Chemistry Reference
In-Depth Information
An acid and its conjugate base, or a base and its conjugate acid, are
often called “conjugate acid-base pairs.” Conjugate acid-base pairs are two
substances that are related by the gain or loss of a single proton (H
+
). In
our example, water (H
2
O) and the hydroxide ion (OH
-
) make up one con-
jugate acid-base pair, and ammonia (NH
3
) and the ammonium ion (NH
4
+
)
make up the other conjugate acid-base pair.
The following summarizes some of the important terms presented in
this section:
Brønsted -Lowry Acids and Bases
Brønsted-Lowry Acid
A substance that donates a proton to
another substance. Examples include
HCl, H
2
SO
4
, NH
3
, and H
2
O.
Brønsted-Lowry Base
A substance that accepts a proton from
another substance. Examples include
KOH, LiOH, NH
3
, and H
2
O.
Conjugate Acid
The substance that is left after a base
accepts a proton. For example, NH
4
+
is
left after NH
3
accepts a proton.
Conjugate Base
The substance that is left after an acid
donates a proton. For example, OH
-
is
left, after H
2
O donates a proton.
Conjugate Acid-Base Pairs
Two substances that are related by the
gain or loss of a single proton. An
example of a pair is NH
3
and NH
4
+
.
If you took a good look at the information just presented, you might
think that I made a couple of mistakes. Did you notice that I included wa-
ter (H
2
O) and ammonia (NH
3
) as examples of both Brønsted-Lowry acids
and Brønsted-Lowry bases? This is no error. Water and ammonia are ex-
amples of amphoteric substances. An amphoteric substance is a substance
that acts as an acid in some cases and as a base in other cases. Look at the
following sets of reactions to see examples of these substances acting as
both acids and bases.
1.
Water acting as an acid (proton donor)
H
2
O
(l)
+ NH
3(aq)
OH
-
(aq)
+ NH
4
+
(aq)
acid
base
conjugate conjugate
base
acid
