James Richard Fromm
The concept of a dynamic equilibrium is central to quantitative discussion of most chemical phenomena. A dynamic equilibrium is a state in which there appears to be nothing happening, a state in which there is no net change, but is also a state in which chemical reactions are taking place, often at rapid rates. These reactions, however, are opposites of each other so that the net change is no change at all. Any chemical system at equilibrium is a dynamic system in which real reactions are occurring with real speed. And, as we shall see in a different section, if that system should be altered in some way by external means the reactions will operate so as to bring it back to equilibrium again.
A dynamic equilibrium, like a balanced teeter-totter in a school playground, is a condition which can be expressed in either of two ways. One of these ways is to say that the condition of equilibrium is one in which the driving forces of the chemical reaction are equal in both of the opposite directions. If two children of equal mass sit on a teeter-totter equally distant from its center, each applies a driving force to rotate the teeter-totter but the forces are equal but opposed and the teeter-totter does not move. The alternative expression of a dynamic chemical equilibrium is to say that the condition of equilibrium is one in which the rates of movement of the system are equal in both of the opposite directions. Again, if the two ends of the teeter-totter are moving equally in opposed directions they are not having any net movement at all. On the teeter-totter, of course, up and up are the opposed directions rather than one side up and one side down.
These two expressions of equilibrium appear also as two alternative statements of the chemical principle called the law of mass action. In its chemical thermodynamic form, the law of mass action is:
The driving force of a chemical reaction is directly proportional to the active masses of the reactants.
The alternative chemical kinetic form of the law of mass action is:
The rate of a chemical reaction is directly proportional to the active masses of the reactants.
Since a dynamic chemical equilibrium is the state in which both the driving forces, and the rates, of opposed reactions are equal, the concept has been approached both from the point of view of driving forces and from the point of view of reaction rates. In either case, however, the concept of active mass appears. In other sections equilibrium will be considered from the point of view of opposed driving forces, while in still others the approach to equilibrium from the point of view of rates of opposing reactions will be used.