Acids and Bases
•  Brønsted Acids and Bases
•  The Acid-Base Properties of Water
•  The pH Scale
•  Strong and Weak Acids and Bases
•  Conjugate Acid-Base Pairs
Brønsted Acids and Bases
When a Brønsted acid donates a proton, what remains of the acid is
known as a conjugate base.
Loses a proton
Gains a proton
HCl(aq)
acid
+
H2O(l)
base
⇌ H3O+(aq)
conjugate
acid
+
Cl–(aq)
conjugate
base
The two species HCl and Cl– are known as a conjugate acid-base pair or
simply a conjugate pair.
Brønsted Acids and Bases
When a Brønsted base accepts a proton, the newly formed protonated
species is known as a conjugate acid.
Gains a proton
Loses a proton
NH3(aq)
base
+
H2O(l)
acid
⇌ NH4+(aq)
conjugate
acid
+
OH–(aq)
conjugate
base
The Acid-Base Properties of Water
A species that can behave either as a Brønsted acid or a Brønsted base
is called amphoteric.
H2O(l) + H2O(l)
⇌ H3O+(aq)
+
Kw = [H3O+][OH–] = 1.0 x 10–14 (at 25°C)
OH–(aq)
The Acid-Base Properties of Water
The relative amounts of H3O+ and OH– determine whether a solution is
neutral, acidic, or basic.
§ When [H3O+] = [OH–], the solution is neutral
§ When [H3O+] > [OH–], the solution is acidic
§ When [H3O+] < [OH–], the solution is basic
Kw = [H3O+][OH–] = 1.0 x 10–14 (at 25°C)
Practice Problem
The concentration of hydroxide ions in the antacid milk of magnesia is
5.0 x 10–4 M. Calculate the concentration of hydronium ions at 25°C.
The pH Scale
The acidity of an aqueous solution depends on the concentration of
hydronium ions, [H3O+].
The pH of a solution is defined as the negative base-10 logarithm of the
hydronium ion concentration (in mol/L)
pH = –log[H3O+]
[H3O+] = 10–pH
In pure water at 25°C, [H3O+] = log1.0 x 10–7
pH = –log(1.0 x 10–7) = 7.00
pH is a dimensionless quantity.
The pH Scale
The pH Scale
A pOH scale analogous to the pH scale can be defined as the negative
base-10 logarithm of the hydroxide ion concentration.
pOH = –log[OH–]
[OH–] = 10–pOH
From the definition of pH and pOH:
pH + pOH = 14.00
The pH Scale
Practice Problem
Determine the pH of a solution at 25°C in which the hydronium ion
concentration is 3.2 x 10–9 M.
Practice Problem
Calculate the hydronium ion concentration at 25°C in which the pOH is
2.74.
Strong Acids and Bases
Strong acid dissociations are not treated as equilibria, rather as processes
that go to completion.
Hydrochloric acid
HCl(aq) + H2O(l)
H3O+(aq) + Cl–(aq)
Hydrobromic acid
HBr(aq) + H2O(l)
H3O+(aq) + Br–(aq)
Hydroiodic acid
HI(aq) + H2O(l)
H3O+(aq) + I–(aq)
Nitric acid
HNO3(aq) + H2O(l)
H3O+(aq) + NO3–(aq)
Chloric acid
HClO3(aq) + H2O(l)
H3O+(aq) + ClO3–(aq)
Perchloric acid
HClO4(aq) + H2O(l)
H3O+(aq) + ClO4–(aq)
Sulfuric acid
H2SO4(aq) + H2O(l)
H3O+(aq) + HSO4–(aq)
Strong Acids and Bases
The list of strong bases consists of the hydroxides of alkali metals and the
heaviest alkaline earth metals.
Group 1A hydroxides
Group 2A hydroxides
LiOH(aq)
Li+(aq) + OH–(aq)
NaOH(aq)
Na+(aq) + OH–(aq)
KOH(aq)
K+(aq) + OH–(aq)
RbOH(aq)
Rb+(aq) + OH–(aq)
CsOH(aq)
Cs+(aq) + OH–(aq)
Ca(OH)2(aq)
Ca2+(aq) + 2OH–(aq)
Sr(OH)2(aq)
Sr2+(aq) + 2OH–(aq)
Ba(OH)2(aq)
Ba2+(aq) + 2OH–(aq)
Practice Problems
1. Calculate the concentration of HNO3 in a solution at 25°C that has a pH
of 2.06.
2. Calculate the pH of a 6.1 x 10–2 M Ba(OH)2 solution at 25°C.
Weak Acids
The ionization of a weak monoprotic acid HA in water is represented by:
HA(aq) + H2O(l)
⇌ H3O+(aq)
+
⎡⎣H3O+ ⎤⎦ ⎡⎣ A − ⎤⎦
Ka =
[HA]
Ka is called the acid ionization constant.
The larger the value of Ka, the stronger the acid.
Solution (at 25 °C)
Ka
pH
0.10 M HF
7.1 x 10–4
2.09
0.10 M CH3COOH
1.8 x 10–5
2.87
A–(aq)
Practice Problem-Weak Acids
Calculate the pH of a 0.50 M HF solution at 25°C.
HF(aq) + H2O(l)
⇌ H3O+(aq)
+
F–(aq)
⎡⎣H3O+ ⎤⎦ ⎡⎣F− ⎤⎦
Ka =
= 7.1 10−4
[HF]
×
Percent Ionization
A quantitative measure of the degree of ionization is percent ionization.
⎡⎣H3O+ ⎤⎦
eq
percent ionization =
× 100%
[HA ]0
Solution (at 25 °C)
pH
% ionization
0.5 M HF
1.72
3.8
1.0 M HF
1.57
2.7
Percent Ionization
HF(aq) + H2O(l) ⇌ H3O+(aq) + F–(aq)
Solution (at 25 °C)
pH
% ionization
0.5 M HF
1.72
3.8
1.0 M HF
1.57
2.7
Practice Problems
Determine the Ka of a weak acid that has a concentration of 0.065 M and a
pH of 2.96 at 25°C.
Weak Bases
The ionization of a weak base is incomplete and is treated in the same way
as the ionization of a weak acid.
B(aq) + H2O(l)
⇌ HB+(aq)
⎡⎣HB+ ⎤⎦ ⎡⎣OH− ⎤⎦
Kb =
[B]
Kb is called the base ionization constant.
The larger the value of Kb, the stronger the base.
+
OH–(aq)
Practice Problems-Weak Base
Calculate the pH of a 0.040 M ammonia solution at 25°C.
NH3(aq) + H2O(l)
⇌ NH4+(aq)
+
OH–(aq)
⎡⎣NH4 + ⎤⎦ ⎡⎣OH− ⎤⎦
Kb =
= 1.8 10−5
⎡⎣NH3 ⎤⎦
×
Conjugate Acid-Base Pairs
A strong acid ionizes completely in water:
HCl(aq)
H+(aq)
+
Cl–(aq)
No affinity for
the H+ ion
Cl–(aq) + H2O(l)
–(aq)
HCl(aq)
+
OH
X
The chloride ion is a weak conjugate base.
Conjugate Acid-Base Pairs
A strong acid ionizes completely in water:
HF(aq)
⇌ H+(aq)
+
F–(aq)
Strong affinity
for the H+ ion
F–(aq) + H2O(l)
⇌ HF(aq) + OH–(aq)
The fluoride ion is a strong conjugate base.
Conjugate Acid-Base Pairs
A strong acid has a weak conjugate base.
A weak acid has a strong conjugate base.
A strong base has a weak conjugate acid.
A weak base has a strong conjugate base.
Conjugate Acid-Base Pairs
A simple relationship between the ionization constant of a weak acid (Ka)
and the ionization constant of a weak base (Kb) can be derived:
CH3COOH(aq)
CH3COO– (aq) + H2O(l)
H2O(l)
⎡⎣H+ ⎤⎦ ⎡⎣CH3COO− ⎤⎦
Ka =
[CH3COOH]
Kb =
[CH3COOH] ⎡⎣OH− ⎤⎦
⎡⎣CH3COO ⎤⎦
−
⇌ H+(aq)
+
CH3COO–(aq)
⇌ CH3COOH(aq)
⇌ H+(aq)
+
+
OH–(aq)
OH–(aq)
⎡⎣H+ ⎤⎦ ⎡⎣CH3COO− ⎤⎦ [CH3COOH] ⎡⎣OH− ⎤⎦
+
−
⎡
⎤
⎡
⎤⎦
×
=
H
OH
⎣
⎦
⎣
−
⎡⎣CH3COO ⎤⎦
[CH3COOH]
Ka x Kb = Kw
Practice Problems
Determine the Kb of the benzoate ion (C6H5COO–).
Diprotic and Polyprotic Acids
Diprotic and polyprotic acids undergo successive ionizations, losing one
proton at a time, and each has a Ka associate with it.
H2CO3(aq)
HCO3– (aq)
⇌ H+(aq) + HCO3–(aq)
⎡⎣H+ ⎤⎦ ⎡⎣HCO3− ⎤⎦
K a1 =
[H2CO3 ]
⇌ H+(aq) + CO32– (aq)
⎡⎣H+ ⎤⎦ ⎡⎣CO32− ⎤⎦
=
⎡⎣HCO3− ⎤⎦
K a2
Ka1 > Ka2
For a given acid, the first ionization constant is much larger than the
second, and so on.
Practice Problem
Calculate the concentrations of all species present at equilibrium in a
0.10 M solution of oxalic acid (H2C2O4) at 25°C.
H2C2O4(aq)
⇌ H+(aq) + HC2O4–(aq)
HC2O4– (aq)
⇌ H+(aq) + C2O42– (aq)
Ka1 = 6.5 x 10–2
Ka2 = 6.1 x 10–5
Acid-Base Properties of Salt Solutions
Salt hydrolysis occurs when ions produced by the dissociation of a salt
react with water to produce either hydroxide ions or hydronium ions.
Basic salts (conjugates of weak acids):
F–(aq) + H2O(l)
⇌ HF(aq) + OH–(aq)
Acidic salts (conjugates of weak bases)
NH4+(aq) + H2O(l)
⇌ NH3(aq) + H3O+(aq)
Acid-Base Properties of Salt Solutions
Calculate the pH of a 0.10 M solution of sodium fluoride (NaF) at 25°C.
Acid-Base Properties of Salt Solutions
The pH of salt solutions can be qualitatively predicted by determining which
ions facilitate hydrolysis.
Examples
A cation that will make a solution acidic is
§ The conjugate acid of a weak base
NH4+ , CH3NH3+ , C2H5NH3+
§ A small, highly charged metal ion (other than Group
1A or 2A)
Al3+ , Cr3+ , Fe3+ , Bi3+
An anion that will make a solution basic is
§ The conjugate base of a weak acid
CN– , NO2– , CH3COO–
A cation that will not affect the pH of a solution is
§ A Group 1A or heavy Group 2A cation (except Be2+)
Li+ , Na+ , Ba2+
An anion that will not affect the pH of a solution is
§ The conjugate base of a strong acid
Cl– , NO3– , ClO4–
Acid-Base Properties of Salt Solutions
The pH of a solution that contains a salt in which both the cation and the
anion hydrolyze depends on the relative strengths of the weak acid and
base.
Qualitative predictions can be made using the Kb (of the salts anion) and
the Ka (of the salts cation).
§ When Kb > Ka, the solution is basic
§ When Kb < Ka, the solution is acidic
§ When Kb ≈ Ka, the solution is neutral or nearly neutral
Lewis Acids and Bases
A Lewis base is a substance that can donate a pair of electrons.
A Lewis acid is a substance that can accept a pair of electrons.
empty
unhybridized 2pz
orbital
Boron trifluoride
a Lewis acid
Ammonia,
a Lewis base
A coordinate covalent bond
Acids and Bases
Brønsted Acids and Bases
The Acid-Base Properties of Water
The pH Scale
Strong Acids and Bases
The Ionization Constant, Ka
Calculating pH from Ka
Using pH to Determine Ka
The Ionization Constant, Kb
Calculating pH from Kb
Using pH to Determine Kb
The Strength of a Conjugate Acid or Base
The Relationship Between Ka and Kb of a Conjugate Acid-Base Pair
Diprotic and Polyprotic Acids
Basic Salt Solutions
Acidic Salt Solutions
Neutral Salt Solutions
Lewis Acids and Bases