Net Reactions and Reaction Rates

The equation for a net chemical reaction represents the overall transformation of reactants into products. Thus, thallium Ill) ions oxidize iron(II) ions according to Eq. (1-1), and a secondary amine reacts with an aryl chloride as in Eq. (1-2). 

A useful convention employs an equal sign for a net reaction, reserving directional arrows for elementary reactions. This practice is not too widely followed, however. 

Thermodynamics defines the relationship governing the concentrations at equilibrium. The exponents of the equilibrium concentrations in the expression for the equilibrium constant are the same as the stoichiometric coefficients in the net equation, as in 

The net equation contains no kinetic information. One cannot infer that the reaction rates (v) are given as vi = [Fe2l-]2[T13+] or v2 = [ArCl][R2NH]2. Although the rates will almost certainly depend upon the concentrations of the reactants, or at least on one of them, these particular power dependences are not required. The actual form must be determined experimentally. Unlike the situation in thermodynamics, the concentration exponents in the expression for the rate of reaction are not predictable from the net chemical reaction. 

Although many industrial reactions are carried out in flow reactors, this procedure is not often used in mechanistic work. Most experiments in the liquid phase that are carried out for that purpose use a constant-volume batch reactor. Thus, we shall not consider the kinetics of reactions in flow reactors, which only complicate the algebraic treatments. Because the reaction volume in solution reactions is very nearly constant, the rate is expressed as the change in the concentration of a reactant or product per unit time. Reaction rates and derived constants are preferably expressed with the second as the unit of time, even when the working unit in the laboratory is an hour or a microsecond. Molarity (mol L-1 or mol dm 3, sometimes abbreviated M) is the preferred unit of concentration. Therefore, the reaction rate, or velocity, symbolized in this book as v, has the units mol L-1 s-1. 

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