Elementary Steps and Mechanisms

Mechanisms. Mechanism is a technical term, referring to a detailed, microscopic description of a chemical transformation. Although it falls far short of a complete dynamical description of a reaction at the atomic level, a mechanism has been the most information available. In particular, a mechanism for a reaction is sufficient to predict the macroscopic rate law of the reaction. This deductive process is vaUd only in one direction, ie, an unlimited number of mechanisms are consistent with any measured rate law. A successful kinetic study, therefore, postulates a mechanism, derives the rate law, and demonstrates that the rate law is sufficient to explain experimental data over some range of conditions. New data may be discovered later that prove inconsistent with the assumed rate law and require that a new mechanism be postulated. Mechanisms state, in particular, what molecules actually react in an elementary step and what products these produce. An overall chemical equation may involve a variety of intermediates, and the mechanism specifies those intermediates. For the overall equation... 

Depending on the oxidation conditions and its reactivity, the inhibitor InH and the formed radical In can participate in various reactions determining particular mechanisms of inhibited oxidation. Of the various mechanisms, one can distinguish 13 basic mechanisms, each of which is characterized by a minimal set of elementary steps and kinetic parameters [38,43 15], These mechanisms are described for the case of initiated chain oxidation when the initiation rate v = const, autoinitiation rate fc3[ROOH] -C vy and the concentration of dissolved dioxygen is sufficiently high for the efficient conversion of alkyl radicals into peroxyl radicals. The initiated oxidation of organic compounds includes the following steps (see Chapter 2). 

D) A catalyst is a substance that increases the rate of a chemical reaction withont itself being consumed. The catalyst may react to form an intermediate, but it is regenerated in a subsequent step of the reaction. The catalyst speeds up a reaction by providing a set of elementary steps (reaction mechanisms) with more favorable kinetics than those that existed in its absence. Choices A and C, even thongh they are true statements, are the resnlts of a lowered activation energy. 

This result may be tabulated as shown in Table VI. In subsequent examples this sort of table will be the principal way of listing direct mechanisms. There are three elementary steps and one independent intermediate, so... 

The experimentally observed rate law for an overall reaction depends on the reaction mechanism—that is, on the sequence of elementary steps and their relative rates. If the overall reaction occurs in a single elementary step, the experimental... 

Reaction mechanism, elementary step and rate-limiting step... 

In this chapter and in Chapter 10, we synthesize or design prototype polymer processing machines using these elementary steps and building blocks. The uniqueness of each prototype machine is determined by the building blocks of the machine itself, and the particular combination of elementary steps and elementary-step mechanisms utilized to create it. 

However, once we have conceptually designed the prototype machine, which involves selecting the elementary steps and the particular mechanisms we wish to use, and describing them in terms of mathematical subsystem models, we can assemble them into a comprehensive mathematical model of the whole machine and proceed through simulation to design-specific machines for given sets of requirements.1... 

Most reaction mechanisms are multi-step processes involving reaction intermediates. Intermediates are chemicals that are formed during one elementary step and consumed during another, but they are not overall reactants or products. In many cases one elementary reaction in particular is the slowest and determines the overall reaction rate. This slowest reaction in the series is called the rate-limiting step or rate determining step. 

Discussions and studies of reaction mechanisms attempt to analyse the way in which a compound A is transformed into a compound B. Varying degrees of sophistication are attached to the phrase reaction mechanism but the aim is generally to define the reaction in terms of elementary steps and stereochemistry. In solution chemistry, the structures of compounds A and B will be known and mechanistic information may be deduced from kinetic studies, solvent effects, stereochemistry, isotopic labelling, and other slight structural modifications. 

This rate law, derived by postulating the two elementary steps and making assumptions about the relative rates of these steps, agrees with the experimental rate law. Since this mechanism (the elementary steps plus the assumptions) also gives the correct overall stoichiometry, it is an acceptable mechanism for the decomposition of ozone to oxygen. 

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