Elementaiy reaction

In deciding whether to write an elementaiy reaction as either a reversible or an irreversible reaction, we take the practical view that if the reverse reaction is negligibly slow on the experimental time scale, the reaction is essentially irreversible. Consider the alkaline hydrolysis of an ester, for which the rate equation is... 

The Arrhenius equation relates the rate constant k of an elementaiy reaction to the absolute temperature T R is the gas constant. The parameter is the activation energy, with dimensions of energy per mole, and A is the preexponential factor, which has the units of k. If Jt is a first-order rate constant, A has the units seconds", so it is sometimes called the frequency factor. 

This description provides information, via conventional structures, about the constitution of reactants, products, and the intermediate. Transition state stmctures are more provisional and may attempt to show the electronic distribution and flow in this region of the reaction path. The curved arrow symbolism is often used, as shown in structure 1 for the first elementaiy reaction. 

Let us examine the equilibrium assumption of transition state theory. Consider a reversible elementaiy reaction at equilibrium. Because the initial and final states are at equilibrium, assuredly the transition state is in equilibrium with each of these. (It follows that for a reaction at equilibrium, transition state theory is exact insofar as the equilibrium assumption is concerned.)... 

Most elementary reactions involve either one or two reactants. Elementaiy reactions involving three species are infrequent, because the likelihood of simultaneous three-body encounter is small. In closed, well-mixed chemical systems, the integration of rate equations is straightforward. Results of integration for some important rate laws are listed in Table 2.7, which gives the concentration of reactant A as a function of time. First-order reactions are particularly simple the rate constant k has units of s , and its reciprocal value (1/k) provides a measure of a characteristic time for reaction. It is common to speak in terms of the half-life ( 1/2) for reaction, the time required for 50% of the reactant to be consumed. When... 

Order follows from the stoichiometry of the reactants in an elementaiy reaction. Note that products do not appear in the rate law of an elementary reaction. See Table 13.3 in the text. 

Figure 7.6 Equilibrium concentrations calculated by the method of false transients for a non-elementaiy reaction.

The application of mass action law to heterogeneous elementaiy reactions is somewhat refined by treating the adsorption of the species S on unoccupied site a(0) as bimolecular combination of S and elementary reaction is thus expressed as proportional to 0 0(0), where <7 is the concentration of S in gas and 0(0) the probability of the site a being unoccupied. The approximation of Langmuir adsorption isotherm is precisely in line with this refinement of the application of the mass action law, which will be called the extended mass action law in what follows. The application of the extended mass action law to the hydrogen electrode reaction leads now to the value of a, which decreases from 2 to 0 with the increase of rj passing through the observed value ea 0.5 (6). The value of tj at a = 0.5 is however far too low and the interval of ij, in which a stays near 0.5, far too short as compared with observations. 

Figure 27.18 Elementaiy reaction network showing the pathways involving CO2, H2O and CO. Steady-state reaction rates (mol s ) in the whole microreactor at 100, 180 and 300°C are also shown. WHSV— 1500 h flow rate (mixture 1) — 5.0 N cm min flow rate (air) =0.5 Ncm min . ...

The Fundamental Constraints on Activation Energies Unimolecular Elementaiy Reactions... 

Consider initially an elementaiy reaction i, which is held in a zone whose space function is Ei, then we get ... 

In an elementaiy reaction, which occurs in a single zone, the space function of this elementary step is the ratio of the volume (or area) of the reaction zone to the reference amount chosen to define the fractional extent ... 

This chapter presents modeling of the elementary steps in the gas phase. Its aim is to determine the reactivity of an elementaiy reaction step, the expression of the rate coefficient and its variation with temperature. 

This represents two opposite elementaiy reactions a reduction toward the left, and an oxidation toward the right. 

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