Showing papers on "Reaction rate published in 1969"

The change in pore size distribution from surface reactions in porous media

Abstract: When a porous solid is penetrated by a reactive fluid which changes the pore geometry, the macroscopic properties of that porous material may be greatly changed. A model is proposed in which the matrix is visualized as being a number of short cylindrical pores dispersed randomly throughout the solid. The change in the distribution of these cylindrical pores is then represented by a integrodifferential equation which is solved for two special cases. The evolution of the pore size distribution is determined by the particular way in which the solid-liquid boundary takes place. The case considered here is that of a surface reaction which dissolves the solid thus continuously enlarging the pores. The rate of reaction is calculated theoretically using a laminar flow diffusion model and this growth rate expression is then taken as the basis for numerical calculations relating to the action of dilute hydrochloric acid on limestone. A comparison is made with experimental results and it is found that the model behaves in much the same way as the real system although the observed rate of pore growth was two to three times that predicted by the diffusion model. Several possible explanations for this discrepancy are being tested. An exact solution of the integrodifferential equation for highly retarded reaction rates has been found with the change in permeability being given in terms of the change in porosity. This result will permit a prediction of the stimulation that can be achieved in acidizing oil wells with retarded acids.

Ion–Molecule Reactions, He++O2 and He++N2, at Thermal Energies and Above

Abstract: The rate coefficients for the reactions of He+ ions with N2 and O2 have been measured in a drift tube–mass spectrometer from thermal energies (300°K) to ∼ 0.1 eV. In addition to reaction rate determinations by the usual product‐ion “arrival spectrum” method, a new technique involving “additional residence time” for the parent ion is described which is applicable even when parent and product ions have equal mobilities. The total rate coefficient (i.e., the sum of dissociative and the ordinary charge transfer) for He++N2 reaction was found to be (1.0 + 0.3, − 0.2) × 10−9 cm3/sec at 300°K, decreasing to ∼ 7 × 10−10 cm3/sec at 0.1 eV. The corresponding coefficient for the He++O2 reaction was measured to be (8.5 + 2.5, − 2.0) × 10−10 cm3/sec and to be independent of energy over the same range. The ratio of dissociative charge transfer (with N+ or O+, respectively, as reaction products) to ordinary charge transfer (with N2+ or O2+, respectively, as products) was found to be 0.55 / 0.45 for He++N2 and 0.8 / 0.2 ...

Thermal Ion–Molecule Reaction Rate Constants at Pressures up to 10 torr with a Pulsed Mass Spectrometer. Reactions in Methane, Krypton, and Oxygen

Abstract: A mass spectrometer for high‐pressure ion–molecule reaction studies was constructed using a pulsed 4‐keV electron beam and pulsed “gate‐open” mass‐spectrometer detection. The ions drift out of the field‐free ion source by diffusion and mass flow. Pressures up to 10 torr can be used and the ion reaction times can be as long as several hundred microseconds. The system is specially suited for study of reactions with very small rate constants, long reaction sequences, and reactions dependent on third‐body deactivation. Also when reverse reactions are operative the establishment of the equilibrium can be directly followed. The thermal rate constants for the reactions of Kr+ with CH4, and CH4+ and CH3+ with CH4, were measured. The reaction of O2+ with O2 to form O4+ is found to be third order, proceeding with a rate constant K = 2.8 × 10−30cm6molecules−2·sec−1 at 298°K. The activation energy for the reaction is negative and equal to 1–2 kcal/mole. The equilibrium constants for the reaction O2+ + 202⇄O4 + +O2 we...

The reaction kinetics of gaseous hydrogen atoms with graphite.

Abstract: : The kinetics of reaction between solid graphite and gaseous atomic hydrogen was studied in the temperature range 450-1200K. The products of reaction are molecular hydrogen and methane. The rate exhibits an activation energy of 5.55 kcal/mole and is a function of the concentration of both hydrogen atoms and hydrogen molecules. Near 800K the rate goes through a maximum value, probably because of the thermodynamic instability of methane. A mechanism for the reaction is proposed. (Author)

The kinetics and mechanism of the hydration of magnesium oxide in a batch reactor

Abstract: The kinetics of the reaction between water and MgO powders has been investigated at 9°, 18°, 28° and 38°C in a stirred batch reactor. Six commercial samples of MgO prepared from Mg(OH)2 plus two of unknown origin, with specific surface areas of 12 to 80 mg/g, were studied. After correcting for particle size distribution, the rate of reaction was found to be directly proportional to the surface area contained in a shell at the surface of the particles. The activation energy is 14.1 ± 0.2 kcal/g-mole indicating chemical reaction as the rate controlling process. A reaction mechanism has been proposed./

Kinetics of the oxidation of sulfur dioxide by aerosols of manganese sulfate

Abstract: On the basis of a four-step reaction a mechanism is proposed for the catalytic oxidation of SO/sub 2/ by aqueous aerosols of MnSO/sub 4/. Data were generated to permit the evaluation of the rate constants. A computer solution of the reaction rates indicates that the proposed model adequately explains the kinetics of the gas-aerosol as well as the bulk phase situations. In principle the mechanism is applicable to reactions in which other metal-salt catalysts are involved.

Collision Dynamics and the Statistical Theories of Chemical Reactions. I. Average Cross Section from Transition‐State Theory

Abstract: By recasting the transition‐state‐theory formula for the reaction rate of a general bimolecular reaction and making use of Laplace transform techniques, an expression for the average reaction cross section as a function of the total system energy is derived; the result for three‐dimensional collisions have been given previously by Marcus. For the collinear model, a reaction probability replaces the cross section. Cross sections and probabilities for a number of special cases are given. It is shown that under certain conditions, improper high‐energy limits result. The relation of the present conclusions to unimolecular rate theory is indicated.

Reaction at Limited Water Concentration 1. Sucrose Hydrolysis

Abstract: SUMMARY —To enable development of a model describing reaction kinetics in dehydrated foods, we studied sucrose hydrolysis at limited water concentration. Saturated sucrose solutions containing various acids and inert solid materials gave identical rate constants and energy of activation as predicted from dilute solutions. Reaction rates in freeze-dried systems humidified to low moisture contents indicated that any equation describing the rate of hydrolysis must include a term for the velocity of dissolution of solid sucrose into the surface water. The rate of hydrolysis was a pseudo first-order reaction obeying the same kinetics as in dilute solution; the rate of dissolution became rate-limiting when the initial supply of dissolved sucrose was exhausted.

Potentiodynamic Examination of Electrode Kinetics for Electroactive Adsorbed Species: Applications to the Reduction of Noble Metal Surface Oxides

Abstract: The relation between reaction rate and potential (or time) for electrochemical surface processes occurring under potentiodynamic control (linear potential-time programme) has been investigated with particular reference to the behaviour of thin surface oxide films on noble metals. The kinetics of processes involving adsorbed electroactive species are treated for several model cases; the rate equations are developed for mechanisms involving various reaction orders or for processes involving adsorbed reactant interactions and surface heterogeneity effects. By examination of the dependence of the reaction rate (current) with time and the effect of potential scan rate, v , on the maximum reaction velocity and the potential at which it occurs, the models may be distinguished. In this manner, the inter­dependence of v and the reaction velocity constants k a and k c for the anodic oxidation and the cathodic reduction processes respectively, can be quantitatively established. The relation between quasi-equilibrium situations where the reverse reaction is significant and irreversible situations where it is not can be demonstrated. Heterogeneity terms introduced into the kinetic relations express deviations from Langmuir adsorption behaviour and may be an intrinsic property of the substrate surface or a property of the adsorbed reactant (induced heterogeneity). Applications of the treatment are made to reduction of surface oxide species at the noble metals and the significance of hysteresis and time effects in the processes of electrochemical formation and reduction of surface oxide at platinum, rhodium, iridium and palladium is investigated.

Kinetics of the Nitrous Oxide–Hydrogen Reaction

Abstract: The reaction of hydrogen with nitrous oxide was studied behind incident shock waves over the temperature range 1700°–2600°K. Experimentally, the concentration profiles of OH and the concentration of NO after the decomposition of N2O were measured by uv absorption. The OH profiles have qualitatively similar features to those determined in the H2–O2 reaction. The concentration of NO after the disappearance of N2O remains constant for several hundred microseconds. Interpretation of the experiments was based on a mechanism which contains the currently accepted data on the H2–O2 reaction and four additional reactions. Computer calculations show that the rate constants for the reaction O+N2O→ lim k32NO as cited in the current literature are too low. The magnitude of k3 determined in this work is consistent with published data on the reverse reaction, and k3 = 6.3 × 1014exp(− 26 700 / RT) cm3mole−1·sec−1 is believed to be correct to within a factor of 3 between 1700° and 4000°K. The present investigation also pe...

Atmospheric ozone: An analytic model for photochemistry in the presence of water vapor

Abstract: An approximate analytic model of ozone photochemistry including reactions with hydrogen compounds is developed for the atmospheric region between 15 and 60 km. The reaction scheme is a simplified version of that used by B. G. Hunt. The model is used to study time-dependent processes, the sensitivity of the equilibrium-concentration values to uncertainties in reaction rates, and the variations in the equilibrium ozone concentration with latitude and season. There are five main results. (1) Ozone loss in the model depends on the ratio of the rate of production of O(1D) and the rate of dissociation of H2O by O(1D), but the loss rate and equilibrium ozone concentration are not very sensitive to this ratio. (2) Below 40 km the model is quite sensitive to the rates of reaction of O3 with OH and HO2; these unknown reaction rates are the weakest link in the theory. (3) Above 40 km the relevant reaction rates are comparatively certain and the ozone concentration is very likely to be controlled by reactions of atomic oxygen with OH and HO2. (4) As a consequence of (3), the ozone concentration near the stratopause is probably not sensitive to temperature, and the dynamical damping sometimes attributed to this temperature sensitivity is likely to be unimportant. (5) If the model is correct, the photochemical time scale for ozone is much less than it would be if only oxygen reactions control ozone; consequently, in low latitudes, ozone may be subject to significant photochemical influence down to as low as 15 km.

Experimental Study of Reaction Kinetics by Light Scattering. I. The Polarized Rayleigh Component

Abstract: Spectral studies of scattered laser light have been conducted in multicomponent solutions where spontaneous fluctuations of the various molecular species occur about the chemical equilibrium. The spectral line broadening observed in these systems reflects the rate of return to equilibrium of these spontaneous fluctuations, and is characteristic of the reaction rates of chemical reactions occurring about the equilibrium. The reaction (Me2+(H2O)SO42−)Aq ⇆ (MeSO4)Aq has been singled out for investigation. Results of the studies on solutions of ZnSO4 and MnSO4 by the scanning Fabry–Perot interferometer and photon counting technique are in reasonable agreement with the data obtained by ultrasonic relaxation spectroscopy. The enthalpies of activation calculated for the process are: ZnSO4, ΔHb‡ = 3.72 kcal/mole; and MnSO4, ΔHb‡ = 6.15 kcal/mole.

Catalysis by Electron Donor-Acceptor Complexes

Abstract: Publisher Summary This chapter provides an overview of the mechanism of hydrogen exchange or hydrogenation over the electron donor–acceptor (EDA) complexes of alkali metals, and of organic electron donor molecules, such as phenothiazine. The hydrogen exchange reaction takes place between D2 or C2D2 and various EDA complexes, and that the H2–D2 exchange reaction to form HD proceeds at a considerable rate over the complexes, while no reaction takes place in the absence of alkali metals even at 200°. A closed circulating system was employed to follow the reaction rate. The reactor was a U-shaped glass tube equipped with two side-arms, each containing electron acceptor (phthalocyanine) and donor (sodium), respectively. When, C2D2 (15 cm Hg) was admitted to the EDA complexes of various phthalocyanines with organic electron acceptors such as 2,3-dicyanoquinone at temperatures between 25 and 90°, the hydrogen exchange reactions of acetylene proceeded at a negligible rate and the components of the complexes were separated from each other above 100° as a result of sublimation.

Ion–Molecule Reactions in Aromatic Systems. I. Secondary Ions and Reaction Rates in Benzene

Abstract: The reactions between benzene and the ions obtained by electron bombardment of benzene have been investigated in a pulsed ion source mass spectrometer. The major secondary ions were found to be C12H11, C12H10+, C12H9+, C12H8+, C11H7+, C10H9+, C10H8+, C10H7+, C9H7+, and C8H7+. Ionization‐efficiency curves were measured for parent and product ions, employing the retarding potential difference (RPD) method. The parent ions leading to most of the product ions were identified. The secondary ions appear similar to ions formed by electron bombardment of biphenyl and related compounds. The ion–molecule reaction rate constants were evaluated for benzene and hexadeuterobenzene. The rate constants varied between 2% and 30% of the values predicted by the Stevenson–Gioumousis theory of ion–molecule reactions. Inverse isotope effects were observed for “condensation” reactions, while normal effects were obtained for most of the other reactions. Variation of the translational energy of the reactant ions in a conventional...

Relative Rates of Reaction of Hydrogen Atoms with Olefins

Abstract: A new technique has been developed and used to redetermine the rates of H atom reactions at room temperature with a number of simple olefins. Hydrogen atoms are generated by the mercury photosensitized decomposition of H2 in the presence of small amounts of ethylene or its mixtures with an added olefin. The relative rate constants are determined from a comparison of the yields of n‐butane in the two cases. The values obtained are compared with the earlier determinations and with another independent recent redetermination in this laboratory. The values obtained by the three independent techniques are in very good agreement.

The preparation of iron powders consisting of submicroscopic elongated particles by pseudomorphic reduction of iron oxides

Abstract: The preparation of submicroscopic elongated iron particles by pseudomorphic reduction of α-FeOOH with hydrogen has been studied. At a temperature of 350°C rapid sintering occurs of the iron particles formed. At temperatures below 300°C sintering is largely absent but the reaction is too slow. It has been found that the reduction rate at low temperatures can be increased, and hence sintering diminished, by absorption of Ag+ or Co+ ions on the surface of the iron oxide particles, leading to iron powders with improved magnetic properties. When Sn2+is adsorbed the reaction rate decreases. Yet the coercivity of the iron powders obtained is considerably increased, probably also due to a diminished sintering.

Competition of two substrates for a single enzyme. A simple kinetic theorem exemplified by a hydroxy steroid dehydrogenase reaction.

Abstract: 1. If two compounds are substrates for a single enzyme, and do not form any ternary complex with the enzyme or combine directly with each other, then the total initial rate of reaction for a mixture of the two compounds may be greater than the rate for either compound alone, or may lie between the rates for the compounds alone. It is the concentration of the compound with the higher maximum velocity that determines which applies, and there is one concentration of the compound of higher maximum velocity at which the total rate of reaction is independent of the presence or absence of the substrate of lower maximum velocity. The values concerned are derived. 2. An example is given of 5α-androstan-3-one and 5α-androstane-3,16-dione as substrates competing for a hydroxy steroid–NAD oxidoreductase (EC 1.1.1.53).