Showing papers on "Reaction rate constant published in 1971"

Reactions of Metastable Nitrogen Atoms

Abstract: The line absorption technique was applied to the kinetic study of the two metastable atomic nitrogen states N (22D) and N (22P) in a flowing afterglow system. The optical absorptions of the NI 1493‐A (2p32D−3s2P) and 1743‐A (2p32D−3s2P) transitions were used for the quantitative measurement of N (22D) and N (22P) concentrations. Deactivation of N (2D) and N (2P) by the Pyrex tube wall was found to be very efficient, i.e., occurs at nearly every collision. The second‐order rate constants at 300°K for the removal of N (2D) by O2, N2O, CO2, NO, N2, Ar, and He were found to be (6±2)×10−12, (3.5±1.2)×10−12, (5±2)×10−13, (7±2.5)×10−11, (1.6±0.7)×10−14, (1±0.6)×10−16, and ≤ 1.6×10−16 cm3 sec−1, respectively. It was established that the process for the first three reactant gases results in chemical reaction rather than physical quenching.

The oxidation of TD NiC (Ni-20Cr-2 vol pct ThO2) between 900° and 1200°C

Abstract: The oxidation of TD NiC (Ni-20Cr-2 vol pct ThO2) has been studied at temperatures between 900° and 1200°C. Chromium is selectively oxidized in TD NiC and the parabolic rate constants for the growth of the resulting Cr2O3 layer are about an order of magnitude lower than those for the growth of Cr2O3 on a simple Ni-Cr alloy. The oxidation rate of TD NiC is initially controlled by the interplay of diffusion through the Cr2O3 scale and the formation of gaseous CrO3 but after extended periods of oxidation the rate corresponds to that of the vaporization process. Comparison of results obtained for the oxidation of TD NiC to those for Ni-Cr alloys indicates that the ThO2 dispersion inTD NiC improves the oxidation properties of this alloy by promoting the selective oxidation of chromium and decreasing the growth rate of the Cr2O3 scale.

Kinetics of Diffusion‐Controlled Reaction between Chemically Asymmetric Molecules. I. General Theory

Abstract: The treatment of diffusion‐controlled bimolecular chemical reactions is extended to molecules with axially symmetrical distributions of reactivity on their spherical surfaces. Whereas an explicit solution exists for the time‐dependent effective rate constant at small times, the physically more interesting solution at large times can be obtained only approximately (though to any required degree of accuracy) by solving sets of linear equations.

Quantum mechanical computational studies of chemical reactions: I. Close-coupling method for the collinear H + H2 reaction

Abstract: A fully converged, close-coupling calculation, using natural collision coordinates, is presented for the collinear H + H2 system, on a Porter-Karplus potential energy surface in the energy range of 9 to 35 kcal/mole. Eight closed channels were required for convergence in the threshold region. Inclusion of the closed channels alters considerably the numerical results. The effects of the vibrational non-adiabatic coupling terms were demonstrated. Particularly pronounced is the influence of the (energy dependent) inertial centrifugal energy due to the curvature of the reaction path. No evidence for tunnelling was obtained. The considerable difference between the quantal and classical reaction probabilities in the threshold energy region is entirely due to the non-adiabatic coupling terms, which tend to reduce the kinetic energy available along the reaction path, more so in the classical case, thereby leading to a higher dynamic threshold. The Arrhenius plot of the collinear rate constant shows more curvature...

Characteristics of Acid Reaction in Limestone Formations

Abstract: A kinetic model for the reaction of hydrochloric acid with limestone has been determined. Reaction order and rate constant for this model were calculated from experiments where acid reacted with a single calcium carbonate plate. Experiments were performed so that acid flow past the plate and mass transfer rate to the rock surface could be calculated theoretically. The resulting model, therefore, accurately represents the acid reaction process at the rock surface and is independent of mass-transfer rate. Combination of this model with existing theory allows prediction of acid reaction during acid-fracturing operations. A model for acid reaction in wormholes created during matrix acidization treatments is presented along with data for reaction of hydrochloric, formic, and acetic acids in a wormhole. Factors limiting stimulation in acid fracturing or matrix acidizing treatments are discussed. (27 refs.)

Pulse radiolysis studies of electron transfer in aqueous quinone solutions

Abstract: The transient absorption spectra observed on pulse radiolysis of neutral aqueous solutions containing p-benzoquinone and excess methanol, ethanol, isopropanol and formate, saturated with nitrous oxide are reported. The spectra are analogous to those obtained in solutions containing excess t-butanol and N2, O2, nicotinamide adenine dinucleotide or thymine and are attributed to the semiquinone radical-anion formed by electron transfer reactions with bimolecular rate constants k=ca. 109-1010 M–1 s–1. This assignment is supported by measurements of the acid dissociation constant of the absorbing product (pKa= 4.1) in solutions containing excess acetone and isopropanol.

Measurements of Rate Constants for Termolecular Reactions of O(3P) with NO, O2, CO, N2, and CO2 Using a Pulsed Vacuum‐uv Photolysis—Chemiluminescent Method

Abstract: Absolute rate constants for a number of termolecular reactions of O atoms were determined at 300°K. Oxygen atoms were generated by pulsed vacuum‐uv photolyses of NO, O2, CO2, and N2O and were monitored by NO2* or CO2* chemiluminescent emission. The rate constants k, in units of cm6 molecule−2·second−1, for the following reactions are: O+NO+NO→NO2+NO, k=1.5×10−31 (± 10 %) O+NO+He→NO2+He, k=6.65×10−32 (± 10%) O+O2+O2→O3+O2, k=6.4×10−34 (± 25 %) O+O2+N2→O3+N2, k=5.4×10−34 (± 25 %) O+O2+CO→O3+CO, k=6.7×10−34 (± 25 %) O+CO+CO→CO2+CO, k=3.2×10−36 (± 25 %) O+CO+N2→CO2+N2, k=2.2×10−36 (± 25 %) O+CO+He→CO2+He, k=1.7×10−36 (± 25 %) O+CO2+CO→CO3+CO, k〈9×10−36 O+N2+N2→N2O+N2, k〈5×10−38.

Picosecond Pulse Radiolysis. III. Reaction Rates and Reduction in Yields of Hydrated Electrons

Abstract: Rate constants for reactions of the hydrated electron with a wide range of compounds, including simple salts and amino acids, have been measured in the time region 20–350 psec. In nearly all cases the second‐order rate constant for reaction with the hydrated electron did not change in the concentration range studied. However, the measured rate constant was in general different to that obtained in dilute solutions, and this was attributed to incomplete formation of the ionic atmosphere around the hydrated electron before reaction. Apart from the hydronium ion, Haq+, nearly all compounds decreased the initial hydrated electron yield: in all cases this decrease showed an exponential dependence on concentration. Compounds most efficient at decreasing the hydrated electron yield were cystine and cadmium salts, in both cases 0.39 mole/liter being needed to reduce the initial yield to 37%. The ability to reduce the initial yield showed no direct correlation with the corresponding hydrated electron rates. This an...

Initial vibrational energy distributions determined by infra-red chemiluminescence: I. The reaction of fluorine atoms with hydrogen and methane† This research has been sponsored in part by the Air Force Cambridge Research Laboratories through the European Office of Aerospace Research (OAR), United States Air Force.

Abstract: Infra-red emission has been detected from hydrogen fluoride formed in the gas-phase atom-molecule exchange reactions of atomic fluorine with hydrogen and methane. Emission was limited to that from vibrational levels v′ˇ-3 in each case. An extrapolation technique has been used to obtain relative rate constants for direct chemical population of the various vibrational levels. The values found were k 3tk2=0·76, k 2tk1=3·4 for the F + H2 reaction and k 3tk2=0·23, k 2 k 1=3·0 for the F + CH4 case.

The Interaction of Cyanide with Cytochrome Oxidase

Abstract: 1 The interaction of cyanide with the oxidised and reduced forms of cytochrome-c oxidase has been investigated by kinetic and equilibrium measurements at 20 °C and pH 7.4. The inhibition by cyanide of the oxidation of cytochrome c has also been studied under different conditions. 2 When the oxidised form of cytochrome oxidase is mixed with cyanide, the heme-absorption bands are changed extremely slowly in a process whose rate is independent of the concentrations of cyanide and protein. Thus, the primary binding site does not appear to be one of the cytochrome components of the oxidase. 3 With reduced cytochrome oxidase, the interaction can be described as a simple secondorder process involving the a32+ form and HCN. The stability constant for the complex is 1.8×103 M−1 and the rate constant for its formation 1.3X102M−1sec−1. 4 The inhibition of the enzyme may occur in two ways. One involves the reduction of the enzyme by cytochrome c and a subsequent reaction between a32+ and the inhibitor. However, if the oxidase is preincubated with cyanide, inhibition has been reported at much lower concentrations of cyanide. This cannot be related to the much slower changes in the heme-absorption bands observed in the reaction of cyanide with the oxidised form of the enzyme, and it is suggested that it involveds binding of cyanide to one of the copper ions in the oxidase.

Collisional Deactivation of O2(1Δg)

Abstract: This paper reports determinations of collisional deactivation rate constants for O2(1Δg) by minor atmospheric constituents and other gases. The benzene–oxygen photochemical system was used to produce O2(1Δg), and relative concentrations were measured by monitoring the intensity of its emission at 1.27 μ. The rate constants for the reaction O2(1Δg)+M→O2+M were 4.53 × 10−18, 5.60 × 10−18, and (0.5–1.0) × 10−19 (cm3 molecule−1·sec−1) for M=H2, H2O, and N2O, respectively. Upper limits were obtained for deactivation by He, Ar, CO2, and SF6 of 8.0 × 10−21, 8.3 × 10−21, 1.5 × 10−20, and 1.2 × 10−20, respectively. The temperature dependence of the deactivation of O2(1Δg) by oxygen has been investigated. The rate constant can be expressed in the form kO2 = 2.22 ± 0.09 × 10−18 (T / 300)n, where n = 0.78 ± 0.32 and T is the temperature in degrees Kelvin.