Chemistry Reference
In-Depth Information
2.
Water acting as a base (proton acceptor)
H
2
SO
4
+ H
2
O
HSO
4
-
+ H
3
O
+
acid
base
conjugate conjugate
base
acid
3.
Ammonia acting as an acid (proton donor)
OH
-
(aq)
+ NH
3(aq)
+ NH
2
-
(aq)
+ H
2
O
(l)
base
acid
conjugate conjugate
base
acid
4.
Ammonia acting as a base (proton acceptor)
HCl + NH
3
NH
4
+
+ Cl
-
acid
base
conjugate conjugate
acid
base
I purposely switched the order of the acid and the base in #3 to point out
a potential mistake that some students make. If your instructor seems to be
in the habit of always writing the acid and base in the same order, don't try to
memorize that order to avoid understanding the definitions. Even if that
trick helped you on one of your quizzes, how is it going to help you on a
standardized test, which your instructor doesn't create? Don't try to look
for patterns that will help you avoid learning the material correctly.
Before we go on to the third, and last, theory of acids and bases, let's
try a couple examples that are typical for this material.
Example 1
What would be the conjugate base for each of the following acids?
A. HF
B. HNO
3
C. H
2
SO
4
D. NH
4
+
Remember: We define the conjugate base as the substance that is left
over when an acid loses a single proton. The key to solving this problem is
to remember that we are removing a bare proton (H
+
), which means that
each acid will change in two ways. First, we will take one “H” off the for-
mula. Second, we must remember to subtract +1 from each formula, be-
cause the hydrogen is leaving its electron behind. Substances with a charge
of +1 become (+1 - 1 = 0) neutral. Neutral substances become (0 - 1 = -1)
negatively charged. So, HF becomes F-; HNO
3
becomes NO
3
-
; H
2
SO
4
be-
comes HSO
4
-
; and NH
4
+
becomes NH
3
. If it helps, you can think of each
change as a balanced reaction, as shown here:
H
2
SO
4
HSO
4
-
+ H
+
Notice that both mass and charge are conserved in the reaction.
