Elementary steps definition

As explained before, a chemical reaction can seldom be described by a single elementary step, and hence we need to adapt our definition of activation for an overall reaction. Since we are not particularly interested in the effects of thermodynamics we define the apparent activation energy as... 

Besides, the structure, nature and reactivity of the chemisorbed molecule could not be unambiguously identified because the physical tools used could not lead easily to a complete understanding of the quasi molecular character of surface chemisorbed species and move precisely to the definition of the elementary steps occurring during the molecular transformations taking place on the surfaces. 

Since all of the above-mentioned interconversion reactions are reversible, any kinetic analysis is difficult. In particular, this holds for the reaction Sg - Sy since the backward reaction Sy -+ Sg is much faster and, therefore, cannot be neglected even in the early stages of the forward reaction. The observation that the equilibrium is reached by first order kinetics (the half-life is independent of the initial Sg concentration) does not necessarily indicate that the single steps Sg Sy and Sg Sg are first order reactions. In fact, no definite conclusions about the reaction order of these elementary steps are possible at the present time. The reaction order of 1.5 of the Sy decomposition supports this view. Furthermore, the measured overall activation energy of 95 kJ/mol, obtained with the assumption of first order kinetics, must be a function of the true activation energies of the forward and backward reactions. The value found should therefore be interpreted with caution. 

At equilibrium the net rate is by definition zero. If we compare the rate equation for an elementary step, e.g the rate equation... 

For our purpose elementary steps can be chosen to include any reaction that cannot be further broken down so as to involve reactions in which the specified intermediates are produced or consumed. Ideally, elementary steps should consist of irreducible molecular events, usually with a molecularity no greater than two. Such steps are amenable to treatment by fundamental chemical principles such as collision and transition state theories. Often such a choice is not feasible because of lack of knowledge of the detailed chemistry involved. Each of these elementary reactions, even when carefully chosen, may itself have a definite mechanism, but theory may be unable to elucidate this finer detail [Moore (2)]. 

A knowledge of the kinetic parameters and, in particular, the orders of reaction of a catalysed reaction is important to the accurate definition of the reaction mechanism. However, catalytic hydrogenation reactions proceed through a series of elementary steps, only one of which may be ratedetermining. In consequence, the observed rate expressions give little or no direct information about most of the steps involved and kinetics alone are not sufficient for a precise description of the mechanism. 

In the isomerisation of the tetra-substituted olefin 3,4-dimethylhex-3-ene over palladium—alumina [146], it has been shown that double bond migration is a necessary precursor to cis—trans isomerisation. This has been interpreted as showing that the mechanism involves a series of elementary steps, each of which is stereospecific, although no definite conclusions were drawn as to whether an addition—abstraction or an abstraction-addition mechanism was involved. 

In contrast to outer-sphere reactions, the simple observation that a reaction occurs by an inner-sphere mechanism necessarily introduces an element of structural definition. The relative dispositions of the oxidizing and reducing agents are immediately established and, except for structurally flexible bridging ligands such as NC5H4(CH2) C5H4N, the internuclear separation between redox sites can be inferred from known bond distances. Even so, bimolecular inner-sphere reactions necessarily occur by a sequence of elementary steps (Scheme 2) and the observed rate constant may include contributions from any of the series of steps. 

The assumption of a definite location for the negative charge on the j8-carbon requires that 59a and 60a both be on the lowest energy path for substitution and isomerization. That this location should be formed anti derives from orbital symmetry, i.e. microscopic reversibility suggests that anti entry of one halide and syw-departure of the other are improbable. We deduce, therefore, that the substitutions in (142) require at least three elementary steps, e.g. cis-1 - 59a -> 60b -v cis-2 isomerizations with exchange require at least three steps, e.g. cis-l 59a —> 59b - trans-2 isomeriza-... 

One conceivable extrapolation of this definition of concertedness is that every elementary chemical reaction is concerted. Every chemical process may be at least conceptually broken up into a sequence of one, two, or more elementary steps or concerted subprocesses. Therefore a reaction may be termed one-step, two-step, or multi-step, and the term "concerted" becomes a redundancy simply equivalent to "one-step . The frequent use of "concerted and "one-step as interchangeable adjectives is one consequence of this extrapolation. 

In this case, the apparent activation energy is not equal to the activation energy of the rate-determining step. By definition, the activation energy for elementary step 2 equals... 

One-step reactions between stable molecules are rare since a stable molecule is by definition a quite unreactive entity. Rather, complicated rearrangements of chemical bonds are usually required to go from reactants to products. This implies that most reactions do not proceed in a single elementary step as illustrated below for NO formation from N2 and O2. 

For more detailed description see also parts 2.1 (Table of Symbols for Scheme 1) and 2.2.1 (Definition of Elementary Steps)... 

The rate coefficients of many of the important elementary steps at high temperatures are now well established, particularly and the functions / /[M] and A a2)2)[M] which describe the recombination kinetics for those gas compositions which have been studied directly. Improved experimental accuracy is apparently needed in the determination of the rate of recombination before more definitive values of the coeflScients for individual collision partners, kf and kf, can be anticipated. Also, more quantitative information is desirable concerning the high-temperature rate coefficients of the other important bimolecular steps, k, kc and k. Some of this can be provided, without major advances in experimental technique, by further study of the nonequilibrium excursions of intermediate species concentrations toward the end of the ignition process under selected conditions in nonstoichiometric mixtures, and from further resolution of the exponential branching behaviour of lean mixtures, as discussed in section 2.3.2. 

Yes, and many do. While some chemical reactions may only involve a single step, others may involve ten or more elementary steps. Of course, chemists working in different subfields may have different definitions of what constitutes a step of a reaction, depending on what aspects of the reaction they focus on. 

---