Showing papers on "Reaction rate published in 1975"

Overall reaction rates of NO and N2 formation from fuel nitrogen

Abstract: From measurements carried out on flat premixed hydrocarbon/oxygen argon (or helium) flames, into which small amounts of ammonia, or cyanogen are added, overall reaction rates of formation of NO and N2 are determined. From similar measurements effected on nitrogen-diluted ethylene/oxygen flames, an overall rate of prompt NO formation is obtained. The discussion of these rate constants indicates that the relative importance of HCN molecules as intermediates in the fuel NO mechanism increases according to the following sequence of primary fuel nitrogen compounds: ammonia, cyanogen and molecular nitrogen; this last is found to behave like a true fuel nitrogen compound in the early flame stages. Experimental values of the total yield of nitric oxide obtained from the added nitrogen compounds have been determined; they are found to be in good agreement with yields calculated by numerical integration of the empirical overall reaction rates of NO and N2 formation, showing almost the same dependence of the NO yield on temperature, initial fuel nitrogen concentration and oxygen concentration.

Mechanism of catalysis of hydrocarbon reactions by platinum surfaces

Abstract: Atomic height steps and kinks in steps have been identified on platinum surfaces as distinct catalytic sites where C–C, C–H and H–H bond scissions occur during the reactions of hydrocarbons. The active catalyst surface is covered with a carbonaceous overlayer whose properties affect both the reaction rates and the production distribution. A model for the catalysis of hydrocarbons on platinum surfaces is proposed which incorporates the effects of the low coordination number active sites and the atomic surface structure.

Gabbro-eclogite reaction rate and its geophysical significance

Abstract: The gabbro-garnet granulite-eclogite transformation may play a significant role in driving the motions of terrestrial lithospheric plates. Whether or not this transformation is in fact important as a driving mechanism for plate tectonics depends on the relationship of the reaction time to geologic time. Solid state diffusion under completely dry conditions is investigated as a possible model for the gabbro-eclogite reaction, with the result that it could not produce the transition in geologically meaningful times at temperatures less than circa 600°–800°C in the earth's upper mantle. Other reaction mechanisms must exist for the geologically rapid occurrence of the phase change at lower temperatures. It is found that one of these mechanisms can be grain interstitial diffusion in a mantle with minute amounts of water. In this model, dissolved ions migrate through water films surrounding mineral grains to sites of reaction. A water-undersaturated mantle contains a small quantity of hydrous phases, such as chlorite, amphibole, or talc, the presence of which implies that interstices within the rock can contain water in equilibrium with these minerals and at a pressure P_(H_2O) which is less than the pressure in the rock. Implicit then is the presence of other gases and/or structural rock integrity. This P_(H_2O) is calculated for serpentine, tremolite, and talc as a function of temperature and rock pressure. Various pertinent cations are sufficiently mobile in aqueous solution that at high temperature and high pressure, diffusion through water will not significantly slow the reaction. Rather, pressure-induced solubility of ions in this water vapor is the important rate-limiting process in the model. Rock pressure and temperature must be such as to generate at least ∼0.5-1 kbar of P_(H_2O) in the presence of the hydrous phases for geologically short reaction times. Under ambient conditions P_(H_2O) is quite small, the cations are relatively insoluble, and the reaction time is geologically long. Upon subduction of a basaltic upper crust or lithosphere, for example, an increase in P_(H_2O) occurs, and with increasing pressure the mineral solubility in this supercritical water increases dramatically, yielding geologically short reaction times; for example, ∼20 m.y. for chlorite-containing rocks with ∼10^(−5)-cm film thickness for ion diffusion at depths of ∼15-30 km and at temperatures of ∼150°–300°C for different heating models of the descending slab. For gabbros in which amphibole (tremolite)-pyroxene equilibria buffer the partial pressure of water, depths of ∼55–70 km and temperatures of 400°–550°C are required for rapid eclogitization, again for different slab heating models. Thus contrary to previous suggestions, the gabbro-eclogite transformation, as it probably occurs in the descending or spreading lithosphere, is not simply rate-controlled by temperature but depends heavily on pressure and on the nature of the minor hydrous minerals present.

Theory of diffusion-controlled reaction between non-simple molecules. II

Abstract: A new vibrational principle is presented for reaction rates of diffusion-controlled chemical reaction. The variational principle is obtained by a modification of the closure approximation of Wilemski and Fixman. The closure approximation is found to correspond to a special choice of the trial function in this formation and to yield a lower bound of the reaction rate. This formulation is found to be especially useful for the analysis of reaction between non-simple molecules, such as macromolecules.

Ruthenium dioxide: a new electrode material. II. Non-stoichiometry and energetics of electrode reactions in acid solutions

Abstract: RuO2 films on Ta and Pt as supports have been prepared by thermal decomposition of RuCl3 at different temperatures in various atmospheres. Bulk and surface electronic properties of these films have been deduced from their behaviour as electrodes with respect to reactions involving or not involving the adsorption of reactants and/or products, (for example Fe2+/Fe3+, I−/I2, H2/H+ and some simple organic reactions). Results indicate that the non-stoichiometry of the oxide film is a function of the temperature of preparation and also of the atmosphere of annealing. In particular, smaller fractions of Ru3+are obtained as the temperature of preparation is increased and/or the atmosphere of annealing is made more oxygenated. Kinetics of electrode reactions suggest an essentially metallic nature for RuO2 films. They behave very well with redox systems but exhibit poor electrocatalytic features in that rates of reactions involving adsorption steps are usually lower on RuO2 films than on noble metals. Possible reasons for this are discussed in detail.

Interactions between catalytic hydrodesulfurization of thiophene and hydrodenitrogenation of pyridine

Abstract: Pyridine hydrodenitrogenation (HDN) is more difficult than thiophene hydrodesulfurization (HDS), and there is a thermodynamic limitation on the first step of the HDN reaction mechanism which occurs, for example, at 5 to 11 bars, at temperatures above about 350°C. Pyridine inhibits the HDS reaction as previously reported, but sulfur compounds have a dual effect on HDN. At low temperatures, thiophene inhibits the reaction by competing with pyridine for hydrogenation sites on the catalyst. This retards the hydrogenation of pyridine to piperidine, reducing the overall reaction rate. At high temperatures the dominant effect is interaction of hydrogen sulfide, an HDS reaction product, with the catalyst to improve its hydrogenolysis (hydrocracking) activity. This increases the rate of piperidine hydrogenolysis, which is rate determining at the latter conditions, and enhances the overall rate of HDN.

Engineering analysis of continuous production of L‐aspartic acid by immobilized Escherichia coli cells in fixed beds

Abstract: The reaction mechanism and decay behavior of aspartase activity for immobilized Escherichia coli cells were investigated by using a sectional packed column. Reaction within the immobilized cell column proceeded at zero-order on substrate solutions ranging in concentration from 0.1 to 1.0M, and the initial reaction rate was found to be 1.556 X 10(-2) mol/min/liter of immobilized cells. The effect of temperature on the reaction rate constant was investigated. The Arrhenius plot was a straight line at temperatures below 43 degrees C, and the activation energy for immobilized cells was calculated to be 12.36 kcal/mol. Aspartase activity in the immobilized cell column decayed exponentially and uniformly in all sections of a column. Its half-life was approximately 120 days. The rate of formation of L-aspartic acid was shown to be independent of column dimensions.

Reactions of O(1D) with atmospheric molecules

Abstract: A statistical model of chemical reaction is applied to collisions of O(1D) atoms with the molecules H2, N2, CO, CO2, N2O, O3, and H2O. Rate constants for reaction and deactivation are computed over the temperature range 100−2100°K. Competition among various product channels is investigated. In most cases quantitative agreement with experiment is achieved.

Reactions of O− with N2, N2O, SO2, NH3, CH4, and C2H4 and C2H2− with O2 from 300 °K to relative kinetic energies of ∼2 eV

Abstract: The energy dependences of the reaction rate constants of O− ions with the neutral species N2, N2O, SO2, NH3, CH4, and C2H4 have been measured from thermal energy to approximately 2 eV relative kinetic energy The energy dependences of the over‐all reaction rate constants do not fit available theories for collision rate constants The reaction of O− with C2H4 produces four different negative‐ion product channels, in addition to the associative‐detachment channel The branching ratios are found to change drastically with reactant energy The reactions establish the following upper limits on electron affinities: EA(:C=CH2) <043 eV and EA(C2H3) ≳04 eV The reactions also establish an upper limit for the heat of formation of the ion C2H3O− (ΔHf°<−062 eV)

The effect of reagent energy on chemical reaction rates: An information theoretic analysis

Abstract: The effect of changing reagent vibrational and rotational energy on the reaction rate has been analyzed for over 20 chemical reactions. In most cases the selectivity in energy requirements could be characterized by a single (’’consumption potential’’) parameter, even when the reactivity varied by many orders of magnitude. The reactions analyzed covered atom–diatom and diatom–diatom collisions and included both simple rearrangement (’’exchange’’) reactions as well as collision induced dissociation (CID) and quenching of electronically excited states. The results were derived both from experiments and classical trajectory computations and include the variation in reactivity at both a given total collision energy and at a given translational (and rotational) temperature. In all cases the analysis was based on evaluating the surprisal of the energy consumption, i.e., the observed (or computed) reaction rate constant was compared to the rate expected on prior grounds when all states (at a given total energy) r...

Protein hydration changes during catalysis: a new mechanism of enzymic rate-enhancement and ion activation/inhibition of catalysis

Abstract: There exists a linear correlation between the effect of a salt on the rate of an enzymic reaction and its effect on the activation volume (delta V++) of the reaction. Salts that increase delta V++ invariably decrease the rate of the reaction, and vice versa. The salt effects on reaction rate are, however, much larger than would be predicted solely on the basis of pressure-volume work changes deriving from the observed alterations in delta V++. Different inorganic salts affect reaction rates and activation volumes in a manner that reflects the salts' positions in the Hofmeister series. These observations, taken in conjunction with data on the effects of salts on protein functional group (aminoacid side-chains and peptide linkages) hydration, lead us to propose the following hypothesis to account for salt activation and inhibition of catalysis. Aminoacid side-chains and peptide linkages located on or near the protein surface change their exposure to water during conformational events in catalysis. These protein group transfers are accompanied by large volume and energy changes that are due largely to changes in the organization of water around these groups. When these transfer processes occur during the rate-limiting step in catalysis, these energy and volume changes can contribute to the free energy of activation (delta G++) and the activation volume of the reaction. By influencing the degree to which water can organize around transferred protein groups, salts can modify both the delta G++ (rate) and the delta V++ of a reaction.

Kinetics of ferrihemoglobin formation by some reducing agents, and the role of hydrogen peroxide.

Abstract: The rate of oxidation by hydrogen peroxide of human hemoglobin, virtually free from catalase, glutathione peroxidase, and superoxide dismutase, was found to be proportional to the concentrations of hemoglobin and hydrogen peroxide, the second-order rate constant at pH 7.4 and 37° being k = 125 M-1 sec-1. Formation of ferrihemoglobin by reduced glutathione in air was found to be slow, gaining its maximal velocity after a lag phase. Kinetic data and the effect of catalase or glutathione peroxidase demonstrated that hydrogen peroxide is an essential intermediate which produces ferrihemoglobin in solutions of hemoglobin and reduced glutathione. The much higher rate of ferrihemoglobin formation by phenylhydroxylamine than by hydrogen peroxide and the failure of catalase to inhibit the reaction showed that hydrogen peroxide is not an important intermediate in the formation of ferrihemoglobin by phenylhydroxylamine. The reaction rate was found to be proportional to the concentrations of phenylhydroxylamine and hemoglobin. The second-order rate constant was calculated to be k = 2350 M-1 sec-1. With the formation of ferrihemoglobin by 4-dimethylaminophenol also, reaction rates and the failure of catalase to inhibit the reaction demonstrated that hydrogen peroxide is of no importance. The lag phase of the reaction suggests that oxidation products of 4-dimethylaminophenol produced by the reaction between oxyhemoglobin and 4-dimethylaminophenol are essential intermediates.

Sucrose inversion by immobilized yeast cells in a complete mixing reactor

Abstract: Using whole cell invertase of Saccharomyces pastorianus, entrapped in spherical agar pellets, sucrose hydrolysis was carried out in a continuously fed fluidized bed reactor. The effective rate of reaction determined experimentally for the catalytic pellet was correlated with particle radius (R), intraparticle concentration of enzyme (Ep) and external concentration of substrate (SR). The results were elucidated by theoretical analysis incorporating internal mass transfer resistance. At high degrees of diffusional resistance, the effectiveness factor was successfully estimted from Bischoff's equation. A dimensionless number, mA R(k2Ep/KmD)0.5(Km/(Km + SR)), was used conveniently to predict the effectiveness factor in those cases wher the intraparticle diffusional effect was less significant. This number was employed to determine critical pellet size for an optimal reaction. The relationship between the properties of the pellet (size and intraparticle enzyme activity) and its apparent kinetic constants (k′2 and K′m), estimated according to Lineweaver-Burk, are discussed.

Energy dependence and isotope effect for the total reaction rate of Cl+HI and Cl+HBr

Abstract: A laser initiated chemical reaction method has been used to determine the total reaction rates for Cl+HI and Cl+HBr and the dependence of the rates on collision energy and H–D isotopic substitution. The rate constants are k=1.64×10−10 cm3 molecule−1⋅sec−1 (σ=33.5 A2) for Cl+HI and k=7.4×10−12 cm3 molecule−1⋅sec−1 (σ=1.44 A2) for Cl+HBr. Measurements were all done at 295 °K in slowly flowing gases. Substituting H by D decreases the rate constant by a factor of 1.84 in the case of HI and by 1.5 in the case of HBr. The isotope effect may be a result of tunneling on corner cutting trajectories. The cross section decreases with increasing collision energy. This, together with the large reaction cross section, indicates the importance of an attractive potential in these systems.

Hydrolysis of particulate tributyrin in a fluidized lipase reactor

Abstract: Pancreatic lipase has been immobilized onto stainless steel beads by adsorption followed by crosslinking, and onto polyacrylamide by covalent bonding. The activities of the two types of immobilized enzyme toward the particulate substrate, tributyrin emulsion droplets, were determined experimentally, and rate constants based on Michaelis-Menten kinetics were calculated. The activity of the stainless steel-lipase was determined for various flow conditions and for various support sizes by the use of a differential fluidized bed recycle reactor. The rate constants calculated indicate that the experimental reaction rate is free from mass transfer influences, since the observed Michaelis constant does not vary with the fluidization velocity or with the support particle size. In addition, the Michaelis constant of the stainless steel-lipase was found to be equal to that of the free enzyme, suggesting that adsorption and subsequent crosslinking does not alter the enzyme-substrate affinity. The emulsion substrate mass transfer rates, calculated from the filtration theory, indicate that each substrate particle which contact the immobilized enzyme is hydrolyzed to a significant extent. The experimentally determined kinetic rate constants may be used directly to predict the size of integral fluidized bed reactors.

Reaction kinetics of ground state fluorine, F 2P, atoms. Part 2.—Reactions forming inorganic fluorides, studied mass spectrometrically

Abstract: Elementary reactions involving F 2P atoms have been studied kinetically in a discharge-flow apparatus at 296–298 K, using mass spectrometric detection with a beam inlet system. Reactions of F with the following molecules have been investigated: Cl2, Br2, I2, ICl, HOF, CO, Xe, Kr, XeF2, XeF4 and BrO3F.The rates of reaction of F with all three halogen molecules at 298 K are close to the hard-sphere bimolecular collision frequency, and we report the following rate constants (cm3 molecule–1 s–1): F + Cl2 [graphic omitted] ClF + Cl …(1); k1=(1.6 ± 0.5)× 10–10;, F + Br2 [graphic omitted] BrF + Br …(2); k2=(3.1 ± 0.9)× 10–10;, F + I2 [graphic omitted] IF + I …(3); k3=(4.3 ± 1.1)× 10–10;. The overall rate constant, k4, for the F + ICl reaction is (5 ± 2)× 10–10 cm3 molecule–1 s–1; the major reaction channel (4a) forms IF + Cl. This result supports a new determination of the dissociation energy, D°0(IF)=(277 ± 2) kJ mol–1. A minor reaction channel in F + ICl forms ClF + I (reaction 4b) and k4a/k4b has been determined to be (3.3 ± 0.7). IF is kinetically unstable, and rapidly forms IF5 via a heterogeneous reaction mechanism.F atoms react with CO and Xe via slow reactions considered to be kinetically third order to yield COF2 and XeF2, respectively, as final products. The third order rate constants (cm6 molecule–2 s–1), at 298 K, defined by k6,M=–[M]–1[CO]–1 d ln[F]/dt and k7,M=–[M]–1[Xe]–1 d ln[F]/dt, with M = Ar and He, are k6,Ar=(5·0± 1·0)× 10–32, k6,He=(3.4 ± 0.5)× 10–32, and k7,Ar≃ 2 × 10–33. No reaction was observed between F atoms and krypton, and an upper limit of 2 × 10–34 was set for the analogous third order rate constant for this reaction with M = He.Abstraction of H from HOF by F was rapid (k 2 × 10–10 cm3 molecule–1 s–1), and FO radicals were detected as a reaction product: F + HOF → HF + FO. On the other hand, no reaction could be observed between F and XeF2, XeF4 or BrO3F (k < 7 × 10–16 cm3 molecule–1 s–1).

A kinetics study of the reaction of OH radicals with two C2 hydrocarbons: C2H4 and C2H2

Abstract: The flash photolysis‐resonance fluorescence technique has been utilized to study the kinetics of hydroxyl radical reactions with ethylene and acetylene at 300 K over a wide range of experimental conditions. (1) OH+C2H4 →k1 Products (e.g., C2H5O), (2) OH+C2H2 →k1 Products (e.g.,C2H2O+H). The bimolecular rate constant for Reaction (1) was observed to increase from (2.24–5.33) × 10−12 cm3 molecule−1⋅ sec−1 as the total pressure varied from (3–300) torr of helium. The rate of Reaction (2) was invarient with total pressure, and the value obtained for k2 was (1.65±0.15) × 10−13 cm2 molecule⋅sec−1. The observed pressure dependency of Reaction (1) brings the existing literature values for k1 into close agreement. Reaction (2) has been shown to be particularly sensitive to secondary processes and this is discussed in some detail.