When chemicals that are capable of reacting with each other are mixed together, a reaction ensues and product is produced. In some cases, the reaction continues to completion--the reactants are used up and only products are present at the completion of the reaction. In many cases, however, the reaction proceeds rapidly at first, slowing down as products accumulate, until reaction ceases, even though there may still be a significant amount of the reactants present. This state of the system is called chemical equilibrium.

In most chemical reactions, if a set of reactants A and B come together and react to form products C and D, then these products are capable of accomplishing the reverse, i.e., product molecules come together and react to form the original reactants. In other words, if the reaction A + B C + D occurs, then the reverse reaction, C + D A + B, is usually...