The Foundations of Chemistry

The double arrow ( 34 ) indicates that the reaction is reversible—that is, both the forward
and reverse reactions occur simultaneously. In discussions of chemical equilibrium, the
substances that appear on the left side of the balanced chemical equation are called the
“reactants,” and those on the right side are called the “products.” In fact, the reaction can
proceed in either direction.When A and B react to form C and D at the same rate at which
C and D react to form A and B, the system is at equilibrium.

Chemical equilibriumexists when two opposing reactions occur simultaneously at

the same rate.

Chemical equilibria are dynamic equilibria;that is, individual molecules are continually
reacting, even though the overall composition of the reaction mixture does not change.
In a system at equilibrium, the equilibrium is said to lie toward the right if more C and
D are present than A and B, and to lie toward the left if more A and B are present.
Consider a case in which the coefficients in the equation for a reaction are all 1. When
substances A and B react, the rate of the forward reaction decreases as time passes because
the concentration of A and B decrease.

AB88nCD (1)

As the concentrations of C and D build up, they start to re-form A and B.

CD88nAB (2)

As more C and D molecules are formed, more can react, and so the rate of reaction
between C and D increases with time. Eventually, the two reactions occur at the same
rate, and the system is at equilibrium (Figure 17-1).

AB 34 CD

If a reaction begins with only C and D present, the rate of reaction (2) decreases with
time, and the rate of reaction (1) increases with time until the two rates are equal.

The dynamic nature of chemical

equilibrium can be proved

experimentally by inserting radioactive

atoms into a small percentage of

molecules and following them through

the reaction. Even when the initial

mixture is at equilibrium, radioactive

atoms eventually appear in both

reactant and product molecules.

17-1 Basic Concepts 709

[C] and [D]

[A] and [B]

te

Concentrations

Time

Equilibrium is

established

See the Saunders Interactive

General Chemistry CD-ROM,

Screen 16.3, The Equilibrium State.

Figure 17-1 Variation in the

concentrations of species present in

the AB 34 CD system as

equilibrium is approached, beginning

with equal concentrations of A and

B only. For this reaction, production

of products is favored. As we will

see, this corresponds to a value of

the equilibrium constant greater

than 1. Brackets, [ ], represent the

concentration, in moles per liter, of

the species enclosed within them.

The time axis may be in any

convenient units—seconds, minutes,

hours, and so on.