In Chapter 13 we were concerned with the question: Will a given reaction ever take place spontaneously, even if one waits forever? The answer was that any reaction that leads to a lower free energy will occur spontaneously. Any reaction that requires an increase in free energy will not; it will be spontaneous in the reverse direction instead. Now we come to a more difficult but very practical question in this and the following chapter: Granted that a given reaction is spontaneous, how far will it go, and will it take place within a reasonable time? What factors determine the rates of chemical reactions?

Some chemical reactions appear to go essentially to completion, ending with products and an undetectable or negligable amount of reactants. Some but not all of these reactions also are very fast (e.g., explosions). Other reactions stop short of completion and remain a mixture of reactants and products after all visible chemical change is over. Still other reactions do not appear to take place at all within a reasonable time, even though their calculated free energy change is quite negative.