In previous science classes, you may have learned that one way to distinguish chemical changes from physical changes is that physical changes—such as the melting and freezing of water—are reversible, but that chemical changes are not. In this chapter, we will see that this simple answer is not necessarily what it seems.
- 9.1: Chemical Equilibrium
- Chemical equilibrium can be attained whether the reaction begins with all reactants and no products, all products and no reactants, or some of both. It may be tempting to think that once equilibrium has been reached, the reaction stops. Chemical equilibrium is a dynamic process. The forward and reverse reactions continue to occur even after equilibrium has been reached. Because the rates of the reactions are the same, there is no change in the relative concentrations of reactants and products.
- 9.2: Le Châtelier's Principle
- The description of how a system responds to a stress to equilibrium has become known as Le Châtelier's principle: When a chemical system that is at equilibrium is disturbed by a stress, the system will respond in order to relieve the stress. Stresses to a chemical system involve changes in the concentrations of reactants or products, changes in the temperature of the system, or changes in the pressure of the system.
- 9.3: The Effect of a Volume Change on Equilibrium
- Changing the pressure or volume of a container enclosing an equilibrium system will only affect the reaction if gases are present.
- 9.4: The Path of a Reaction and the Effect of a Catalyst
- Adding a catalyst to a reaction will not affect the position of an equilibrium. A catalyst speeds up both the forward and the reverse reactions, so there is no uneven change in reaction rates. Generally, a catalyst will help a reaction to reach the point of equilibrium sooner, but it will not affect the equilibrium otherwise.