Showing papers on "Reaction rate published in 1973"

Kinetics and thermal characterization of thermoset cure

Abstract: The differential scanning calorimeter (Perkin-Elmer DSC-1) is used to characterize the cure of a general-purpose polyester during isothermal and scanning experiments. The technique is based on a new proposed model for the kinetics of isothermal cure. The model yields results which are in good agreement with experimental isothermal rate of reaction and integral heat of reaction data. It also gives some information about the residual reactivity of the sample after an isothermal cure experiment. With the aid of the proposed kinetic model, it is possible to obtain integral heats of reaction and rates of heat generation at different temperatures during a scanning experiment. The difference between the rate of heat input to the sample and the heat of reaction at any instant during scanning may be used to calculate the specific heat of the sample at the same instant. Specific heat data show two maxima during each scanning experiment. These maxima may be associated with transitions occurring during cure in the melt and rubbery states.

Effect of changing reagent energy on reaction probability and product energy-distribution

Abstract: Experiment and theory have been used to ascertain the effect of various forms of reagent energy on the dynamics of three reactions; (a) F + HCl → HF + Cl, (b) F + D2→ DF + D and (c) H + Cl2→ HCl + Cl. The principal experimental technique was the arrested relaxation form of the infra-red chemiluminescence method, coupled with a quite wide variation in the temperature of the atomic or the molecular reagent. In addition, a novel approach was employed in which the “fluorescence depletion” of specified vibrational-rotational (v, J) states was measured, and hence the relative rate of reaction of these quantum states was obtained. The theoretical technique was the 3D classical trajectory method using LEPS (London, Eyring, Polanyi, Sato) potential-energy hypersurfaces. Some support was obtained from experiment and theory for the following generalisations. (1)ΔT and ΔV enhance the reaction rate-constant, but the former is more effective for these substantially-exothermic reactions. (ΔT and ΔV denote reagent translation and vibration enhancements, in excess of the activation-barrier). (2) On the average ΔT→ΔT′+ΔR′(the primed energies refer to reaction products; ΔR′ is enhancement in product rotational energy). (3) On the average ΔV→ΔV′. (1)–(3) are in accord with findings from earlier trajectory studies. The effect of substantial increase in collision-energy can be understood in terms of a contribution from “induced repulsive energy-release”, and that of increased reagent vibrational energy in terms of both induced repulsive energy-release and (more important)“induced attractive energy-release”. For the reaction F + HCl the increase in rate constant with reagent vibrational quantum number, k(v), was obtained for ν= 0–2, experimentally and theoretically. The fluorescence-depletion method was used for a preliminary measurement of the rotational dependence of reaction rate, k(J), in F + HCl(ν= 1). An estimate was obtained for the variation in threshold collision-energy, Ev′J′0, with the v′J′ state being formed in the reaction F + HCl(v= 0).

Displacement reactions in the solid state

Abstract: Simple displacement reactions in the solid state are considered with the purpose of predict-ing the morphologies and the reaction rates from a knowledge of pertinent thermodynamic and diffusion data. The theoretical predictions are substantiated by experimental observa-tions for four reaction couples [Ni/Cu2O, Co/Cu2O, Fe/Cu2O, and Fe/NiO] reacted at 1000°C. Displacement reactions are classified according to the product morphology; layered and ag-gregate arrangements of the product phases were observed, with two modifications (lamellar and interwoven) occurring within the aggregate morphological class. Parabolic kinetics for the growth of the product phases are observed for each couple. The magnitudes of the para-bolic rate constants for the couples which exhibit the layered arrangement are comparable with calculated values. A technique for controlling the product morphology is discussed, and a process for producing porous metal or oxide screens is introduced.

Kinetics of diffusion-controlled reaction between chemically asymmetric molecules. II. Approximate steady-state solution

Abstract: The previously published equation for the rate of a diffusion-limited bimolecular reaction between chemically asymmetric molecules is studied numerically for the case that one of the reactant molecules is uniform. The results are reproduced quite well by a simple approximate chemical-kinetic steady-state scheme and, in principle, allow estimates of the size of the reactive region and of the activation-controlled rate to be made from the observed dependence of rate on solvent viscosity. The simple scheme is easily generalized to the case of two nonuniform reactants. In general, restriction of reactivity to some fraction of the molecular surface (i.e., a steric factor) must reduce the observable reaction rate, but to an extent which is moderated by the rotational diffusion of the reactant molecules.

Flow‐drift technique for ion mobility and ion‐molecule reaction rate constant measurements. III. Negative ion reactions of O− with CO, NO, H2, and D2

Abstract: A new combined flowing afterglow‐drift tube experimental appartus has been used to measure the reaction rate constants as a function of ion kinetic energy for the following negative ion reactions: O−+CO→ CO2+e, O−+NO→ NO2+e, O−+H2→ H2O+e → OH−H, O−+D2→ D2O+e → OD−+D. Reaction (1) has been measured from thermal energy to ∼ 3 eV, (2) from thermal to ∼ 1.0 eV, (3) from thermal to ∼ 0.5 eV, and (4) from thermal to ∼ 0.8 eV. Reactions (1) and (2) are found to have rate constants that decrease by an order of magnitude with increasing ion kinetic energy, while Reactions (3) and (4) have total rate constants independent of energy. The channels (3b) and (4b) increase markedly with energy at the expense of the associative‐detachment channels (3a) and (4a). The present results are compared with existing available data on these reactions.

Reaction kinetics of tungsten thin films on silicon (100) surfaces

Abstract: The rate of reaction between Si (100) surfaces and tungsten films deposited by rf diode sputtering depends on the preparation of the silicon surface. If rf substrate bias is used to clean the silicon, then the rate of reaction in the temperature range 700–850 °C is independent of time, with an activation energy of 3 eV/mole W. The native oxide layer between the silicon and tungsten, that exists when sputter cleaning is not used, can act as a barrier to WSi2 formation. In this case, the time‐independent region is preceded by a period when the reaction rate increases with time. The rate is then controlled by two‐dimensional spreading of discontinuous WSi2 regions that originate at sites where the reaction barrier can be penetrated. After a continuous WSi2 layer is formed, additional growth can produce a stage where the increased path length for silicon diffusion causes the transport step to control the over‐all rate of the reaction. Quantitative models are presented for each of the three stages in the reaction. The models explain some of the macroscopic observations made on reacted layers.

The influence of fine structure on the pyrolysis of cellulose. I. Vacuum pyrolysis

Abstract: The effects of crystallinity, orientation and degree of polymerization on the vacuum pyrolysis of cellulose have been investigated. Natural, modified and manmade celluloses were characterized and the kinetics of their pyrolysis at 251°C studied. The high-temperature pyrolysis of these samples was also investigated by means of DSC and TGA. At 251°C all of the samples showed a rapid, initial weight loss followed by a linear (weight loss)1/2 versus time dependency. Both the initial weight loss and the subsequent reaction rate were dependent upon the crystallinity of the sample. The reaction rate was shown to be inversely proportional to (DP)1/2 and related to the orientation. Effects of crystallinity and orientation could also be seen in the DSC thermograms and in the apparent energies of activation. Results are interpreted in terms of probable mechanisms of the uncatalyzed pyrolysis of cellulose.

Classical dynamical investigations of reaction mechanism in three‐body hydrogen‐halogen systems

Abstract: A theoretical investigation of the reaction mechanisms in eight different three‐body hydrogen‐halogen reactions has been accomplished by means of Monte Carlo averages over classical trajectories computed on reasonable semiempirical potential‐energy surfaces. Two of the surfaces employed contain a single adjustable parameter characteristic of a halogen core charge while six others result from unadjusted computations. The analysis shows that dynamic effects resulting from momentum transfer require the reactions M2+X=MX+M (M=H,D, or T; X=Br or I) to proceed through excited vibrational states of M2. Reaction of the ground vibrational state is dynamically forbidden in each case. Reaction rate coefficients, associated activation energies, pre‐exponential factors, and kinetic isotope effects calculated from rotationally averaged reaction cross sections are in reasonable agreement with experiment. The computations further suggest an origin for the isotope effect that is significantly different from that derived f...

Kinetics of Nitridation of Si Powder Compacts

Abstract: The kinetics of the reaction of Si powder compacts and N2 to form Si3N4 were investigated thermogravimetrically from 1300° to 1450°C. The results indicate that trace amounts of O2 inhibit the rate of nitridation by forming a thin protective layer of SiO2, whereas Fe impurities catalytically accelerate the rate of reaction by liquid-phase formation. Complicating factors that account for difficulties in interpreting the reaction kinetics and for the poor agreement among previous investigations are discussed.

Kinetics of the uranium‐hydrogen system

Abstract: The reaction kinetics of U + 3/2 H2⇄UH3 was studied with fine powders on an ultrahigh‐vacuum microbalance. The kinetics were found to be first order with respect to uranium for the hydriding reaction and nearly zero order with respect to uranium hydride for dehydriding. A new reaction mechanism is proposed to account for the experimental observations which involves diffusion in the reactant phase by hydrogen before nucleation to form the hydride phase. This reactant phase diffusion and phase transformation reaction with its associated solid state mathematics accounts for all the observations made to date on the kinetics of this system.

Quantum Low-Temperature Limit of a Chemical Reaction Rate

Abstract: The radiation-induced polymerization of formaldehyde has been studied in the solid state. The time of addition of one new link to a polymer chain increases exponentially in accordance with the Arrhenius law at 140 to 80 K, but approaches a constant value (approximately 10 -2 second) at temperatures below 10 K. Thus, a low-temperature limit to a chemical reaction rate has been observed. It is interpreted as a quantum effect caused by tunneling from the zero vibration level of the initial state, and a semiquantitative theory is given. The phenomenon should be taken into account for understanding tunneling of electrons in biological systems when such tunneling is accompanied by conformational changes. It could also be significant in slow, exothermic chemical reactions at low and ultralow temperatures, which may have had a role in chemical and biological evolution (cold prehistory of life?).

Theory of reaction rates as based on the stoichiometric number concept

Abstract: The order of elementary processes bears an obvious relation to the total number of participants, insofar as they behave individually, statistically independent except at the moment of the elementary process; the elementary process of first, second, or third order is thus that of one, two, or three participants, respectively, and so on. This rule has proved to be a useful key for the analysis of homogeneous reactions that are favored with statistical independence of the participants. That is, however, often carelessly applied to heterogeneous reactions and encounters difficulties. Elementary processes are termed steps, and the complete set of its participants at the state before or after the step are the initiai or the final system; overall reactions are each considered to be composed of steps. The hydrogen electrode reaction

Kinetics of the reaction of hydroxyl radicals with ethylene and with C3 hydrocarbons

Abstract: The rates of reaction of hydroxyl radicals with ethylene, propane, propylene, methylacetylene, and allene have been measured at room temperature in a discharge-flow system using electron spin resonance detection. The stoichiometries (n=Δ[OH]/Δ[R]) were obtained by mass spectrometric analysis of the reacted gas under similar, although not completely identical conditions. The primary rate constants for the C3-hydrocarbons obtained by combining the two are given as: OH + propane k6=(5.0 ± 1.0)× 1011 cm3 mol–1 s–1+ propylene k7=(3.0 ± 1.0)× 1012 cm3 mol–1 s–1+ methylacetylene k8=(5.7 ± 1.0)× 1011 cm3 mol–1 s–1+ allene k9=(2.7 ± 1.5)× 1012 cm3 mol–1 s–1. The values of n as well as the nature of the products provide some information on the mechanisms involved.A value of k5=(1 ± 0.3)× 1012 cm3 mol–1 s–1 was obtained for the reaction of OH + ethylene.

Formation of Polysulfides in Aqueous Solution

Abstract: Polysulfides are found to be the reaction intermediates in the oxygenation of aqueous sulfide. The pH dependence of the reaction rates is directly related to the formation of polysulfides. In general, the peak of polysulfide concentrations fall within the range pH 7.0 ± 0.4, and decrease drastically in both acidic and alkaline solutions. Under one atmospheric pressure of oxygen and sulfide concentrations of 10-3 to 10-2M, the corresponding maximum yield of polysulfide conversion varies from 14% to 8%. Usually, for solutions ranging from slightly acidic to neutral, the peak of polysulfide formations comes within half an hour to three hours after the preparation of the reaction mixture. Polysulfides disappear thereafter to form sulfur and sulfur oxyanions. The results confirm a previously proposed reaction mechanism on the oxygenation of sulfide in neutral solution environment involving sulfide, polysulfide, sulfur and sulfur oxyanions.

Atomic hydrogen concentrations in the mesosphere and the hydroxyl emissions

Abstract: It is generally accepted that the hydroxyl emissions observed in the airglow spectrum originate from the hydrogen-ozone reaction. Hence it is logical to use the observed emission intensities to infer the atomic hydrogen concentrations in the mesosphere. The volume reaction rate between O3 and hydrogen can be calculated from the measured intensity of a particular band if the vibrational spectrum of the complete Meinel system is known. This spectrum has recently been calculated by Evans and Llewellyn [1972];the same calculation has also indicated that 4.8 photons are produced from each reaction between H and O3.

Immobilized glucoamylase on porous glass

Abstract: A study was made to determine the controlling mass transfer resistance in the overall reaction rate for conversion of maltose to glucose, catalyzed by glucoamylase immobilized onto porous glass. For normal operation of a packed column and air-stirred batch reactor, the rate controlling step was found to be the internal resistance of simultaneous pore diffusion and chemical reaction. Experimental effectiveness factors were determined and are compared with those derived from a theoretical diffusion model based on Michaelis-Menten kinetics. Also given are temperature and pH relationships for the free and immobilized glucoamylase.

Variational theory of reaction rates: Application to F+H2⇄HF+H

Abstract: The classical variational theory of reaction rates is coupled with a Monte Carlo trajectory analysis to yield reaction rates and energy distributions of reactants and products for the forward and reverse reactions: F+H2⇄HF+H at 300°K. The results are compared with those of a standard quasiclassical trajectory analysis of the forward reaction using the same potential energy surface. Differences are attributed to the quantized distributions of reactants in the quasiclassical calculations. The combination of the two methods allows a substantial reduction in computation requirements for determining the characteristics of reactions in systems with reactants in thermal equilibrium.