Chemical Reactions The Rate Concept

Even though chemical reactions may go far to completion, the reactions never go in only one direction. In fact, reactions reach an equihbrium in which the rates of reactions in both directions are equal. In this chapter we review the equilibrium concept and the equilibrium constant and describe general approaches for calculations using equihbrium constants. We discuss the activity of ionic species along with the calculation of activity coefficients. These values are required for calculations using thermodynamic equihbrium constants, that is, for the diverse ion effect, described at the end of the chapter. They are also used in potentiometric calculations (Chapter 13). 

In 1863 Guldberg and Waage described what we now call the law of mass action, which states that the rate of a chemical reaction is proportional to the active masses of the reacting substances present at any time. The active masses may be concentrations or pressures. Guldberg and Waage derived an equihbrium constant by defining equihbrium as the condition when the rates of the forward and reverse reactions are equal. Consider the chemical reaction 

According to Guldberg and Waage, the rate of the forward reaction is equal to a constant times the concentration of each species raised to the power of the number of molecules participating in the reaction that is,  

At equilibrium, the rate of the reverse reaction equals the rate of the forward reaction. 

Rearranging these equations gives the molar equJlibrimn constant (which holds for dilute solutions) for the reaction, K  

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