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Equilibrium
IN THIS CHAPTERSummary: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 reaction’s 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 reactants and then stopping. However, most reactions behave in a different
way. Consider the general reaction:
a A + b B →c C + d D
Reactants A and B are forming C and D. Then C and D start to react to form
A and B:
c C + d D → a A + b B
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 occurring simultaneously in the same container, the amounts of
A, B, C, and D become constant. A chemical equilibriumhas 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 dynamicequilibrium 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 bB++cC dDCHAPTER
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