Activities and Equilibrium Calculations

As mentioned above, in solving equilibrium problems, both concentrations and activity coefficients of the substances participating in the equilibrium must be evaluated. Implicit in Equation 3-3 is the basis for a very important approach to the incorporation of activity coefficients in all equilibrium calculations. As will be discussed below, it will be possible to arrive at a sufficiently good value of the activity coefficient quotient, Q. in terms of a single experimental parameter, the ionic strength. Hence, a useful strategy for taking activities into account in equilibrium calculations consists of  

Obtain the value of the ionic strength of the solution. This, together with the tabulated K value, are combined in a suitable one of the equations from 3-11 to 3-15 to arrive at the value of K which applies to the problem at hand. 

The remainder of the equilibrium calculation is conducted as described in subsequent chapters, but without the need of referring ever again to activity coefficients until after the concentrations have been calculated. 

When this is done, use of the appropriate activity coefficient readily converts the equilibrium concentrations of the species to their activities as needed. 

THROUGHOUT THE REMAINDER OF THIS BOOK, EXCEPT WHERE THE DISCUSSION DEMANDS IT, WE WILL FOLLOW THE PRACTICE OF USING THE SYMBOL K, RATHER THAN K, TO REPRESENT THE APPROPRIATE VALUE. IN NUMERICAL SOLUTIONS TO THE EXAMPLES AND PROBLEMS, USE OF K OR THE ACTIVUY-CORRECTED K, WILL BE OPTIONAL. Such corrections are essential for reasonably accurate answers, but their incorporation into the course will be left in the instructor s hands. 

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