23
Acid-Base Equilibria
In Chapter 16, we apply the fundamental general equilibrium expression to
gaseous equilibrium reactions. In this chapter, we apply the same expression
to the equilibria that involve weak acids and bases in aqueous solution, the
principal difference being that all concentrations are expressed in moles/liter
(rather than in atmospheres as for gases). All the general conclusions given in
Chapter 16, and summarized in the principle of Le Chatelier, apply to
equilibria in solutions as well as to those in gases.
WEAK ACIDS
When we put a strong electrolyte (such as HO) into solution, essentially all
the molecules dissociate to ions—in this case, H"
1
" and Cl~. But when we put
into solution a weak electrolyte, such as acetic acid (HC 2 H 3 O 2 ), only a small
fraction of the molecules dissociate. The equation is
HC 2 H 3 0 2 ?± H
+ + C 2 H 3 0 2 -
Because this reaction is at equilibrium, we can apply the mathematical
expression
[H
+ ][C 2 H 3 0 2 -] _
[HC 2 H 3 0 2 ]
Acid-Base Equilibria
In Chapter 16, we apply the fundamental general equilibrium expression to
gaseous equilibrium reactions. In this chapter, we apply the same expression
to the equilibria that involve weak acids and bases in aqueous solution, the
principal difference being that all concentrations are expressed in moles/liter
(rather than in atmospheres as for gases). All the general conclusions given in
Chapter 16, and summarized in the principle of Le Chatelier, apply to
equilibria in solutions as well as to those in gases.
WEAK ACIDS
When we put a strong electrolyte (such as HO) into solution, essentially all
the molecules dissociate to ions—in this case, H"
1
" and Cl~. But when we put
into solution a weak electrolyte, such as acetic acid (HC 2 H 3 O 2 ), only a small
fraction of the molecules dissociate. The equation is
HC 2 H 3 0 2 ?± H
+ + C 2 H 3 0 2 -
Because this reaction is at equilibrium, we can apply the mathematical
expression
[H
+ ][C 2 H 3 0 2 -] _
[HC 2 H 3 0 2 ]