For bases, the concentration of OH− must exceed the concentration of H+ in the solution. This imbalance can be created in two different ways.
First, the base can be a hydroxide, which merely dissociates to yield hydroxide ions:
where M represents the cation, usually a metal. The most familiar bases are such hydroxides. (See Table
1 .)
|
Base
|
Formula
|
Ions
|
|
|
Sodium hydroxide
|
NaOH
|
Na+
|
OH−
|
|
Potassium hydroxide
|
KOH
|
K+
|
OH−
|
|
Calcium hydroxide
|
Ca(OH)2
|
Ca2+
|
2OH−
|
|
Aqueous ammonia
|
NH3
(aq)
|
|
OH−
|
|
The second type of base acts by extracting a hydrogen ion from a water molecule, leaving a hydroxide ion:
An example of this second type of base that is not a hydroxide can be an ammonia molecule in water (aqueous ammonia):
Ammonia acts as a base by stripping a proton from a water molecule, leaving an increased OH− concentration. Notice in the equilibrium reaction that
and NH3 are a
conjugate acid-base pair, related by transferring a single proton.
Alternatively, the base may be a particular kind of negative ion with a high attraction for a hydrogen ion:
In 1923, the English chemist Thomas Lowry and the Danish chemist Johannes Brønsted defined an acid and base in another way. An acid is a substance that can donate a proton, and a base is a substance that can accept a proton.
Elements
Acids and Bases
may serve as either a Brønsted-Lowry acid or base. When it acts as an acid, what is its conjugate base? When it behaves as a base, what is its conjugate acid?
