The Thermodynamic Form of Kinetic Equations

It was just shown that fluxes of thermodynamic parameters that describe transformations in chemically reactive systems are in direct relationships with the rate of chemical reactions. The relationship between the rate of a chemical reaction and physicochemical parameters (reactant concentra tions, temperature, etc.) of the system is the subject of a special branch of physical chemistry called chemical kinetics. 

In chemical kinetics, the key parameters are traditionally the rate con stants and the concentrations of reactants. At the same time for thermody namic analysis, the parameters such as chemical potential appear to be the most useful. That is why the most convenient way to consider chemical transformations in terms of thermodynamics of nonequihbrium processes is a thermodynamic form of kinetic equations. The main elements of the application of this form are given following. 

Let us consider a spontaneous (thermally initiated) elementary chemical reaction ij 

In chemical kinetics, when a reaction proceeds at a given temperature and pressure with the maintenance of the Maxwell Bolztmann distribution 

Notice that the dimension of parameter 8y is s while the parameter fij is dimensionless irrespective of the actual molecularity of the reaction under consideration. The concomitance of equation (1.25) and the other equa tions that follow to the necessary dimensions is made, as is common in ther modynamics, by choosing the respective standard states of the reactants. 

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The thermodynamic form of kinetic equations is helpful for providing the kinetic thermodynamic analysis of the effect of various thermodynamic parameters on the stationary rate of complex stepwise processes. Following are a few examples of such analyses in application to the noncatalytic reac tions. The analysis of the occurrence of catalytic transformations is more specific because the concentrations and, therefore, the chemical potentials and thermodynamic rushes of the intermediates are usually related to one another in the total concentrations of the catalyticaUy active centers. (Catalytic reactions are discussed in more detail in Chapter 4.)... 

The thermodynamic form of kinetic equations aUows us to easily find the apparent activation energy, EaS, for the stationary occurrence of stepwise processes, especiaUy in the case of their kinetic irreversibility. For instance, if stepwise process (1.54) goes from left to right and is kineticaUy irrevers ible in whole—that is, R P—and if Sum is the smaUest quantity of aU 8 in the transformation chain, then... 

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