When we discussed energy diagrams, we mentioned that chemical reactions can move in the backward direction as well as the forward direction, forming reactants from the products. Such a reaction is referred to as being "reversible," and has the equation A ⇔ B.
When the forward and reverse reactions for a reversible reaction take place at the same rate, the system is at equilibrium and the concentrations of both the products and reactants stay the same.
We learned that as the concentration of reactants increases, the rate of the reaction increases. As you might imagine, the same is true for the products—the reverse reaction will tend to speed up as the concentrations of the products increases. As a result, the concentrations of the reactants and products in a reversible reaction change in the following way over time:
A lot of new chemists think that when a reaction has reached equilibrium, the process completely stops. Pardon my French, but non! Both the forward and reverse reactions are still taking place—they just take place at the same rate. As a result, the reaction may appear to the casual observer to have stopped even though molecules are still making the conversion between products and reactants.
Excerpted from The Complete Idiot's Guide to Chemistry © 2003 by Ian Guch. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.