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Pre-exponential Factor
In chemical kinetics, the pre-exponential factor or A factor is the pre-exponential constant in the Arrhenius equation (equation shown below), an empirical relationship between temperature and rate coefficient. It is usually designated by A when determined from experiment, while Z is usually left for collision frequency. The pre-exponential factor can be thought of as a measure of the frequency of properly oriented collisions. It is typically determined experimentally by measuring the rate constant k at a particular temperature and fitting the data to the Arrhenius equation. The pre-exponential factor is generally not exactly constant, but rather depends on the specific reaction being studied and the temperature at which the reaction is occurring. A=\frac=ke^ The units of the pre-exponential factor A are identical to those of the rate constant and will vary depending on the order of the reaction. For a first-order reaction, it has units of s−1. For that reason, it is often ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Chemical Kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is different from chemical thermodynamics, which deals with the direction in which a reaction occurs but in itself tells nothing about its rate. Chemical kinetics includes investigations of how experimental conditions influence the speed of a chemical reaction and yield information about the reaction's mechanism and transition states, as well as the construction of mathematical models that also can describe the characteristics of a chemical reaction. History The pioneering work of chemical kinetics was done by German chemist Ludwig Wilhelmy in 1850. He experimentally studied the rate of inversion of sucrose and he used integrated rate law for the determination of the reaction kinetics of this reaction. His work was noticed 34 years later by Wilhelm Ostwald. In 1864, Peter Waage and Cato Guldberg published the law ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Arrhenius Equation
In physical chemistry, the Arrhenius equation is a formula for the temperature dependence of reaction rates. The equation was proposed by Svante Arrhenius in 1889, based on the work of Dutch chemist Jacobus Henricus van 't Hoff who had noted in 1884 that the Van 't Hoff equation for the temperature dependence of equilibrium constants suggests such a formula for the rates of both forward and reverse reactions. This equation has a vast and important application in determining the rate of chemical reactions and for calculation of Activation energy, energy of activation. Arrhenius provided a physical justification and interpretation for the formula.Keith J. Laidler, Laidler, K. J. (1987) ''Chemical Kinetics'', Third Edition, Harper & Row, p. 42 Currently, it is best seen as an empirical relationship.Kenneth Connors, Chemical Kinetics, 1990, VCH Publishers It can be used to model the temperature variation of Mass diffusivity, diffusion coefficients, population of Vacancy defect, crystal ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Reaction Rate Constant
In chemical kinetics, a reaction rate constant or reaction rate coefficient () is a proportionality constant which quantifies the rate and direction of a chemical reaction by relating it with the concentration of reactants. For a reaction between reactants A and B to form a product C, where :A and B are reactants :C is a product :''a'', ''b'', and ''c'' are stoichiometric coefficients, the reaction rate is often found to have the form: r = k mathrmm mathrm Here is the reaction rate constant that depends on temperature, and and are the molar concentrations of substances A and B in moles per unit volume of solution, assuming the reaction is taking place throughout the volume of the solution. (For a reaction taking place at a boundary, one would use moles of A or B per unit area instead.) The exponents ''m'' and ''n'' are called partial orders of reaction and are ''not'' generally equal to the stoichiometric coefficients ''a'' and ''b''. Instead they depend on the reacti ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Collision Frequency
Collision frequency describes the rate of collisions between two atomic or molecular species in a given volume, per unit time. In an ideal gas, assuming that the species behave like hard spheres, the collision frequency between entities of species A and species B is: : Z = N_\text N_\text \sigma_\text \sqrt\frac, which has units of olumetime]−1. Here, * N_\text is the number of A molecules in the gas, * N_\text is the number of B molecules in the gas, * \sigma_\text is the collision cross section (physics), cross section, the "effective area" seen by two colliding molecules, simplified to \sigma_\text = \pi(r_\text+r_\text)^2 , where r_\text the radius of A and r_\text the radius of B. * k_\text is the Boltzmann constant, * T is the temperature, * \mu_\text is the reduced mass of the reactants A and B, \mu_\text = \frac Collision in diluted solution In the case of equal-size particles at a concentration n in a solution of viscosity Viscosity is a mea ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Collision Theory
Collision theory is a principle of chemistry used to predict the rates of chemical reactions. It states that when suitable particles of the Reagent, reactant hit each other with the correct orientation, only a certain amount of collisions result in a perceptible or notable change; these successful changes are called successful collisions. The successful collisions must have enough energy, also known as activation energy, at the moment of impact to break the pre-existing bonds and form all new bonds. This results in the products of the reaction. The activation energy is often predicted using the transition state theory. Increasing the concentration of the reactant brings about more collisions and hence more successful collisions. Increasing the temperature increases the average kinetic energy of the molecules in a solution, increasing the number of collisions that have enough energy. Collision theory was proposed independently by Max Trautz in 1916 and William Lewis (physical chemis ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Transition State Theory
In chemistry, transition state theory (TST) explains the reaction rates of elementary chemical reactions. The theory assumes a special type of chemical equilibrium (quasi-equilibrium) between reactants and activated transition state complexes. TST is used primarily to understand qualitatively how chemical reactions take place. TST has been less successful in its original goal of calculating absolute reaction rate constants because the calculation of absolute reaction rates requires precise knowledge of potential energy surfaces, but it has been successful in calculating the standard enthalpy of activation (Δ''H''‡, also written Δ‡''H''ɵ), the standard entropy of activation (Δ''S''‡ or Δ‡''S''ɵ), and the standard Gibbs energy of activation (Δ''G''‡ or Δ‡''G''ɵ) for a particular reaction if its rate constant has been experimentally determined (the ‡ notation refers to the value of interest ''at the transition state''; Δ''H''‡ is the difference between the ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Entropy Of Activation
In chemical kinetics, the entropy of activation of a reaction is one of the two parameters (along with the enthalpy of activation) that are typically obtained from the temperature dependence of a reaction rate constant, when these data are analyzed using the Eyring equation of the transition state theory. The standard entropy of activation is symbolized and equals the change in entropy when the reactants change from their initial state to the activated complex or transition state ( = change, = entropy, = activation). Importance Entropy of activation determines the preexponential factor of the Arrhenius equation for temperature dependence of reaction rates. The relationship depends on the molecularity of the reaction: * for reactions in solution and unimolecular gas reactions *: , * while for bimolecular gas reactions *: . In these equations is the base of natural logarithms, is the Planck constant, is the Boltzmann constant and the absolute temperature. is the ideal gas ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
