Showing papers on "Reaction rate published in 1982"

Dynamical polar solvent effects on solution reactions: A simple continuum model

Abstract: The influence of polar solvent dynamics in solution reactions is investigated for a simple model. In this model, a charge is subject to a chemical free energy barrier, the successful crossing of which constitutes reaction. The retarding influence of the time dependent solvent polarization is described at the continuum Debye relaxation level. The reaction rate constant k is determined as a function of the barrier curvature, the charge‐solvent interaction strength, and the solvent polarization relaxation time. The reduction of k with increasingly slower solvent relaxation is found to depend sensitively on the magnitude of charge‐solvent interactions. When the latter are weak, the behavior of k differs qualitatively from a standard Kramers prediction. The analogous reaction problem for a dipole is discussed briefly. Directions for a more realistic treatment of polar solvent effects on reaction rates are described.

Diffusion‐controlled vinyl polymerization. I. The gel effect

Abstract: It is well known that the reaction rate and molecular weight of vinyl polymers can change markedly during the course of polymerization and that these changes are due to the influence of diffusion on the termination reaction. The chain length dependence of the termination rate constant has been considered in this work and has resulted in a general method of treating the polymerization kinetics and molecular weight distribution. This method is independent of the form of the chain length dependency and is capable of dealing with both disproportionation and recombination modes of termination. A specific model for the termination rate constant with chain length dependence is proposed and is based on free volume theory and entanglement coupling. Master curves for the characteristics of the reaction rate and molecular weight distribution are presented with the application of this model.

Reaction Rate Studies for the Corrosion of Metals in Acids—I, Iron in Mineral Acids

Abstract: A detailed kinetic study of corrosion reactions of iron in HCl, H2SO4, and H3PO4 is made by a modified version of the gasometric technique. Reaction rates are measured at different tempera...

Thermal nitridation of silicon dioxide films

Abstract: Thermal SiO2 films, ranging in thickness from 200 to 1200 A, were thermally nitrided using NH3 at temperatures between 800 and 1160 °C and for times varying between 3 min and 5 h. The resulting films were analyzed with Auger electron spectroscopy in combination with argon ion sputtering, Rutherford backscattering spectrometry, spectroscopic ellipsometry, and infrared spectroscopy. The nitridation results in the formation of oxynitride layers in the oxide, the largest nitrogen concentration being found in the surface region and at the SiO2/Si interface. The reaction proceeds via a replacement of oxygen by nitrogen atoms under the action of hydrogen. Although the amount of nitrogen incorporated in the surface region is independent of the original oxide thickness, the interface nitrogen concentration is lower the larger the oxide thickness. This can be explained by assuming that the diffusion of NHx (0

Negative ion chemistry and the electron affinity of SF6

Abstract: An extensive study of negative ion–molecule chemistry involving SF6 and SF−6 utilizing the room temperature flowing afterglow technique is reported. Based on a series of limit determinations, the recommended value of E.A.(SF6) is 1.0±0.2 eV. The discrepancy between this value and several previous determinations of E.A. (SF6) is discussed in terms of systematic experimental conditions and the formation of an excited state of SF−6. A large number of reaction rate coefficients as well as qualitative observations of reaction mechanisms are presented.

Kinetics of the reaction of hydroxyl radical with methane and with nine chlorine- and fluorine-substituted methanes. 1. Experimental results, comparisons, and applications

Abstract: The rates of reaction of OH with CH/sub 4/ and with all nine Cl- and F-substituted methanes that contain at least one abstractable H atom are measured over wide temperature ranges (mainly 250-480 K) by the discharge-flow method under strictly comparable experimental conditions. Curvature is found in all but one Arrhenius plot, that of CHF/sub 3/, which is slower reacting and was studied over a smaller temperature range. Optimized three-parameter expressions require a temperature exponent, n, of 2.0-2.2. They are reduced to Arrhenius A's and E's centered at 300 K. Substantial variation is found in the A's for related compounds, less so in the E's which correlate roughly with the bond energy of the abstracted C-H. The results are compared with all published data, and their use in atmospheric science and combustion is briefly discussed.

Temperature dependence of the C2(X1Σg+) reaction with H2 and CH4 and C2(X1Σg+ and a 3Πu equilibrated states) with O2

Abstract: Temperature dependence measurements of the rates of reaction of C 2 (X 1 Σ g + ) with H 2 and CH 4 are reported along with similar measurements for an equilibrated mixture of C 2 (X 1 Σ g + and a 3 Π u spin states with O 2 over the 300–600 K range. All the reactions are characterized by low activation energies. Singlet C 2 likely reacts with hydrocarbons by an insertion mechanism followed by fragmentation.

Avidin is a slow-binding inhibitor of pyruvate carboxylase

Abstract: In the presence of avidin, progress curves for the pyruvate carboxylase (EC 6.4.1.1) reaction show a marked decrease in the rate of reaction over a period of several minutes. These curves are quantitatively described by an equation in which the rate of catalysis by the enzyme undergoes a firstorder decay from a high initial velocity to a final velocity which is close to zero. The initial velocity of the reaction is independent of avidin concentration while the decay constant increases linearly with increasing concentrations of avidin. These findings are in agreement with a model in which there is slow equilibration between free enzyme and the enzyme-avidin complex. Alternative models involving slow isomerization of the free enzyme or of an enzyme-avidin complex are not consistent with the data. From the slope of the decay constant vs. avidin concentration plot, the rate constant for formation of the enzyme-avidin complex was calculated to be (1.42 ± 0.09) X 105 M-1 s-1 for the enzyme isolated from chicken liver. The intercept of this plot, which is equal to the rate constant for dissociation of the enzyme-avidin complex, was not well-defined by the data but was estimated to be less than 2 X 10-3 s-1. In view of this uncertainty, an alternative procedure was used to obtain a more reliable value. This procedure involved following the regeneration of enzymic activity upon incubation of the enzyme-avidin complex with biotin and yielded a value of (1.45 ± 0.13) X 10-5 s-1 for the dissociation rate constant. The enzyme-avidin complex undergoes a reaction which renders the inhibition of the enzyme irreversible and which occurs at a rate that is 5 times greater than the rate of dissociation of the avidin from the enzyme. This observation accounts for the widely held view that avidin inhibition is irreversible.

The Dehydrogenation of Cyclohexane by the Use of a Porous-glass Reactor

Abstract: The dehydrogenation of cyclohexane was studied by the use of a double tubular reactor; the inner tube was made of porous glass, and the outer tube, of non-porous glass. The Pt/Al2O3 catalyst was placed inside the inner tube. It was demonstrated that dehydrogenation proceeded beyond the equilibrium conversion to be expected in the absence of a porous tube, by separating the hydrogen produced from the central reaction zone through the porous tube. When the reaction rate was so high that the reactant gas reached equilibrium near the inlet of the reactor, the dehydrogenation conversion became greater (more than twice the equilibrium conversion) as the flow rate in the inner path decreased and the flow rate in the outer path increased. The experimental data obtained by using the porous reactor were well reproduced by a computer simulation based on a simple model, in which such parameters as the flow rate, the gas permeability, and the reaction rate, were considered. The potential application of the present por...

Stratospheric negative‐ion reaction rates with H2SO4

Abstract: A recently measured value of the dipole moment of H2SO4 necessitates a recalculation of stratospheric negative ion reaction rate constants with H2SO4. The measured reaction rate constants were normalized to a theoretical limit which depends on this dipole moment. The revised values are increased by the factor 2.7. Deduced values of stratospheric vapor phase sulfuric acid from ion composition measurements are inversely proportional to the reaction rate constants.

Reaction rate constants for O−2(H2O)n ions n = 0 to 4, with O3, NO, SO2, and CO2

Abstract: Reaction rate constants for O−2(H2O)n ions with n = 0 to 4 have been measured in a variable temperature flowing afterglow apparatus with a novel ion source configuration. The ions have been reacted with O3,NO, SO2, and CO2. The reaction with O3 is charge‐transfer to produce O−3 with simultaneous transfer of water ligands. The reactions with NO and SO2 are ligand switching reactions in which NO or SO2 displaces one or more water molecules clustered to O−2 leaving NO−3 and SO−4 as core ions. In these cases, the reaction rate constants are not decreased measurably by an increase in n. CO2 rapidly displaces H2O in reaction with O−2(H2O) but does not react with O−2(H2O)3,4. Isotopically labelled O−2 ions were used to elucidate several reaction mechanisms. The rapid destruction of O−2(H2O)4 ions by O3,NO, and SO2 insure that O−2(H2O)n ions cannot be dominant small air ions in the earth’s lower atmosphere.

Fluorimetric analysis of the binding of warfarin to human serum albumin. Equilibrium and kinetic study.

Abstract: Binding of warfarin to human serum albumin results in a red shift of the UV absorption maximum, suggesting that the binding site is a hydrophobic area of the protein. The enhancement of the fluorescence of warfarin upon binding to human serum albumin was used to study the binding equilibrium and the kinetics of this drug-protein interaction. From equilibrium fluorescence measurements, contributions from free and bound warfarin could be evaluated. From the resulting Scatchard plots, equilibrium constants ranging from 4.2 X 10(5) to 3.5 X 10(5) M-1 for temperatures from 8 degrees to 37 degrees were calculated. The reaction is slightly exothermic (delta H = -1.2 kcal m mole-1) and strongly entropy-driven (delta S = +21 cal . mole-1 . K-1). The reaction rate constants of the warfarin-albumin interaction were determined by the stopped-flow technique. The association rate constant varies from 2.2 X 10(5) to 7.7 X 10(5) M-1 sec-1 from 10 degrees to 32 degrees. The corresponding activation enthalpy is 9.0 kcal . mole-1. These values are not consistent with a diffusion-controlled reaction. The dissociation of the complex was studied by making use of the direct competition between warfarin and phenylbutazone for the same binding site. The dissociation rate constant varies from 2.5 to 10.8 sec-1 in the same temperature range. Activation parameters obtained in the kinetic experiments correspond very well with the thermodynamic parameters calculated from the equilibrium study, validating the fluorescence approach to the equilibrium studies.

On the temperature and reaction rate of burning pulverized fuels

Abstract: A two-color pyrometer technique has been used to measure the temperature of burningpulverized fuel particles flowing in dilute suspension on the centerline of a laminar flow reactor. The transparent reactor is fed by a premixed flat flame fueled with CH 4 −H 2 −O 2 −N 2 mixtures, which permit widely variable post-flame temperatures and oxygen concentrations to be obtained. The two-color system employs an optical-electronic system which includes two monochromators to acquire light emission intensity at two wavelengths simultaneously from individual burning particles passing through a 1 mm square area on the centerline of the reactor. Planck's law and the gray body assumption (verified here by measurements) are used to compute particle temperatures, and the average temperature is then determined by averaging the values for some 500 individual particles. Particle temperatures have been determined for various gas temperatures over a range ofoxygen mole fractions from 0 to 0.2 for size-graded (90 μm mean diameter) samples of a petroleum coke and a flash pyrolysis char. Particle burning rates per unit external surface area were determined from the measured temperatures by a heat balance analysis. The functional variation in burning rate with temperature was then used to infer whether the burning rate was controlled by chemical rates (Zone I), the combined effects of chemistry and pore diffusion (Zone II), or bulk diffusion (Zone III). For the petroleum coke, both zone I and Zone II conditions were observed, and chemical rate coefficients for each zone were derived. For the char, Zone II and a transition to Zone III conditions were observed, and a chemical rate coefficient for Zone II was derived. Reaction orders and activation energies were in the range expected from previous oxidation studies, and the chemical rates for both samples were in good agreement with previous measurements by a different method in another laboratory. The results show that careful temperature measurements can provide a sensitive method for the determination of pulverized fuel burning rates.

Proton-induced quenching and hydrogen-deuterium isotope-exchange reactions of methoxynaphthalenes

Abstract: Proton-induced quenching, photochemical, and thermal isotope-exchange reactions of methoxynaphthalenes in H/sub 2/O (or D/sub 2/O)-CH/sub 2/CN mixtures at moderate acid concentrations were studied by means of emission, /sup 1/H NMR, and mass spectroscopy and measurements of reaction quantum yields. It is demonstrated that the proton-induced fluorescence quenching of 1-methoxynaphthalene (..cap alpha..-RH) proceeds via electrophilic protonation at the proper carbon atom of the aromatic ring in the lowest excited singlet state (/sup 1/L/sub a/) in polar media, leading to proton exchange or isotope exchange mainly at position 5 (slightly at position 8) of the naphthalene ring. The rate constant /sup 1/k/sub R/ for electrophilic protonation to the carbon atom of the aromatic ring in the excited state is almost equal to that of /sup 1/k/sub q/ (the rate constant for proton-induced quenching). The isotope-exchange reaction via the triplet state is negligibly small (about 5% of that via the excited singlet state). For 2-methoxynaphthalene (..beta..-RH) both proton-induced quenching and isotope exchange in the excited state (/sup 1/L/sub b/) scarcely occur. The intramolecular CT structure in the excited state is responsible for the quenching. At higher temperatures (greater than or equal to318 K), the thermal isotope-exchange reactions of ..cap alpha..- and ..beta..-RH takemore » place at positions 2 and 1, respectively; the exchange rate for the latter is faster than that for the former. It is shown that the reaction rate for protonation in the excited state is extremely fast compared with that in the ground state. Activation parameters for the reactions in the excited and ground states are determined. An H-D isotope-exchange reaction mechanism is proposed on the basis of the experimental results with the aid of the usual MO calculations.« less

Combustion properties of carbon slurry drops

Abstract: A theoretical and experimental investigation of the combustion properties of carbon slurry fuels is described The combustion of individual drops (400-1000 /*m in diameter) supported at various positions within an open turbulent diffusion flame was observed When a slurry drop was exposed to the flame, the liquid fuel evaporated in the first stage of the process leaving a porous carbon agglomerate formed from the carbon particles in the slurry The second stage involved heat-up and reaction or quenching of the agglomerate Consumption of the agglomerate was the slowest step in the process, requiring 90-95% of the drop lifetime, even in regions where maximum agglomerate reaction rates were observed An analysis was developed to provide predictions of both liquid and agglomerate heat-up and gasification The analysis yielded good predictions of both particle size and temperature variations for flame equivalence ratios of 0272-1350 The use of a catalyzed slurry was found to increase agglomerate burning rates in the lean portions of the flame, extending the lean limit of agglomerate reaction Nomenclature ai - area/reactivity factor At = pre-exponential factor Cp - specific heat d - flame jet diameter dp = particle diameter D = effective binary diffusivity Ei - activation energy / = enthalpy if = enthalpy of formation K = dimensionless mass burning rate, Eq (15)

Nylon 6 polymerization in the solid state

Abstract: The postcondensation of nylon 6 in the solid state was studied. The reactions were carried out on fine powder in a fluidized bed reactor in a stream of dry nitrogen in the temperature range 110-205°C and during 1-24 h. The solid-state polymerization (SSP) did not follow melt kinetics, but was found to be limited by the diffusion of the autocatalyzing acid chain end group. Factors thought to influence SSP were studied, e.g., heat treatment, starting molecular weight, and remelting. Surprisningly, heat treatment had little effect, but the starting molecular weight had a strong effect on the reaction rate. The higher the starting molecular weight, the faster the reaction. This could be explained as a changing concentration distribution of the reactive groups in the solid state on SSP. The kinetics of the SSP had more than one region, and the rate of reaction for conversions of over 30% could be expressed as - dc/dt = k(c/t), where k is a dimensionless constant independent of temperature with a value of 0.28. The integrated form has the form - In(c/co) = k In(t/), where co is the acid end-group concentration at the start, t is the reaction time, and is the induction time. The value of is both dependent on the starting concentration co and the reaction temperature and has an activation energy of 105 kJ/mol.