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6.11: Some Final Thoughts on Equilibrium Calculations

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  • In this chapter we have developed several tools for evaluating the composition of a system at equilibrium. These tools differ in how accurately they allow us to answer questions involving equilibrium chemistry. They also differ in their ease of use. An important part of having several tools available to you is knowing when to use them. If you need to know whether a reaction if favorable, or to estimate the pH of a solution, then a ladder diagram will meet your needs. On the other hand, if you require a more accurate estimate of a compound’s solubility, then a rigorous calculation that includes activity coefficients is necessary.

    A critical part of solving an equilibrium problem is knowing what equilibrium reactions to include. The importance of including all relevant reactions is obvious, and at first glance this does not appear to be a significant problem—it is, however, a potential source of significant errors. The tables of equilibrium constants in this textbook, although extensive, are a small subset of all known equilibrium constants, making it easy to overlook an important equilibrium reaction. Commercial and freeware computational programs with extensive databases are available for equilibrium modeling. Two excellent freeware programs are Visual Minteq (Windows only) and ChemEQL (Windows, Mac, Linux, and Solaris). These programs also include the ability to account for ionic strength.

    Finally, a consideration of equilibrium chemistry can only help us decide if a reaction is favorable. It does not, however, guarantee that the reaction occurs. How fast a reaction approaches its equilibrium position does not depend on the equilibrium constant. The rate of a chemical reaction is a kinetic, not a thermodynamic, phenomenon. We will consider kinetic effects and their application in analytical chemistry in Chapter 13.