone, oxalic acid, humic acid, and a dimer were identified from mass spectra as products of the ozonization reaction. A free radical mechanism, with initiation of an electrophilic reaction for the formation of catechyl radical, has been proposed to explain the experimental data for phenol-ozone reaction in aqueous solutions. According to the proposed mechanism, the reaction path through the attachment to ortho position is much more favorable compared with that through the paraposition because of geometric advantage, though quinones and hydroxyphenol can be formed through parallel paths. The products of reaction detected in this research, therefore, are accountable by this mechanism. A rate equation derived on the basis of this mechanism also agrees well with that observed from the kinetic experiments.

Particle size of aggregate and its influence upon the expansion caused by the alkali–silica reaction

Abstract: Synopsis Measurements of the expansion behaviour of mortar bars containing various proportions of a reactive porous op aline rock are described. It is shown that the age at which cracking occurred is essentially independent of particle size, and that the alkali-silica reaction rate is primarily a function of particle volume.

Temperature dependence of the reaction HO2+NO→OH+NO2

Abstract: The title reaction has been studied using laser magnetic resonance detection of HO2 radicals in a discharge flow system. For the range 403≳T≳232 K, k1= (7.9±1) (T/300)−0.83±0.18 ×10−12 cm3 molecule−1 s−1. Upper limits are placed on the rate constants for the reactions of (HO2+NO) that yield HNO and HOONO: k<0.0012 k1. Measurements have also been made that indicate that the reactions of HO2 with CO and N2O are very slow at 300 K. These rate constants are less than about 5×10−17 cm3 molecule−1 s−1.

Kinetic equations for reactions in concentrated aqueous acids based on the concept of "excess acidity"

Abstract: Kinetic equations, applicable to A-1, A-SE2, and A-2 reactions in concentrated aqueous acids, are derived. The variation in reaction rate with varying acid concentration is treated in terms of the "excess acidity" of the medium (X-function), rather than in terms of Hammett-type acidity functions or the water activity. The parameters obtained are the medium-independent rate constant k0, in the aqueous standard state, as an intercept, and a slope parameter m≠ hydration parameters (r-values) are also obtained, for A-2 reactions. The equations derived are shown to apply to A-1 acetal hydrolyses, A-SE2 electrophilic aromatic substitutions, and mixed A-2/A-1 ester hydrolyses. In a general discussion of available methods for analyzing rate data in these media, it is shown that the X-function method encompasses most, if not all, of the others, and that classical acidity functions are no longer necessary.