Rate Constants Of Chemical Reactions Research Articles

Applications of power-law transition state theory to the reaction of a hydroxyl radical with molecular hydrogen and the respiration in Camellia Japonica

The reaction of a hydroxyl radical with molecular hydrogen and the respiration in Camellia Japonica are used as examples for studying chemical reaction rate constants by the power-law transition state theory. In the first example, the rate constants are numerically obtained at different temperature and activation energies based on the Morse potential at the potential well and improved potential energy surface 5 (I5B) at the saddle point respectively, and the relationship between the rate constants and the power-law parameter is plotted as well. In the second one, especially, the power-law parameter presents an abrupt change at temperature 290 K around, which may be explained that the Camellia Japonica is the ombrophyte, and the activity of biological enzyme reaches the maximum at this temperature. This theory allows accordance with experimental values by a power-law parameter. It is found that the values of the power-law parameter in both examples deviate from 1 but not too much and lie in the 0–1 range and proved that power-law transition state theory shows a good agreement with experiments.

Electrochemical Oxidation of Catechols in the Presence of Triethyl Phosphite

The mechanism of electrochemical oxidation of catechol and some of its derivatives have been studied in the presence of triethyl phosphite as a nucleophile in aqueous solution. Voltammetric studies indicate that the quinones derived from catechol, and its derivatives, participate in Michael addition reaction with triethyl phosphite. The reaction mechanism consists of electron transfer followed by a chemical reaction which is named as an EC mechanism. The homogeneous rate constants (kobs) were estimated by comparing the experimental cyclic voltammograms with the digitally simulated voltammograms based on EC mechanism. Also the effects of nucleophile concentration and substituted group of catechols on voltammetric behavior and the rate constants of chemical reactions were examined.

Structure-reactivity relationships in terms of the condensed graphs of reactions

An approach for the prediction of rate constants of chemical reactions, based on the representation of a chemical reaction as a condensed graph, has been tested on more than 1000 bimolecular nucleophilic substitution reactions with neutral nucleophiles in 38 solvents. Molecular fragment descriptors, temperature, and solvent parameters characterizing solvation power have been used in the reaction modeling. The obtained models ensure a good correlation between the predicted and experimental values; the corresponding deviations are comparable with interlaboratory measurement errors.

Impedance Insight in Catalytic Properties of Long-Lived Free-Radical Oxidizer Based on Silver(II)

Novel functional materials are of significant relevance to the development of numerous industrial technologies such as electronics, renewable energy sources and environmental protection. Compounds of divalent silver are particularly interesting due to their unusual physicochemical properties. Recently we have found that the formal potential of the Ag(II)/Ag(I) redox couple can reach as much as 2.9V vs. NHE[1] in superacidic media, which makes this system the strongest fluorine free oxidizer known. Electrooxidation of silver(I) in sulfuric acid solutions proceeds according to EqCi’ mechanism, in which one-electron transfer leading to solvated Ag(II) species is followed by a homogeneous chemical step with regeneration of depolarizer. These results indicate that silver(I) oxidation in superacidic media leads to the formation of exceptionally strong oxidizing, and relatively long-lived, species containing Ag(II) [2]. These powerful free-radical Ag(II) oxidizers can be used for 1e– oxidation of various kinetically inert systems and in general for initialization of free radical reactions in superacidic media.Here we report electrochemical impedance studies of this interesting system [3]. Typical impedance spectrum is shown in Fig. 1. For data analysis we developed analytical model for impedance of catalytic processes together with potential dependence of all impedance parameters derived for general case in rigorous treatment [4]. The model is valid for various systems regardless the kinetics of the heterogeneous and homogeneous reactions and also for system with unequal diffusion coefficient. Resistance of the charge transfer was found to be potential independent at far anodic or far cathodic overpotentials for electrocatalytical oxidation and reduction, respectively. Resistance of the charge transfer in silver(I) oxidation is shown in Fig. 2 (top). The equivalent circuit for catalytic systems, in which the kinetics of the homogenous chemical step can be described by Gerisher element is shown in the inset to Fig. 1 (bottom). By fitting impedance spectra both the heterogeneous and the homogeneous rate constants for silver electrooxidation were obtained in various experimental conditions. We have found that the half-life of the free-radical oxidizer based on silver(II) is the largest in 100% sulfuric acid solutions (Fig.2 bottom) and it substantially decreases both for diluted sulfuric acid and as well as for oleums. The influence of water and SO3 on the silver(II) chemistry will be discussed.Fig. 1. Impedance spectra of Ag(I) oxidation in 18 M (95%) H2SO4. Green continuous line (−) represent the fit result of experimental points (◆) by using equivalent circuit shown in the inset.Fig. 2. (Top) Potential dependence of the charge transfer resistance for Ag(I) oxidation in 18 M(95%) H2SO4. (Bottom) Rate constant of homogenous chemical reaction in function of sulphuric acid concentration.

Zirconia Sensor Device for In Situ Monitoring of Metalpowder Oxidation for Energy Storage Applications

A zirconia oxygen sensor was designed to monitor the gas-solid oxidation reaction characteristics of metal/metal-oxide powder beds for energy storage applications. Energy is stored in the metallic state and released during oxidation. This in-situ monitoring device allows for determination of the oxidation state of the metal oxides during reactions in real time, as well as calculation of the chemical reaction rate constants. The signal output of the sensor was analyzed for oxidation of and diffusion into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the sensor. Two metal/metal-oxide systems were examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility, and the complex W/WO3 system was chosen to demonstrate versatility of the sensor for actual energy storage applications.

Zirconia Sensor Device for In Situ Monitoring of Metalpowder Oxidation for Energy Storage Applications

A zirconia oxygen sensor was designed to monitor the gas-solid oxidation reaction characteristics of metal/metal-oxide powder beds. This in-situ monitoring device allows for determination of the oxidation state of the metal oxides during reactions in real time, as well as calculation of the chemical reaction rate constants. The signal output of the sensor was analyzed for oxidation of and diffusion of oxygen into the powder bed. The oxygen transport mechanisms occurring inside the sensor were described to further understand the signal outputs from the powder bed. A model metal/metal-oxide system is examined using this device to demonstrate its performance. The simple Ni/NiO system was chosen to demonstrate feasibility.

Ab Initio Investigation of the Abstraction Reactions by H and D from Tetramethylsilane and Its Deuterated Substitutions

Thermal rate constants for chemical reactions using the corrections of zero curvature tunneling (ZCT) and of small curvature tunneling (SCT) methods are reported. The general procedure is implemented and used with high-quality ab initio computations and semiclassical reaction probabilities along the minimum energy path (MEP). The approach is based on a vibrational adiabatic reaction path and is applied to the H + Si(CH3)4 → H2 + Si(CH3)3CH2 reaction and its isotopically substituted variants. All of the degrees of freedom are optimized, and harmonic vibrational frequencies and zero-point energies are calculated at the MP2(full) level with the cc-pVTZ basis set. Single-point energies are calculated at a higher level of theory with the same basis set, namely, CCSD(T,full). The influence of the basis set superposition error (BSSE) on the energetics is tested. The method is further exploited to predict primary and secondary kinetic isotope effects (KIEs and SKIEs, respectively). Rate constants computed with the ZCT and SCT methods over a wide temperature range (180-2000 K) show important quantum tunneling effects at low temperatures when compared to rates obtained from the purely classical transition-state theory (TST) and from the canonical variational transition state theory (CVT). For the H + Si(CH3)4 reaction, they are given by the following expressions: k(TST/ZCT) = 9.47 × 10(-19) × T(2.65) exp(-2455.7/T) and k(CVT/SCT) = 7.81 × 10(-19) × T(2.61) exp[(2704.2/T) (in cm(3) molecule(-1) s(-1)). These calculated rates are in very good agreement with those from available experiments.

Cyclic multipulse voltammetric techniques. Part II: EC mechanism

A theoretical analysis of electrode processes controlled by kinetics of follow-up homogeneous chemical reaction under conditions of (simple) cyclic multi pulse voltammetry (CMPV), cyclic differential multi pulse voltammetry (CDMPV) and cyclic square-wave voltammetry (CSWV) is presented. The study was performed under linear diffusion conditions and for equal diffusion coefficients of all participants of the electrode process. Chemical reaction changes the shape and position of CMPV response, compared to a simple reversible electrode reaction. As the chemical process becomes faster, forward/reduction wave decreases whereas a pronounced maximum appears on it. On the other hand, the influence of following chemical reaction on the shape of CDMPV and CSWV reduction signal is not significant, while their voltammograms shifted towards more positive potentials as well. Experimentally, the rate constant of follow-up chemical reaction (kf) can be estimated from the linear dependence of limiting to maximum reduction currents ratio (il,c/imax,c) on log(tp) in CMPV, as well as of anodic to cathodic net peak currents ratio (Δip,a/Δip,c) on log(tp) in CDMPV and on log(f) in CSWV. Combination of two or more cyclic multipulse voltammetric techniques gives more information about kinetics i.e. more precise determination/estimation of the rate constant of follow-up chemical reaction, compared with application of only one technique.

A computational methodology for accurate predictions of rate constants in solution: Application to the assessment of primary antioxidant activity

The accurate prediction of rate constants for chemical reactions in solution, using computational methods, is a challenging task. In this work, a computational protocol designed to be a reliable tool in the study of radical-molecule reactions in solution is presented. It is referred to as quantum mechanics-based test for overall free radical scavenging activity (QM-ORSA) because it is mainly intended to provide a universal and quantitative way of evaluating the free radical scavenging activity of chemical compounds. That is, its primary antioxidant activity. However, it can also be successfully applied to obtain accurate kinetic data for other chemical reactions in solution. The QM-ORSA protocol has been validated by comparison with experimental results, and its uncertainties have been proven to be no larger than those arising from experiments. Further applications of QM-ORSA are expected to contribute increasing the kinetic data for free radical-molecule reactions relevant to oxidative stress, which is currently rather scarce.

The Approximation of the Chemical Reaction Rate by Solving the Integral Equation

The paper discusses possible methods of approximation of the chemical reaction rate con- stant for the range of values that lie outside of the experimental temperature range: direct approximation of chemical reaction rate constants obtained by pro- cessing experimental values; approximation based on an analytical model of dependence of the integrated process cross-section on energy; and approximation based on the direct solution of the chemical reaction rate constant equation with arbitrary dependence of the integrated process cross-section on energy. The second-order reactions CH4 + M ! CH3 + H + M, CH3 +M! CH2 +H +M, CH3 +M! CH +H2 +M were explored. To solve the integrated equation, the variational Tikhonov's regularization method was used. It was shown that this method allowed both estimating the threshold energy value and re-establishing the cross- section form. By using the calculated cross-section we can obtain estimated chemical reaction rate constants over a wide temperature range. The data obtained can be used in various calculations in applied elds, in particular, in hypersonic gas dynamics problems, as well as for lling information system databases.

Kinetics of pseudo-boehmite sol coagulation

The kinetics of coagulation of spherical drops of the sol obtained in an ammonia solution by peptization of pseudo-boehmite with nitric acid has been investigated. The chemical reaction rate constants and coefficients of the ammonia cations diffusion in the formed gel were calculated from the experimental data in accordance with the model of a particle with noninteracting core.

Ni/ZSM‐5 Zeolite Modified Carbon Paste Electrode as an Efficient Electrode for Electrocatalytic Oxidation of Formaldehyde

AbstractIn this study, we prepared a modified carbon paste electrode consisting of Nickel entrapped in synthesized ZSM‐5 zeolite (Ni/ZMCPE). Then Ni(II) ions were incorporated to electrode by immersion of modified electrode in 1 M Ni(II) ion solution. Cyclic voltammetry and chronoamperometry experiments were used for electrochemical study of this modified electrode; a good redox behavior of Ni(OH)2/NiOOH couple at the surface of electrode can be observed, the excellent capability of this modified electrode for catalytic oxidation of formaldehyde was demonstrated during the anodic potential sweep in alkaline solution. The amount of transfer coefficient (α), surface coverage (Γ*) of the redox species and catalytic chemical reaction rate constant (k) for formaldehyde were evaluated. Thus, it can be a candidate as an anode for fuel cell application.

Reconciling NOx emissions reductions and ozone trends in the U.S., 2002–2006

Dynamic evaluation seeks to assess the ability of photochemical models to replicate changes in air quality as emissions and other conditions change. When a model fails to replicate an observed change, a key challenge is to discern whether the discrepancy is caused by errors in meteorological simulations, errors in emission magnitudes and changes, or inaccurate responses of simulated pollutant concentrations to emission changes. In this study, the Community Multiscale Air Quality (CMAQ) model is applied to simulate the ozone (O3) change after the NOx SIP Call and mobile emission controls substantially reduced nitrogen oxides (NOx) emissions in the eastern U.S. from 2002 to 2006. For both modeled and observed O3, changes in episode average daily maximal 8-h O3 were highly correlated (R2 = 0.89) with changes in the 95th percentile, although the magnitudes of reductions increased nonlinearly at high percentile O3 concentrations. Observed downward changes in mean NOx (−11.6 to −2.5 ppb) and 8-h O3 (−10.4 to −4.7 ppb) concentrations in metropolitan areas in the NOx SIP Call region were under-predicted by 31%–64% and 26%–66%, respectively. The under-predicted O3 improvements in the NOx SIP Call region could not be explained by adjusting for temperature biases in the meteorological input, or by considering uncertainties in the chemical reaction rate constants. However, the under-prediction in O3 improvements could be alleviated by 5%–31% by constraining NOx emissions in each year based on observed NOx concentrations. This demonstrates the crucial need to accurately characterize changes in precursor emissions when dynamically evaluating a model's ability to simulate O3 responses to those changes.

6PM3-PMN-030 化学発光を用いたマイクロチャンネルにおける対向流拡散反応場の評価(マイクロ流体,ポスターセッション)

This paper reports the local chemical reaction in the counter flow diffusion reactor investigated by using luminol chemiluminescenee (CL). The local chemical reaction is important phenomena to control the performance of the micro-fluid device. The chemical reaction rate can be estimated by applying the luminol CL reaction at an interface between two liquids in the microchannel. However, the chemical reaction rate constant and the mass diffusivity are necessary to estimate the local chemical reaction rate quantitatively. In this study, the local chemical reaction the counter flow channel was investigated to determine the chemical reaction rate constant.

Calculation of chemical reaction rate constants using on-the-fly high level electronic structure computations with account of multidimensional tunneling

The semiclassical instanton approach to the calculation of reaction rate constants at arbitrary temperatures in multiatomic systems is combined with high-level ab initio calculations of reactive potential energy surface (PES). The number of required ab initio calculations weakly depends on system size and allows on-the-fly evaluations of PES with high accuracy. The approach can be efficiently parallelized and provides a practical way of calculating quantum reaction rate constants with account of nuclear quantum effects such as multidimensional tunneling and zero point energies, which are rigorously incorporated in the theory. An algorithm for the search of instanton trajectories is explained. Application of the approach is illustrated for H + H(2) → H(2) + H and D + D(2) → D(2) + D bimolecular reactions in the wide temperature range with on-the-fly evaluation of PES at the ab initio full configuration interaction (FCI), coupled-cluster single double (CCSD), and single and double excitation configuration interaction (CISD) levels.

Development and validation of a reduced combined biodiesel–diesel reaction mechanism

In this study, a compact combined biodiesel–diesel (CBD) reaction mechanism for diesel engine simulations is proposed through the combination of three component mechanisms using a chemical class-based approach. The proposed mechanism comprises the reaction mechanisms of methyl crotonate (MC), methyl butanoate (MB) and n-heptane which are the surrogate fuel models of unsaturated fatty acid methyl ester, saturated fatty acid methyl ester and straight chain hydrocarbon (HC), respectively. The MC and MB mechanisms are adopted to represent biodiesel fuels, while n-heptane is utilised to characterise the combustion of fossil diesel. Here, the MC and MB mechanisms are reduced before integrating with a compact n-heptane mechanism. CHEMKIN-PRO is used as the solver for the zero-dimensional, closed homogenous reactor with a constant volume in this study. In the first phase, the mechanisms of MC and MB are methodologically reduced. The MC mechanism by Gail et al. with 301 species and 1516 reactions is reduced to 47 species and 210 reactions, while the MB mechanism by Brakora et al. with 41 species and 150 reactions is reduced to 33 species and 105 reactions. The mechanisms are reduced from a combination of methods, including peak molar concentration analysis, reaction flux analysis and the removal of individual species. In the second phase, the reduced MC and MB mechanisms are combined with the n-heptane mechanism by Pang et al. with 46 species and 112 reactions. Upon the combination of the component mechanisms, parametric adjustments to the Arrhenius rate constants of pertinent chemical reactions are performed for better ignition delay (ID) prediction. The final mechanism developed comprises 80 species and 299 reactions. The compact-sized CBD mechanism is validated against 234 test conditions ranging from initial temperatures of 750–1350K, pressures of 40–60bar and equivalence ratios of 0.4–1.5. The mechanism is generally found to accurately predict the timing and duration of ID for the combustion of each surrogate fuel. This model is also shown to be feasible for use with multidimensional computational fluid dynamics studies involving a light-duty diesel engine fuelled with biodiesel of different feedstock types, diesel as well as their blends.

Kinetic study of electrochemically induced C[sbnd]P bond formation of catechols with trialkylphosphites

The reactions of electrochemically generated o-quinones, as Michael acceptors, from oxidation of catechols with trialkylphosphites as nucleophiles have been studied using cyclic voltammetry. The reaction mechanism is an EC mechanism and the products of reaction are believed to be dialkylphosphonate derivatives of catechol. The observed homogeneous rate constants (kobs) for reactions were estimated by comparing the experimental voltammetric responses with the digitally simulated results based on the proposed mechanism. A quantitative relation between half-wave potentials of catechols and the reactivities of their corresponding o-quinone were derived. Also the effects of substituted group of trialkylphosphites on rate constants of chemical reactions were studied.

Kinetic analysis of iron oxide reduction in top gas recycling oxygen blast furnace

Reduction experiments of sinter in a top gas recycling oxygen blast furnace (TGR-OBF) were studied at temperatures ranging from 1173 to 1373 K. The changes in iron oxide reduction were evaluated, and based on the ‘unreacted core model’, the kinetic behaviour of this reduction was also analysed. The results show that with increasing temperature, the reduction rate of sinter increased, the reduction time decreased and the final degree of reduction increased. At 1173, 1273 and 1373 K, the reduction process is controlled by interfacial chemical reactions, and the chemical reaction rate constants are 0·363, 0·591 and 0·787 cm s−1 respectively at an early stage of reduction. Reactions are controlled by interface chemical reactions and internal diffusion at the final stage of reduction, and the chemical reaction rate constants are 0·541, 0·682 and 0·866 cm s−1 respectively, and the diffusion co-efficients of the reduction reaction are 0·463, 1·970 and 2·412 cm2 s−1 respectively. In the TGR-OBF, the range of temperatures is from 1173 to 1373 K. The apparent activation energy is 52·07 kJ mol−1, and the frequency factor is 77·26 cm s−1 at an early stage of reduction. At the final stage, the apparent activation energy is 31·41 kJ mol−1, and the frequency factor is 13·46 cm s−1.

Electrochemical Study of Esculetin Nitration by Digital Simulation of Cyclic Voltammograms

The reaction of electrochemically generated o‐quinones from oxidation of esculetin as Michael acceptor with nitrite ion as nucleophile has been studied using cyclic voltammetry. The reaction mechanism is believed to be EC, including oxidation of catechol moiety of esculetin followed by Michael addition of nitrite ion. The observed homogeneous rate constants (kobs) for reactions were estimated by comparing the experimental voltammetric responses with the digitally simulated results based on the proposed mechanism. Also the effects of pH and nucleophile concentration on voltammetric behavior and the rate constants of chemical reactions were described.

IMC growth of Sn-3.5Ag/Cu system: Combined chemical reaction and diffusion mechanisms

The growth kinetics of intermetallic (IMC) compound layers formed between Sn-3.5Ag solders and Cu substrate in soldering process are investigated experimentally and analytically. In order to determine the governing mechanisms of IMC formation, an experiment is designed in which samples are fabricated with bonding temperature varied at three levels of 260, 310 and 360°C while time is varied at seven levels between 1 and 240 min. Microstructural analysis is conducted to analyze the IMC thickness and morphology. A physics-based analytical model (Dybkov), in which net growth of each IMC phase is modeled as result of diffusion and partial chemical reaction, is modified for a case of liquid–solid state for two IMC layers. Dissolution of solid IMCs into liquid solder is included in the model. The model is then used to evaluate governing mechanisms of IMC formation and growth and determine the activation energies for formation and growth of different IMC compounds. The chemical reaction and diffusion rate constants have also been evaluated for different intermetallic compounds of Sn-3.5Ag/-Cu-substrate soldering system. Two intermetallic phases were observed during soldering at the interface: η-phase (Cu6Sn5) and ε-phase (Cu3Sn) IMC layers. The thickness of the η and ε IMC phases increase with increasing the soldering time and/or soldering temperature. The increase in the IMC layer thickness during the process is found to obey linear relationship with time during the chemical reaction-controlled stage of formation of Cu3Sn IMC layer and it obeys a parabolic relationship with time during the diffusion-controlled growth stage of formation of both IMC layers. It was shown that as temperature increases, the chemical reaction and diffusion rate constants monotonically increase. Modeling results indicate clear distinction of governing mechanisms of growth at different IMC growth stages with chemical reaction-controlled growth at initial stage and diffusion-controlled growth at final stage of IMC formation. The apparent activation energy calculated for the chemical reaction-controlled growth stage is 20.59 (KJ/mol) for ε IMC phase. For diffusion-controlled growth stage, the activation energy values obtained are 41.98 (KJ/mol) for η IMC phase and 50.10 (KJ/mol) for ε IMC phase. These values slightly differ from values reported in literature where a simple power-law fit is used to extract those quantities. This is expected because unlike present study, the power-law fit does not explicitly incorporate the interaction between the chemical reaction and diffusion mechanisms in IMC growth. The values obtained in the current study are still within the range of the values found in the literature.