5

A

MAJOR THEME IN CHEMISTRY is chemical equilibrium:that point

during the course of a chemical reaction where there is no further net

change in the chemical composition of the system. One of the triumphs of

thermodynamics is that it can be used to understand chemical equilibria.

When you stop and think about it, very few chemical processes are actually

atchemical equilibrium. Consider the chemical reactions going on in your

body’s cells. If they were at equilibrium, you wouldn’t even be alive! Many

chemical reactions that occur on the industrial scale aren’t at equilibrium, or

else chemical producers wouldn’t be making new chemicals for sale.

Then why do we put so much stock in equilibria? For one thing, a system

in equilibrium is a system we can understand using thermodynamics. Also,

though almost all chemical systems of interest aren’t at equilibrium, the idea

of equilibrium is used as a starting point. The concept of chemical equilibrium

is the very basis for understanding systems that are notat equilibrium. An

understanding of equilibrium is a central part of understanding chemistry.

5.1 Synopsis

In this introductory chapter, we will define chemical equilibrium. The Gibbs

free energy is the energy that is most useful to us, because processes at con-

stant Tand p(conditions that are easily established) have dGas a spontaneity

condition. Therefore, we will relate the idea of chemical equilibrium to the

Gibbs free energy. Chemical reactions go only so far toward completion, and

we will define extent as a means of expressing how far a reaction proceeds as

pure reactants proceed towards products. We will use extent to help define

chemical equilibrium.

Since Gis related to the chemical potential, we will see how chemical po-

tential is related to equilibrium. We will see how the equilibrium constant be-

comes a characteristic for any chemical process. We will find out why solids

and liquids do not contribute numerically to values of most equilibrium con-

stants, and why concentrations of solutes in solutions do. Finally, we will

consider the fact that the values of equilibrium constants do change with

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5.1 Synopsis

5.2 Equilibrium

5.3 Chemical Equilibrium

5.4 Solutions and Condensed
Phases

5.5 Changes in Equilibrium
Constants

5.6 Amino Acid Equilibria

5.7 Summary

Introduction to

Chemical Equilibrium