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204 CHEMISTRY FOR THE UTTERLY CONFUSED
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Our goal in this chapter is to help you master the concept of chemical equilibria,
the mathematical representations that we use in equilibrium systems and the
manipulation of equilibrium by factors such as temperature and pressure.
Chapters 15 and 16 will rely on the basic concepts presented in this chapter.
Mastering them here will make things much easier later. Mastering these concepts
will require, you guessed it, Practice, Practice, Practice.
14-1 Equilibrium
Very few chemical reactions proceed to completion, totally using up one or
more of the reactants and then stopping. Most reactions behave in a different
way. Consider the general reaction:
a A b B l c C d D
Reactants A and B are forming C and D. The reaction proceeds until appreciable
amounts of C and D form. Then it is possible for C and D to start to react
to form A and B.
c C d D l 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 equilibrium results when 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 a dynamic equilibrium state to emphasize
the fact that the reactions are still occurring—it is a dynamic, not a static state.
A double arrow instead of a single arrow indicates an equilibrium state. For the
reaction above it would be:
a A b B K c C d D