Elementary reaction act

In general, the potential dependence of the current is determined by both the potential dependence of the concentrations of the reacting particles near the electrode surface and the potential dependence of the reaction rate constant itself (i.e., the probability of the elementary reaction act per unit time, W). 

The concentrations of the reactants and reaction prodncts are determined in general by the solution of the transport diffusion-migration equations. If the ionic distribution is not disturbed by the electrochemical reaction, the problem simplifies and the concentrations can be found through equilibrium statistical mechanics. The main task of the microscopic theory of electrochemical reactions is the description of the mechanism of the elementary reaction act and calculation of the corresponding transition probabilities. 

The surface concentration Cq Ajc in general depends on the electrode potential, and this can affect significantly the form of the i E) curves. In some situations this dependence can be eliminated and the potential dependence of the probability of the elementary reaction act can be studied (called corrected Tafel plots). This is, for example, in the presence of excess concentration of supporting electrolyte when the /i potential is very small and the surface concentration is practically independent of E. However, the current is then rather high and the measurements in a broad potential range are impossible due to diffusion limitations. One of the possibilities to overcome this difficulty consists of the attachment of the reactants to a spacer film adsorbed at the electrode surface. The measurements in a broad potential range give dependences of the type shown in Fig. 34.4. 

There is no change in the chemical composition of the reacting species in reactions (a) and (b) nevertheless, it is possible to measure the rate of these reactions by studying the process of the change in the spin state of the nuclei (ortho-para conversion). Theoretically, reactions (a)-(d) are of special interest because for them rather accurate non-empirical calculations of the potential energy surface, as well as detailed, up to quantum mechanical, calculations of the nuclear dynamics during an elementary reaction act can be carried out. 

The theory of steady-state reactions operates with the concepts of "a path of the step , "a path of the route , and "the reaction rate along the basic route . Let us give their determination in accordance with ref. 16. The number of step paths is interpreted as the difference of the number of elementary reaction acts in the direct and reverse directions. Then the rate for the direct step is equal to that of the paths per unit time in unit reaction space. One path along the route signifies that every step has as many paths as its stoichiometric number for a given route. In the case when the formation of a molecule in one of the steps is compensated by its consumption in the other step, the steady-state reaction process is realized. If, in the course of this step, no final product but a new intermediate is formed, then it is this... 

The main reactions (A-G) and the corresponding pathways resulting in the formation, or subsequent functionalization, of tetrazole rings are shown in Scheme 83. As seen from the scheme, only several elementary reaction acts leading to a five-membered heteroaromatic ring with four nitrogen atoms (tetrazole ring) are possible. These reactions follow the mechanisms depicted in Scheme 83. These are ... 

And is called kinetic equation or reaction rate law. Here r. is rate of reactions normalized over volume, C.,. is molar concentrations of reac-tants, k. is constant value characterizing the rate reactions at reactants concentration equal to 1, which is called reaction rate constant or intrinsic reaction rate, v.. is stoichiometric coefficient of the component i usually called partial order of reaction. Sum of one reaction partial order determines order of the reaction overall or order of its rate law. Elementary reactions (acts) dominate, which are subject to the rate law of zero, first and second order. For instance, for an elementary direct reaction... 

Chemical reactions are at the heart of chemistry, making possible the achievement of its ultimate goals, which include synthesizing materials with desired properties. What happens in the chemist s flask is a complex phenomenon that consists of an astronomical number of elementary reactions of individual molecules. In order to control the reactions in the flask, it would be good to understand first the rules which govern these elementary reaction acts. This is the subject of this chapter. 

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