Chemical Rate Constants Research Articles

A study of the reaction of CO on liquid iron alloys

In all previous studies of the CO reaction on liquid iron the rate was controlled by liquid phase mass transfer. In this study an isotope exchange technique was used to eliminate liquid phase mass transfer as a possible rate-controlling mechanism. The rate in the present work is over an order of magnitude faster than previously measured. The isotope exchange rate for C13O18 in normal CO on liquid iron was not a function of temperature or sulfur content up to 0.43 pct S, but was a function of gas flow rate, indicating that the rate is controlled by gas phase mass transfer. the mass transfer parameter for the specific experimental condition was determined and the calculated rate for gas phase mass transfer of C13O18 in CO controlling the rate is in agreement with the experimental results. At low temperatures (1523 K) and sulfur contents greater than 0.015 pct the rate may be controlled by mixed control, gas phase mass transfer, and chemical kinetics in series. The chemical rate constant is estimated to be 1.5×10−5 moles/cm2 sec atm for these conditions.

A model of the regional uptake of gaseous pollutants in the lung: II. The sensitivity of ozone uptake in laboratory animal lungs to anatomical and ventilatory parameters

An O 3 dosimetry model is used to simulate the local absorption of O 3 in the lower respiratory tract of rats and guinea pigs. The model takes into account lower respiratory tract anatomy, transport in the lumen and air spaces, and transport and chemical reactions in the mucous and surfactant layers and in the underlying tissue and capillaries. For each species two anatomical models were used to investigate their influence in predicting absorption. Results with all four anatomical models and various ventilatory parameters showed a qualitative similarity in the shape of the dose versus airway number curves but significant differences in predicted percentage total and percentage pulmonary uptake. The percentage uptake was also sensitive to breathing frequency and tidal volume. Rat lobe models were used to study absorption in lobes and show that O 3 tissue dose in centriacinar regions decreases with increasing distance from the trachea. The effect on results of values used for functional residual capacity and of values used for the chemical rate constants for O 3 reactions in mucous were explored. Results differed quantitatively but not qualitatively.

Fluidized bed combustion kinetics of spent Nagoorin carbonaceous oil shale

The combustion kinetics of Nagoorin carbonaceous spent shale, prepared by Fischer assay, have been measured using a fluidized bed batch technique at 600–700 °C bed temperature. The kinetics were shown to conform to a reaction-controlled, shrinking-core model. The burnout time of a particle in the bed was almost independent of bulk bed temperature. The overall chemical reaction rate constant was only slightly dependent on bulk bed temperature, with a maximum for particles with a diameter close to the bulk bed size, the rate constant reducing for both smaller and larger particles. It was concluded that the particle combustion temperature was also almost independent of bulk bed temperature. An empirical equation was derived to calculate the burnout time for a given particle size between 600 and 700 °C thus enabling the estimation of the time for combustion of a particle to a given burnup.

Comparative fluidized bed combustion kinetics of some Australian spent oil shales

The combustion kinetics of spent oil shales from seven major Australian deposits were examined using a fluidized bed batch technique at 600–700 °C and chemical and diffusional kinetics for six of them were determined using a generalized form of the grain model. Chemical rate constants varied between the shales and were less than those extrapolated from data from American spent oil shales. The effective diffusivity also varied widely between the shales.

A dynamical‐chemical model of wave‐driven fluctuations in the OH nightglow

Wave‐induced temporal fluctuations in the intensity of the OH nightglow are related to the temperature oscillations of the wave field by a model that incorporates a five‐reaction photochemical scheme and the complete dynamics of linearized acoustic‐gravity waves in an isothermal, motionless atmosphere. The intensity I and rotational temperature T oscillations, δI and δT, are conveniently related by the ratio, where the overbar refers to time‐averaged quantities. The ratio η is a complex quantity that depends on the properties of the basic state atmosphere (temperature, thermodynamic parameters, major constituent O2, N2 and minor constituent O, O3, OH, H, HO2 concentrations, and scale heights), chemical reaction rate constants, wave period, horizontal wavelength, and direction of wave energy propagation (upward or downward). The intensity‐temperature oscillation ratio η is evaluated for a nominal case corresponding to an altitude of about 83 km in a nightside model atmosphere with an atomic oxygen scale height of −2.8 km; horizontal wavelength λx is 100 km, and wave energy propagation is upward. Over a broad range of acoustic periods |η| varies between 7 and 8, and η is approximately in phase with the temperature fluctuations. At gravity wave periods, |η| decreases with increasing period from a maximum value of about 7.0; at a period of about 3 hours, |η| is about 1.8. The phase of η and δT are within 45° in the gravity wave regime. The main effect of order of magnitude changes in λx is the modification of the location and width (in period) of evanescent regions. At hour periods, |η| increases as the magnitude of atomic oxygen scale height decreases; at periods of several hours, |η| is about 1/3 greater for an atomic oxygen scale height of −2 km than for the nominal scale height. The amplitude of η is essentially independent of the direction of wave energy propagation, but the phase of η relative to that of δT depends on the upward or downward sense of energy propagation at periods in close proximity to the evanescent regime. The magnitude of η at gravity wave periods can depend sensitively on the altitude of the OH emission layer; higher OH emission heights give smaller values of |η| at 10‐min periods, providing the O3 scale height is not too great. Neglect of minor constituent photochemistry in computing η is a tolerable approximation at acoustic wave periods, but it is entirely inadequate at gravity wave periods. Inclusion of dynamical effects is absolutely essential for a valid assessment of η at any period.

Rate Controlling Mechanism of Nitrogen Desorption by Ar Injection into Molten Iron

A study was made on the rate of nitrogen desorption from molten iron by Ar injection. Argon was injected into the melt through a submerged alumina nozzle of 0.1cm in ID. and 0.3cm in O.D. The immersion depth of the nozzle was 4.6-5.2cm. The gas flow rate was in the range of 74-96Ncm3/min. The oxygen content in the melt was varied as 0.004-0.066%. During the experiment, the bubble formation time was measured by a pressure pulse technique.The measured desorption rates are compared with those calculated by a mixed control model derived by considering that the mass transfers in both liquid and gas phases and the chemical reaction at the bubble-metal interface are the rate controlling steps. Nitrogen desorption during the bubble formation is taken into account in the model. From comparison between calculation and experiment, the chemical reaction rate constant is determined. It is shown that the chemical reaction rate constant decreases with increasing [%O]+[%S]/2. The rate constant obtained in the present study agrees with that obtained in the previous study on nitrogen absorption in molten iron. It is confirmed that both nitrogen absorption and desorption in molten iron could be explained by the mixed control model.

Analysis of atmospheric photooxidation mechanisms for organosulfur compounds

Reaction mechanisms are formulated for the atmospheric photooxidation of the organosulfur compounds, dimethyl sulfide, CH3SCH3, methanethiol, CH3SH, and diethyl sulfide, C2H5SC2H5. Predictions of the mechanisms are compared with available data on laboratory photooxidations of these three species to identify critical uncertainties in chemical pathways and reaction rate constants. The sensitivity of product yields to these uncertainties is investigated.

Quantum tunnelling of muonium reactions in solutions

Muonium spin rotation experiments have been performed in a system of benzene in liquid isopentane solutions in the temperature range between 136 K and 295 K. The observed chemical reaction rate constants show a concave curve in an Arrhenius plot. These results are interpreted in terms of to a significant quantum tunnelling effect for muonium reaction in solutions. The oscillating frequency and the width of barrier for such reactions were found to be 400 cm−1 and 2.1 A respectively by curve-shape analysis. The reaction barrier (6.0 kJ/mol) is found to be less than the activation energy due to viscous flow of the solvent.

KINETICS OF COAL SLURRY ELECTROXIDATION

Expressions for limiting currents are obtained to describe the heterogeneous electroxidation of a suspended solid particle in a stirred electrolyte. The electrochemical reaction involves oxidation of a dissolved cation (mediator) in the electrolyte followed by oxidation of the solid coal particle by the cation via an irreversible pseudo first order chemical reaction. This model is applied to bulk electrolysis of the acidic coal slurries containing dissolved Fe(II), and the observed limiting currents permit evaluation of the chemical rate constant of coal oxidation. In addition to Fe(III), multivalent cation of Cu(II) was found to enhance the rates of electrochemical oxidation of coal.

Influence of potassium on the redox kinetics of wustite in its homogeneity range

In view of the numerous previous interpretations of the role of potassium in the reduction of wustite, experiments have been carried out under conditions that allow a more straightforward conclusion: instead of working with a diphasic solid (wustite partially reduced to iron), wustite single crystals were submitted to a series of redox cycles in CO-CO 2 at 890°C, inside its homogeneity range, with and without potassium. With pure wustite, the initial rate of the reaction was shown to be controlled by the surface chemical reaction, as it is linearly dependent on the total gas pressure. Such a conclusion seems valid for the main part of the reaction. Therefore, the chemical rate constant was derived as a function of the solid stoichiometry. Doping with 1 atom-% of potassium brings about an increase in the chemical rate constant which is multiplied by factors of roughly 5 and 10 for reduction and oxidation, respectively. However its behaviour as a function of the solid composition is more complicated. These results indicate that potassium acts directly on the surface exchange between wustite and a CO-CO 2 gas phase, and rule out several other interpretations.

Photochemical formation of peroxides in the boundary layer

Boundary layer gas phase concentrations of H2O2 and CH3OOH are calculated using a multilevel Lagrangian photochemical model. A range of emission conditions and resultant pollutant levels are considered, such as occur in the northeastern United States during summer months. The dependence of peroxide concentrations on chemical rate constants and meteorological variables is analyzed in terms of a budget equation for the formation and removal of radicals in the atmosphere. For representative summer conditions (solar intensity, humidity, and ambient O3 level) the production of radicals, primarily from photolysis of O3, occurs significantly faster than the average NOx emission rate in the northeastern United States. A necessary condition of this imbalance is that only a fraction of the radicals introduced into the atmosphere can be removed by reactions with NO and NO2. The remaining radicals are removed primarily by recombination reactions forming peroxides. Variations in radical production, such as might reasonably be caused by changes in precursor concentration, latitude, season, cloud cover and/or humidity, can reverse the sense of the inequality between NOx emissions and radical production and lead to an order of magnitude decrease in the amount of peroxides formed.

Derivation of diffusion controlled chemical rate constants with the help of Einstein's original derivation of the diffusion constant

The purpose of this article is to show that the reaction rate constants during the steady state phase of diffusion-controlled reactions can be derived from simple considerations of osmotic pressure forces on the reacting species.

Time domain spectroscopy: an application to electrochemistry

Electrochemistry uses current and impedance data to derive chemical rate constants and to obtain information about reaction mechanisms, usually with techniques operating below 100 kHz. By the application of time-domain spectroscopy this region can be extended to nano- or picoseconds and information about the electronic structure of cell components, e.g. electrodes, can be obtained. Fast transient faradic and nonfaradic currents of electrochemical cells can be measured conveniently by a new apparatus which combines the conventional time domain spectroscopy configuration with a potentiostat. The system is applied to semiconductor electrodes in contact with an aqueous electrolyte and multiexponential relaxation spectra are observed.

Kinetic model studies of atmospheric droplet chemistry: 2. Homogeneous transition metal chemistry in raindrops

Considerable evidence indicates that dissolved transition metal ions (TMI) are capable of catalyzing oxidations in atmospheric water droplets, at least in certain circumstances. Wide variations in the importance of TMI chemistry are expected in these systems because concentrations of transition metals in water droplets range over at least 4 orders of magnitude. In the present work we perform an extensive series of model calculations for TMI chemistry in raindrops. The specific TMI discussed are iron, manganese, and copper. The present treatment is restricted to homogeneous processes, that is, those involving dissolved molecules and ions. Results are presented for studies at pH 3 and pH 4, for both daytime and nighttime conditions. Among the results are the following: (1) At pH 3 and pH 4, Fe(III) is present largely as photosensitive hydroxide complexes. Our model results indicate that under atmospheric conditions the photolysis of these complexes is the primary daytime source of reactive free radicals within the droplets, even at quite low TMI concentrations. (2) TMI complex photolysis is not, of course, operative at night. At those times the presence of TMI continues to control the concentration of free radicals in raindrops through “Fenton type” reactions with hydrogen peroxide, for example, Fe(II) + H2O2 → Fe(III) + OH + OH−. (3) The oxidation of S(IV) to S(VI) by H2O2 is the most important daytime sulfite oxidation process in raindrops, but S(IV) oxidation catalyzed by Mn(II) can be significant under certain conditions, particularly at night or in the winter months. (4) Solution nitrogen chemistry is relatively unaffected by TMI. Its most important daytime chemical process is the (rather inefficient) photodissociation of the nitrate ion, a process which generates ozone (and hence HOx radicals). (5) Organic chemistry in atmospheric water droplets is very sensitive to the presence of hydroxyl radicals. Since OH· production is strongly influenced by catalysis involving iron complexes, the presence of soluble iron is a major stimulus for organic chemical processes (such as the conversion of alkyl aldehydes to carboxylic acids). (6) For cases where S(IV) concentrations exceed those of H2O2 the S(IV) effectively “titrates out” the H2O2. If the H2O2 concentration dominates, however, residual H2O2 remains to initiate a variety of solution oxidation chains, particularly those producing organic acids. The presence of H2O2 in the gas phase thus implies acid production in the aqueous phase, the form of the acid depending upon the particular oxidizable species available. (7) The chemical reactions and rate constants used in the calculations are relatively well determined, but our results are quite sensitive to the assumed concentrations of TMI and other species. Increased attention to measurement of species concentrations in fog, clouds, and rain is therefore indicated.

Analytical affinity chromatography in studies of molecular recognition in biology: A review

Measuring macromolecular and cellular interactions remains central to the study of recognition in biology and its application in biotechnology. Analytical affinity chromatography provides a versatile methodology to detect and quantitate such interactions. Both zonal and frontal elution approaches have been developed, essentially in parallel, for analytical affinity chromatography. A close quantitative relatedness of chromatographically obtained equilibrium constants and analogous constants determined fully in solution has been found for a growing number of proteins. This consistently observed correlation has formed the basis for extending theoretical treatments in order to evaluate not only monovalent molecular systems of varying types but also multivalently interacting macromolecules, including those which exhibit cooperativity, The potential to measure chemical rate constants by affinity chromatography also has been recognized, and experimental tests of the available theory are being made. As a micromethod, the quantitative use of affinity chromatography has important applicability for biochemical analysis of an increasing array of biologically active molecules being discovered and isolated but available in only relatively small amounts. Analytical affinity chromatography thus provides a means to use matrix-mobile interactant systems to study mechanisms of biomolecular interactions and therein to attain an understanding of such interactions which often is not easily achieved by solution methods alone.

31P-NMR studies on ATP X Mg2+, p21 X nucleotide, and adenylate kinase X nucleotide complexes. Chemical shifts, rate and equilibrium constants.

Article 31P-NMR Studies on ATP·Mg2⊕, p21·Nucleotide, and Adenylate Kinase·Nucleotide Complexes. Chemical Shifts, Rate and Equilibrium Constants was published on January 1, 1986 in the journal Biological Chemistry (volume 367, issue 2).

The effect of added dissolved calcium on calcite dissolution kinetics in aqueous solutions at 25°C

The kinetics of calcite dissolution in solutions containing dissolved Ca 2+ has been investigated at 25°C, using a rotating disc apparatus. In acid solutions no effect of Ca 2+ in solution is observed. The rate is dependent on the transport of H + to the surface. In neutral to alkaline solutions the dissolution reaction is controlled by mixed kinetics and the conventional empirical representation of the dissolution rate results through the interaction of chemical and transport gradients in the diffusion boundary layer. The chemical reaction rate is a function of the gradient between the equilibrium and the surface concentrations of calcium carbonate, whereas the transport reaction can be described in terms of a series of gradients between the surface and the bulk of dissolved calcium and carbonate species. The presence of dissolved Ca 2+ decreases the rate of the transport reaction, making the dissolution process more transport-controlled. The chemical rate constant is independent of the Ca 2+ concentration in solution. The chemical rate constant for Carrara marble dissolution is determined to ∼2·10 −2 cm s −1 and the diffusion coefficient for the transport reaction to ∼7.6·10 −6 cm 2 s −1. In natural systems with high Ca 2+ concentrations and in absence of inhibitors of the surface chemical reaction, the dissolution of calcite may approach a transport-controlled reaction, especially in environments with restricted flow.

The effect of emergent dislocations on the kinetics of decomposition of solid surfaces under conditions of chemical reaction control

Using a simplified model of an oxide surface containing emergent dislocations, it has been shown that the overall chemical reaction rate constant for the reduction of an iron oxide reaction surface varies during the initial period of oxygen removal due to the increase in the number of reaction sites. The removal of any effective barrier to ledge formation ensures that the maximum number of reaction sites eventually becomes active,i.e., the maximum chemical reaction rate constant is achieved. The transient time or time to achieve the maximum rate constant has been shown to depend on the ledge velocity, which is directly related to the chemical reaction rate per reaction site, and the number of dislocation sources per unit area on the surface. The form of the transient and the transient time are estimated using a simple nucleation and growth model.

The direct determination of rate constants and onsager flow coefficients for non-first-order coupled chemical reactions of any degree of complexity by multispectral chemical relaxation spectrometry

In an earlier paper, the formulation was given for the direct determination of chemical rate constants for coupled first-order ??? multispectral chemical relaxation. This paper gives the theory for the direct derivation of rate constants and the ??? ??? ???, ??? multispectral relaxation data for chemical systems of any degree of complexity in terms of ??? chemical thermodynamics. The procedures only require in integration of auto- and cross-relaxation data and ??? ???.

The vertical distribution of ozone in the mesosphere and lower thermosphere

An assessment is made of the ability of current theory to explain the phenomenology of upper atmospheric ozone as revealed by the sizeable body of measurements presently available. The chemistry of ozone in the mesosphere and lower thermosphere is closely coupled to the chemistries of other oxygen/hydrogen‐containing species, which must be considered concurrently. To provide insight into the sensitivity of model calculations to the choice of values used for key chemical rate constants and climatological parameters, simple analytical expressions for ozone are derived for those situations when it is in photochemical steady state, the mesosphere during daylight hours and the lower thermosphere throughout the full diurnal period. The model is found to reproduce the detailed Aladdin 74 rocket measurements of ozone from 50 to 95 km, numerous other measurements of mid‐latitude ozone in the lower mesosphere, the secondary maximum in the ozone vertical distribution at the mesopause, and the diurnal variability of ozone seen in the radio measurements of Wilson and Schwartz (1981) and other observations. The agreement with the Aladdin 74 data results from adjusting some key parameters within the uncertainties of laboratory measurements or known natural climatological variability. The variety of mid‐latitude observations can be understood in terms of the estimated variability of environmental factors: diurnal, seasonal, and solar cycles in the solar illumination; the abundance of water vapor; and the details of the thermal profile of the atmosphere. The ozone secondary maximum results from the onset of the coupling between active‐hydrogen and active‐oxygen chemistry and its observed variability may be a consequence of secular changes in mesopause dynamics. Above ∼95 km, ozone observations are consistently higher than model results and cannot be accounted for by the set of reactions currently included in the model.