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Steady-State Approximation, Reaction Mechanism and Rate Law of Chain Reactions
Published in Eli Usheunepa Yunana, Calculations in Chemical Kinetics for Undergraduates, 2022
Elementary reactions are characterized by the following: They are either unimolecular (involves a single reactant species) or bimolecular (involves two reactant molecules), but termolecular (involves three reactant molecules colliding simultaneously) are very rare.The order of an elementary reaction corresponds with the stoichiometric coefficients in the balanced equation for that step, although this is not always true for the overall rate law and overall balanced equation in an experiment.The overall rate law of a reaction depends on a single elementary step which is usually the slowest step in the reaction mechanism.They can also be reversible reactions having both forward and reverse processes at equilibrium.
Chemical Kinetics
Published in Franco Battaglia, Thomas F. George, Understanding Molecules, 2018
Franco Battaglia, Thomas F. George
A reaction of the type (13.1) occurs because the reactant species undergo collision processes evolving toward the reaction products. It does not proceed through a collision of α molecules of species A with β molecules of species B, etc. Such a collision process is so improbable that its occurrence is out of question. Rather, a reaction proceeds through a sequence of elementary steps which all together constitute the mechanism of the global reaction. Each elementary step typically consists of either a two-species collision (bimolecular reaction) or the reactive decomposition of a single species (unimolecular reaction). Occasionally, but more rarely, it may be necessary to make appeal to a less-probable three-body collision (trimolecular reaction).1
Hydrodesulfurization Catalysis Fundamentals
Published in Deniz Uner, Advances in Refining Catalysis, 2017
where rA is the rate of disappearance of reactant A, kA is the reaction rate constant, PA is the partial pressure of A, Ki is the equilibrium constant for adsorption of species, Pi is the partial pressure of species in equilibrium, and n is a constant (1 or 2), reflecting the number of adsorbed species involved in the rate determining step [23]. Based on the quasi-equilibrium assumption, one elementary step is considered as being slow and it determines the rate, while other steps are fast and they are all in equilibrium. The rate-limiting step is generally the reaction between the adsorbed organosulfur compounds and the adsorbed hydrogen. The f(PH) term usually changes linearly with hydrogen pressure, but when hydrogen adsorbs on a different site from that of the organosulfur compounds, it depends on both hydrogen pressure and the equilibrium constant of hydrogen adsorption (KH). The general rate equation includes the effect of strong inhibition by the adsorption of the organosulfur compounds and H2S that is formulized by the term of KiPi. If these compounds adsorb on the surface strongly, this will lead to the decrease in the HDS rate [23].
Methanol synthesis revisited: reaction mechanisms in CO/CO2 hydrogenation over Cu/ZnO and DFT analysis
Published in Petroleum Science and Technology, 2019
From a historical perspective, the early research in methanol synthesis chemistry/technology is perhaps the origin of this controversy (the source of “C” in product CH3OH) as both principal viewpoints – CO vs. CO2–were proposed and actively endorsed/advocated for a very long time. In the United States, the research works of Klier et al. (Klier et al., 1982; Klier, 1982; Herman et al., 1979; Bulko et al., 1979) initially proposed that the hydrogenation of CO was the dominant pathway in methanol synthesis chemistry, and also that adsorption of CO on Cu sites was the first step (elementary step) in the surface reaction mechanisms. At the same time, in Russia and Poland (and United States), a counter view was proposed that CO2 is the principal precursor and the main C source in methanol synthesis. It is important to note that the parallel developments and the exactly counter viewpoints were based on experimental studies that were carried out at very similar conditions, i.e., 50–70 atm, 225–275 °C, and with an industrial Cu/Zn/Al2O3 catalyst with a nominal Cu/Zn ratio of 30/70 at%. It is interesting to note that in a somewhat parallel and an independent development, the research groups of Chinchen et al. (1984) presented this controversial area in a concise form, in the following 5 questions:Is methanol synthesized from CO or CO2?What is the state of the copper in a working catalyst?What roles are played by the ZnO and Al2O3 components in the commercial catalyst?What is the reaction mechanism and what elementary step is rate determining?What is/are the active site(s) on a practical Cu/ZnO/Al2O3 catalyst?