Dr. Earley
Acid-Base Summary
Kent Stark
Acids and Bases
+
Acid According to the Brφnsted-Lowry definition, an acid is defined as a proton (H + ) donor. Note that H+
aq ≡ H3 Oaq
due to the reaction:
GGGGGG
B
H+ + H2 O F
G H3 O+
Common strong acids include: H3 O+ , H2 SO4 , HNO3 , HCl, HBr, and HI
Base According to the Brφnsted-Lowry definition, a base is a proton (H + ) acceptor.
Conjugate Pairs In a chemical reaction, the product obtained from and acid-base reaction of an acid is a molecule
with one fewer H+ atoms. In the reverse reaction, this product acts as a base. These two compounds, which
differ by a single H+ , are a conjugate acid/base pair.
Example
CH3 CO2 H
H+ donor
acid
***
***
———- Conjugate Pair ———GGGGGB
CH3 CO2 −
+
NH3
F
GG
+
H acceptor
H+ acceptor
base
base
———- Conjugate Pair ———***
+
***
NH4 +
H+ donor
acid
Dissociation The splitting apart of an acid into H+
aq and an anion. Alternatively, the reaction of an acid with water to
produce H3 O+
aq and an anion. For example:
GGGGGG
B
HNO3 F
G H+ + NO3 −
GGGGGG
B
HNO3 + H2 O F
G H3 O+ + NO3 −
Acid Dissociation constant (Ka ) and pKa Measure of acid strength. Large values for Ka and small values for pKa
indicate stronger acids. For the dissociation of HNO3 given above:
Ka =
[H + ][N O3− ]
[HN O3 ]
≡
[H3 O + ][N O3− ]
[HN O3 ]
pKa = –log10 (Ka )
pKa Values for Common Weak Acids
CH3 COOH
4.8
H2 CO3
6.4
Weak Acid
HCO3 − H3 PO4 H2 PO4 −
10.2
2.2
7.2
pKa
HPO4 −2
12.7
NH4 +
9.2
pH Measure of acid/base strength. pH = 7 is neutral. pH < 7 is acidic. pH > 7 is basic. pH is defined as:
pH = -log10 ([H+ ])
Buffers A buffer solution is a mixture of a weak acid and its conjugate weak base. Buffers are important because they
prevent drastic pH changes from occurring. The pH of a buffer solution can be calculated using the HendersonHasselbalch equation:
− [A ]
pH = pKa + log10 [HA]
where HA is the acid and A− is the conjugate base
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Dr. Earley
Acid-Base Summary
Kent Stark
Acid-Base Equilibrium In an acid-base reaction, the products will normally be a new acid and a new base. The
position of the equilibrium for these reactions can be predicted by recognizing that the equilibrium will shift
away from the stronger acid. In other words, the stronger acid will react (going away). If one of the two acids
is a strong acid, the equilibrium will be away from this strong acid. For weak acids, the equilibrium will be
away from the acid with the smaller pKa value (away from the stronger acid). Using these ideas, the equilbrium
position for each of the following reactions can be predicted.
=⇒ because HNO3 is a strong acid
GGGGGG
B
HNO3 + CO3 −2 F
G HCO3 − + NO3 −
GGGGGG
B
NH4 + + CO3 −2 F
G NH3 + HCO3 −
=⇒ because NH4 + is a stronger acid than HCO3 −
GGGGGG
B
CH3 COOH + H2 PO4 − F
G CH3 COO− + H3 PO4
⇐= because H3 PO4 is a stronger acid than CH3 COOH
Amino Acid Structure The general structure for amino acids is shown below. The only difference between the amino
acids is the difference in the R group, which is commonly referred to as the side chain. Amino acids contain
both an acidic functional group (the carboxylic acid), and a basic functional group (the amine). The acid and
base forms of these functional groups are shown below. In addition, a few of the amino acids have functional
groups in the side chain that can act as acids or bases.
H
H
N
H
C
R
O
C
.
OH
Carboxylic Acid
Amine
Acid Form
–COOH
H 3 N+ –
Basic Form
–COO−
H2 N–
Acid/Base Forms of Amino Acids Amino acids (and proteins) found in the body are almost always present in a fluid
that is buffered. If we know the pH of a buffer solution, we can predict which structural form of each of the
functional groups will be most common (predominant) in amino acids and proteins. Several methods can be
used to understand this.
1. Chemical Equation Approach
If we assume the buffer can be approximated as the conjugate pair HA/A − and that the pKa of HA is approximated by the buffer pH, the following equilibrium expressions can be written.
GGGGGG
B
R–COOH + A− F
G R–COO− + HA
GGGGGG
B
H3 N+ –R’ + A− F
G H2 N–R’ + HA
If RCOOH is the stronger acid, the equilibrium will shift to the right. If HA is the stronger acid (estimated as
the buffer solution pH), the equilibrium will shift to the left (the acid form is predominant). If HA is the weaker
acid, the equilibrium will shift to the right, and the base form will be predominant.
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Dr. Earley
Acid-Base Summary
Kent Stark
2. Procedural Approach
• Compare the solution pH with the pKa of a functional group.
• If the solution pH > pKa (solution pH on basic side of pKa ), then the base form of the functional group
will predominate.
• If the solution pH < pKa (solution pH on acidic side of pKa ), then the acid form of the functional group
will predominate.
• Repeat for each functional group with acid/base properties.
3. Graphical Approach
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