Showing papers on "Elementary reaction published in 1975"

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...

Absolute rate constants for the reactions O(3P) atoms with HCl and HBr

Abstract: A flow tube method has been used to determine rate constants for the elementary reactions: Oxygen atoms were produced by adding a small excess of NO to a stream of partially dissociated nitrogen, and their reaction with hydrogen halide was monitored by observing the intensity of the NO + O afterglow Experiments were carried out at temperatures from 293 to 440°K with HCl, and from 267 to 430°K with HBr The role of secondary reactions was minimised and the residual effects were allowed for The rate constants for the primary reactions could be matched by Arrhenius expressions: where the units are cm3/molec·sec and the errors correspond to a standard deviation

Mass spectrometric determinations of the rates of elementary reactions of NO and of NO2 with ground state N4S atoms

Abstract: Kinetic studies of the reactions of NO, (1) and of NO2, (2), with N 4S atoms have been made using direct mass spectrometric detection of N atoms in a discharge flow system. The rate constant k1(cm3 molecule–1 s–1) for the rapid reaction (1), N + NO [graphic omitted] N2+ O (1), has been determined with pseudo first-order kinetic analysis ([NO]0/[N]0 1). The mean value for k1 was (2.2 ± 0.2)× 10–11 at 298 K, and between 298 and 670 K, k1 was given by the expression (8.2 ± 1.4)× 10–11 exp[–(410 ± 120) K/T].Similar kinetic studies of the N + NO2 reaction, using pseudo first-order analysis with very large excesses of NO2([NO2]0/[N]0 > 80), showed the rate constant for this reaction to be an order of magnitude less than the literature value. However, at lower values of [NO2]0/[N]0, much greater apparent rate constants for the N + NO2 reaction were obtained, similar to those found previously. These high values are attributed to a rapid catalytic cycle capable of removing both N atoms and NO2, i.e., N + NO2 [graphic omitted] N2O + O (2A), O + NO2 [graphic omitted] NO + O2(3), N + NO [graphic omitted] N2+ O (1). Reactions (1)+(3) have the stoichiometry, N + NO2→ N2+ O2: No evidence was found from N2O yields in the N + NO2 reaction for any reactive channel involving N + NO2 other than reaction (2A). The results give a mean value for k2A equal to (1.4 ± 0.2)× 10–12 cm3 molecule–1 s–1 at 298 K.

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).

Kinetic studies for diatomic free radicals using mass spectrometry. Part 3.—Elementary reactions involving BrO X2Π radicals

Abstract: Elementary reactions involving BrO X2Π radicals have been studied at 298 K in a discharge-flow system linked to a mass spectrometer with collision-free sampling. Detection of BrO was made directly by means of the BrO+ ion current, calibrated in terms of absolute BrO concentrations using the rapid reaction (1), NO + BrO [graphic omitted] NO2+Br.(1) Rate constants at 298 K (cm3 molecule–1 s–1) are reported for the second order decay reaction (2), BrO + BrO [graphic omitted] 2 Br + O2(2) as well as for reaction (1): k1=(2.2 ± 0.4)× 10–11; k2=(6.4 ± 1.3)× 10–12 cm3 molecule s–1. In addition, the rate constant k3 for reaction of Br 2P3/2 atoms with O3(3), has been determined at 298 K: Br + O3 [graghic omitted] BrO + O2(3), k3=(1.2 ± 0.2)× 10–12 cm3 molecule–1 s–1.

Flame structure studies of CF3Br-Inhibited methane flames: II. Kinetics and mechanisms

Abstract: Composition profiles for atomic, radical, and stable species, as well as temperature and area expansion ratio profiles, have been determined for a nearly stoichiometric CH4−O2−Ar flame and for one to which 0.3% CF3Br inhibitor had been added. Net reaction rate profiles were calculated for all the observed species. For the normal flame, these and the mole fraction profiles gave rate coefficient information about the elementary reactions in the methane flame, viz., C H 3 + O → 4 H 2 C O + H , k 4 = 1.05 × 10 14 c m 3 m o l e − 1 sec ⁡ − 1 for 1350≤T≤1750°K. Comparison between the inhibited and normal flame showed that [H] and [CH3] were significantly reduced at the lower temperatures in the inhibited flame even though in the hot gas region the [H], [OH], and [O] were the same in both flames. The CF3Br disappears very early in the flame, relative to the fuel, and the reaction primarily responsible for its disappearance is H + C F 3 B r → 7 H B r + C F 3 where k7 is found to be 2.2×1014 exp (−9460/RT), 700–1550°K. Reaction, of the inhibitor with methyl radicals provides for the relatively small amounts of CH3Br observed, but CH3+Br2→CH3Br+Br must also occur. The HBr formed reacts rapidly with H atoms to form H2 and Br, but the reaction is soon “balanced” in the flame as demonstrated by calculation of the equilibrium constant at various temperatures. The fluorocarbon fragment produced in reaction (7) also reacts rapidly, in part with methyl radicals to give the observed elimination product CH2CF2. The magnitude of the net reaction rate for both HF and F2CO early in the flame indicates that these, too, are formed by rapid reactions involving CF3. Later in the flame, above ∼1400°K, F2CO is formed from the reaction C H 2 C F 2 + O → 18 F 2 C O + C H 2 and k18∼1.5×1013 at 1600°K. The rather slow decay of carbonyl fluoride is attributed to reaction with H atoms, and the sequence F2CO+H→HF+FCO and FCO+H→HF+CO plus reaction (6) provides an additional radical recombination route in the inhibited flame.

Rate constant of the elementary reaction of carbon monoxide with hydroxyl radical

Abstract: Using a supersonic molecular beam sampling technique coupled with a mass spectrometer, the concentrations of all stable and unstable species have been measured in the reaction zone of a lean carbon monoxide-hydrogen-oxygen flame (9.4%CO, 11.4%H2, 79.2%O2) burning at 40 Torr. Reaction (1) CO+OH→CO2+H is the main process for CO conversion to CO2. From radical concentration profiles, it was determined that reaction (4) CO+HO2→CO2+OH is negligible as compared to (1). The rate constant k1 was determined from the CO2 mole fluxes over a large temperature range (400°–1800°K). The experimental data exhibit a marked and significant curvature in the plot of logk1 vs 1/T. From 400° to 800°K, k1 (8×1010 cm3 mole−1s−1) increases only slightly but above 1000°K the Arrhenius expression k1=2.32×1012 exp (−5700/RT) cm3 mole−1s−1 up to 1800°K. The rate constant of reaction (9) H2+OH→H2O+H was determined similarly and found to be 7×1012 exp (−4400/RT) cm3 mole−1s−1 in the temperature range of 600° to 1300°K. A curvature, less pronounced than for k1, was observed.

Kinetics of the hydrogen atom reactions with benzene, cyclohexadiene and cyclohexene: hydrogenation mechanism and ring cleavage

Abstract: The hydrogen atom reaction with benzene and the subsequent elementary reactions with H-atoms were studied in detail, using a fast gas flow in a linear reactor at pressures in the mbar region, with a mass spectrometer for the product analysis. The rate-constant determinations were based on a kinetic model, which includes the strong catalytic H-atom recombination on the wall, caused by adsorbed reactant molecules, and also corrects for the pressure drop within the reactor. The H-atom concentration was determined by scavenging with NO2. The method was checked by determining the rate constant k(H + trans-butene-2) = (4.6 ± 1.2) × 108 M−1 s−1, which agrees with the literature value of Daby et al. within experimental error limits. The rate constants determined are: H + benzene is the rate determining step for benzene hydrogenation. From the rate constant for H + C6D6 it is concluded, that benzene is reformed from some intermediate reaction products (C6H*7 and/or C6H*8). These back reactions should be suppressed at high pressures, in agreement with results by Sauer and Ward (1–54 bar). The mass spectra show that H + benzene at mbar pressures predominantly initiates ring cleavage to form methyl radicals, methane, and C2-hydrocarbons as the main products. However for H + cyclohexene 85% of the products is cyclohexane. The results for H + cyclohexadiene are intermediate to these extremes. It is argued that accumulation of vibrational energy over two consecutive reactions must be responsible for the ring cleavage, which most likely occurs from C6H**8 and C6H**10.

A Shock Tube Study of the Thermal Decomposition of Nitric Oxide

Abstract: Nitric oxide decomposition has been studied in a shock tube, time-of-fiight mass spectrometer system at 2700 to 4700 K and 1.5 to 3.5 atm using neon as diluent. The overall decomposition rate was found to be second order in NO concentration and in good agreement with previously reported rates. N2, O2 and O-atoms were the only observed reaction products. The concentration-time profiles of the observed species lend support to a mechanism of primary NO decomposition to N2 and O2 with slower decomposition to N2O and O-atoms. A mathematical simulation of the reaction consisting of eight elementary reactions was deduced by fitting experimental data to simulated concentration-time profiles.

Effects of Kinetic Mechanism on Diffusion-Controlled Structure of Hydrogen-Halogen Reaction Zones

Abstract: Some theoretical computations are completed, illustrating possible influences of reaction mechanism on the structure of the reaction zone, for reactions which occur by a straight-chain process. The occurrence of single and of double reaction fronts is identified. It is demonstrated that while a double front may occur for hydrogen-fluorine systems, in the case of the hydrogen-bromine reaction only a single front is obtained.

Energy partitioning in the products of elementary reactions involving OH-radicals

Abstract: An experimental program is described in which we looked for vibrational excitation in the products of the reactions (1) OH+H 2 →H 2 O † +H, (2) OH+CO→CO 2 † +H, and (3) OH+OH→H 2 O † +O. No infrared emission was observed in any of the potentially radiating fundamental vibrational modes or strong combination bands of CO 2 or H 2 O by our calibrated detection system, thus enabling upper bounds to be placed on the amount of vibrational excitation occurring in these products. Our results, expressed as a bound on the percent of the exothermicity which could have been deposited in the vibrational states of interest, are as follows: Reaction (1)−H 2 O (2.7μ)≤11%, H 2 O (6.3μ)≤18%; Reaction (2)−CO 2 (2.7 μ) ≤3%, CO 2 (4.3μ)≤0.6%; Reaction (3)−H 2 O (2.7μ)≤2%.

Chemical reactions in shock waves

Abstract: : Experimental and computational studies were carried out on a number of high-temperature chemical reactions. The experiments were done using a variety of spectroscopic techniques, coupled with shock-wave initiation of reaction. The principal results comprise mechanism explorations and determinations of elementary reaction rate constants among the reaction species H2, CO, CO2, O2, D2, C2H2, and CH4. In the course of obtaining these results, several significant advances in the technology of shock-tube experiments and in computer simulation of reactive flow were achieved. Discoveries of particular note include coupling between vibrational excitation and reaction rate for several bimolecular reactions, non-Arrhenius behavior of rate constant expressions, and participation of radical-radical processes in decomposition and isotope exchange reactions.