Steps, elementary sequential

The strong emphasis placed on concentration dependences in Chapters 2-5 was there for a reason. The algebraic form of the rate law reveals, in a straightforward manner, the elemental composition of the transition state—the atoms present and the net ionic charge, if any. This information is available for each of the elementary reactions that can become a rate-controlling step under the conditions studied. From the form of the rate law, one can deduce the number of steps in the scheme. In most cases, further information can be obtained about the pattern in which parallel and sequential steps are arranged. 

Mechanism I illustrates an important requirement for reaction mechanisms. Because a mechanism is a summary of events at the molecular level, a mechanism must lead to the correct stoichiometry to be an accurate description of the chemical reaction. The sum of the steps of a mechanism must give the balanced stoichiometric equation for the overall chemical reaction. If it does not, the proposed mechanism must be discarded. In Mechanism I, the net result of two sequential elementary reactions is the observed reaction stoichiometry. 

The solution phase is modeled explicitly by the sequential addition of solution molecules in order to completely fill the vacuum region that separates repeated metal slabs (Fig. 4.2a) up to the known density of the solution. The inclusion of explicit solvent molecules allow us to directly follow the influence of specific intermolecular interactions (e.g., hydrogen bonding in aqueous systems or electron polarization of the metal surface) that influence the binding energies of different intermediates and the reaction energies and activation barriers for specific elementary steps. 

If we want to understand and describe the influence of environmental factors, especially temperature, on chemical reaction rates, and if we want to see how transformation rates vary as a function of the chemical structure of a compound, we need to take a closer look at these reactions on a molecular level. As mentioned already, a chemical reaction often proceeds in several sequential elementary steps. Frequently, one step in the reaction sequence occurs at a much slower rate than all the others. 

A multielectron electrode reaction may also occur by a number of mechanistic routes including sequential and parallel pathways, which in complex electrokinetics may also be analysed individually in terms of elementary chemical and electrochemical steps. Figure 7 depicts plots of log j vs. Tj for (a) sequential and (b) parallel paths for multielectron transfer reactions. It is apparent that, at a given electrode potential, in... 

An elementary example of diis process is the reaction of an organometallic reactant widi a ketone (or aldehyde) followed by dehydration of the resulting alcohol to die olefin. This is truly a sequential process in that the product alcohol is dehydrated in a second, independent reaction step. It suffers as a useful synthetic method because regioisomers are often formed hi die elimination step. 

The Bodenstein approximation recognises that, after a short initial period in the reaction, the rate of destruction of a low concentration intermediate approximates its rate of formation, with the approximation improving as the maximum concentration of intermediate decreases (see Chapters 3 and 4). Equating rates of formation and destruction of a non-accumulating intermediate allows its concentration to be written in terms of concentrations of observable species and rate constants for the elementary steps involved in its production and destruction. This simplifies the kinetic expressions for mechanisms involving them, and Scheme 9.3 shows the situation for sequential first-order reactions. The set of differential equations... 

Nature accomplishes many syntheses-even those of complex molecules-by sequences of elementary steps. In the last few decades, the blueprint of catalyzed cascade reactions has found fertile soil through the advent of transition metal catalysis in laboratories. Scrutinizing catalytic cycles and mechanistic insight has paved the way for designing new sequential transformations catalyzed by transition metal complexes in a consecutive or domino fashion. In particular, transition metal-catalyzed sequences considerably enhance structural complexity by multiple iterations of organometalhc elementary steps. All this has fundamentally revolutionized synthetic strategies and conceptual thinking. 

When a reaction rate is measured in a chemical reactor, the reaction is generally a composite reaction comprised of a sequence of elementary reactions. An elementary reaction is a reaction that occurs at the molecular level exactly as written (Laidler, 1987). The mechanism of the reaction is the sequence of elementary reactions that comprise the overall or composite reaction. For example, mineral dissolution reactions generally include transport of reactant to the surface, adsorption of reactant, surface dilfusion of the adsorbate, reaction of the surface complex and release into solution, and transport of product species away from the surface. These reactions occur as sequential steps. Reaction of surface complexes and release to solution may happen simultaneously at many sites on a surface, and each site can react at a different rate depending upon its free energy (e.g., Schott et al., 1989). Simultaneous reactions occurring at different rates are known as parallel reactions. In a series of sequential reactions, the ratedetermining step is the step which occurs most slowly at the onset of the reaction, whereas for parallel steps, the rate-determining step is the fastest reaction. 

The chemical mechanism of a reaction is a proposed set of elementary (molecular) reactions, which provide a sequential path or a number of parallel paths that account for both the stoichiometry and the observed rate law of the overall reaction. If the reaction mechanism is simple, it consists of a single elementary step (apart from molecular diffusion of reactants and products, which is always a step in aqueous reactions) capable of accounting for the rate. 

Wei and Iglesia54 57 recently reported isotopic and kinetic studies for the SMR and C02 reforming of methane over Ni- and noble metal-based catalysts. They considered the sequence of elementary steps involved in the steam reforming and C02 reforming of methane as well as methane decomposition and WGS reactions as shown in Figure 2.5. Accordingly, CH4 decomposes to chemisorbed carbon (C ) via sequential elementary H-abstraction steps, which becomes faster as H atoms are sequentially abstracted from the CH4 reactant. This cascade process leads to a low... 

The spread of site energies in systems of diagonal disorder is of considerable impact on the space-time evolution of an elementary excitation. Energy-dispersive excited-state walks are involved in a hierarchy of sequential steps of gradually decreasing event times which have been recognized to produce nondiffusive, i. 

Thus, the reason for the stability of the T1(CN) " complexes is obviously their kinetic inertness. It is generally assumed that reduction-oxidation reactions often consist of several elementary, one-electron steps. This assumption is based on the fact that the transfer of several electrons in a single elementary discharge act is less probable than the sequential transfer through one-electron consecutive steps (however, see the discussion in the beginning of this section). In the present case, the reaction... 

Most chemical reactions are sequential chains of the simplest actions, which are called acts or steps. They are either a simultaneous merger of two, rarely three particles (atoms, ions, molecules, radicals, etc.) into one compound or, on the contrary, decomposition of one compound into several. Reactions in one act are called elementary reactions, and in several acts, complex reactions. 

Most chemical reactions in nature include several mutually associated mono- or bimolecular steps and for this reason have a complex, often reversible character. The entire sequence and interconnection of elementary acts of the complex reactions is called reaction mechanism. These mechanisms differ not only in the number of elementary acts but also in the nature of their sequence and direction. Most common in hydrochemistry are sequential, parallel, and reversible mechanisms. 

In reality, most chemical reactions consist of numerous elementary reactions combined in reaction networks that are much more difficult to describe. An interesting elementary reaction is a 2-step sequential reaction which is relevant e.g. in modelling thermal separation processes ... 

Reactions don t just occur singly they occur sequentially or in parallel. We will consider how several processes occurring simultaneously affect the amounts of products and reactants. Finally, we recognize that most chemical reactions occur in discrete steps. The overall combination of these steps, called elementary processes, is what makes up the mechanism of a reaction. A proposed mechanism must be consistent with the experimentally determined rate law of a reaction. This requirement puts some restrictions on how we can expect a chemical reaction to occur on an atomic and molecular scale. 

---