Equilibrium
IN THIS CHAPTER
Summary: We’ve been discussing chemical reactions for several chapters. In the
Kinetics chapter you saw how chemical reactions take place and some of the factors
that affect the reactions’ speed. In this chapter, we will discuss another aspect of
chemical reactions: equilibrium.
A few chemical reactions proceed to completion, using up one or more of the reac-
tants and then stopping. However, most reactions behave in a different way. Consider
the general reaction:
aA + bB → cC + dD
Reactants A and B are forming C and D. Then C and D start to react to form A and B:
cC + dD → aA + bB
These two reactions proceed until the two rates of reaction become equal. That
is, the speed of production of C and D in the first reaction is equal to the speed of
production of A and B in the second reaction. Since these two reactions are occur-
ring simultaneously in the same container, the amounts of A, B, C, and D become
constant. A chemical equilibrium has been reached, in which two exactly opposite
reactions are occurring at the same place, at the same time, and with the same rates
of reaction. When a system reaches the equilibrium state, the reactions do not stop.
A and B are still reacting to form C and D; C and D are still reacting to form A and B.
But because the reactions proceed at the same rate, the amounts of each chemical
species are constant. This state is sometimes called a dynamic equilibrium state to
emphasize the fact that the reactions are still occurring—it is a dynamic, not a static
state. An equilibrium state is indicated by a double arrow instead of a single arrow.
For the reaction above it would be shown as:
aA++bBcC dDCHAPTER
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