Acids and Bases
Chapter 15
15.1-15.10 and 15.12
BrØnstead Acids and BrØnstead
Bases
• Recall from chapter 4:
– BrØnstead Acid- a substance capable of
donating a proton.
– BrØnstead Base- a substance capable of
accepting a proton.
– Conjugate Acid-Base Pair- an acid and its
conjugate base or a base and its conjugate
acid.
BrØnstead Acids and BrØnstead
Bases
NH3 + H2O ↔ NH4+ + OH-
BrØnstead Acids and BrØnstead
Bases
The Acid-Base Properties of Water
autoionization of water
H+ (aq) + OH- (aq)
H2O (l)
H
O
H
+ H
[H
O
H
]
H
+ H
H
base
H2O + H2O
acid
O
+
conjugate
acid
H3O+ + OHconjugate
base
O
-
The Ion Product of Water
• Hydrogen Ion concentration is key
• Ion Product Constant (Kw)- the
product of the molar concentrations of H+
and OH- ions at a particular temperature.
At 250C
Kw = [H+][OH-] = 1.0 x 10-14
The Ion Product of Water
[H+] = [OH-]
[H+] > [OH-]
[H+] < [OH-]
Solution is
neutral
acidic
basic
The Ion Product of Water
What is the concentration of OH- ions in a HCl solution
whose hydrogen ion concentration is 1.3M?
Kw = [H+][OH-] = 1.0 x 10-14
[H+]= 1.3M
Kw
[OH ] =
[H+]
-14
1
x
10
[OH-] =
1.3
[OH-] = 7.7 x 10-15 M
The Ion Product of Water
pH a measure of Acidity
• pH- the negative logarithm of the
hydronium ion concentration (mol/L).
pH = -log [H+]
pOH = -log [OH-]
pH + pOH = 14.00
pH calculations
The pH of rainwater collected in a certain region of the
northeastern United States on a particular day was 4.82.
What is the H+ ion concentration of the rainwater?
pH = -log [H+]
[H+] = 10-pH
[H+]= 10-4.82
[H+]= 1.5 x 10-5 M
pH Calculations
The OH- ion concentration of a blood sample is 2.5 x 10-7 M.
What is the pH of the blood?
pH + pOH = 14.00
pOH = -log [OH-]
pOH = -log (2.5 x 10-7)
pOH = 6.60
pH = 14.00 – pOH = 14.00 – 6.60 = 7.40
pH = 7.40
Strength of Acids and Bases
• Strong Acids- are strong electrolytes that
are assumed to ionize completely in water.
• Weak Acids- acids that ionize only to a
limited extent in water.
• Strong Bases- are strong electrolytes
that ionize completely in water.
• Weak Bases- weak electrolytes.
Strength of Acids and Bases
Conjugate Acid-Base Pairs
Conjugate Acid-Base Pairs
Conjugate acid-base pairs:
•
The conjugate base of a strong acid has no
measurable strength.
•
H3O+ is the strongest acid that can exist in aqueous
solution.
•
The OH- ion is the strongest base that can exist in
aqueous solution.
Weak Acids and Acid Ionization
Constants
• Acid Ionization Constant (Ka)- the
equilibrium constant for the ionization of
an acid.
– The larger the ka, the larger the stronger the
acid.
HA (aq) + H2O (l)
HA (aq)
H3O+ (aq) + A- (aq)
H+ (aq) + A- (aq)
[H+][A-]
Ka =
[HA]
Weak Acids and Acid Ionization
Constants
What is the pH of a 0.5 M HF solution (at 250C)?
HF (aq)
H+ (aq) + F- (aq)
[H+][F-]
= 7.1 x 10-4
Ka =
[HF]
ICE
Weak Acids and Acid Ionization
Constants
HF (aq)
Initial (M)
Change (M)
Equilibrium (M)
H+ (aq) + F- (aq)
0.50
0.00
0.00
-x
+x
+x
0.50 - x
x
x
Weak Acids and Acid Ionization
Constants
x2
= 7.1 x 10-4
Ka =
0.50 - x
[HF] = 0.50 – x = 0.48 M
Ka << 1
[H+] = [F-] = 0.019 M
0.50 – x  0.50
pH = -log [H+] = 1.72
Ka 
x2
= 7.1 x 10-4
0.50
x2 = 3.55 x 10-4
x = 0.019 M
When can we use the
Approximation Method?
When can I use the approximation Method?
When x is less than 5% of the value from which it is subtracted.
x = 0.019
0.019 M
x 100% = 3.8%
0.50 M
Less than 5%
Approximation ok.
The Quadratic Equation
• If approximation method does not work,
use the quadratic equation.
– Write the ionization expression in terms of x.
-b ± b2 – 4ac
x=
2a
Solving Weak Acid Ionization
Problems
• Solving weak acid ionization problems:
• Identify the major species that can affect the pH.
–
–
In most cases, you can ignore the autoionization of water.
Ignore [OH-] because it is determined by [H+].
• Use ICE to express the equilibrium concentrations in
•
•
terms of single unknown x.
Write Ka in terms of equilibrium concentrations. Solve
for x by the approximation method. If approximation is
not valid, solve for x exactly.
Calculate concentrations of all species and/or pH of the
solution.
Percent Ionization
• Percent Ionization- measure of the
strength of an acid.
Ionized acid concentration at equilibrium
x 100%
percent ionization =
Initial concentration of acid
For a monoprotic acid HA:
Percent ionization =
[H+]
[HA]0
x 100%
Percent Ionization
Weak Bases and Base Ionization
Constants
• Base Ionization Constant- the
equilibrium constant for the ionization of a
base.
NH3 (aq) + H2O (l)
NH4+ (aq) + OH- (aq)
[NH4+][OH-]
Kb =
[NH3]
Weak Bases and Base Ionization
Constants
Relationship between Ionization
Constants and their Conjugate
Bases
KaKb = Kw
Weak Acid and Its Conjugate Base
Kw
Ka =
Kb
Kw
Kb =
Ka
Diprotic and Polyprotic Acids
• An ionization constant expression can be
written for each ionizable proton.
• Two or more expressions may be
necessary to calculate the concentrations
of all species present in the acid solution.
Diprotic and Polyprotic Acids
Diprotic and Polyprotic Acids
Molecular Structure and Strength of
Acids
• Strength of an acid depends on:
– Structure of the acid
– Strength of the bond
– Polarity of bond
H X
H+ + X-
The
stronger
the bond
The
weaker
the acid
Molecular Structure and Strength of
Acids
Hydrohalic Acids
• Order of relative acid strength:
H X
H+ + X-
HF << HCl < HBr < HI
Acid-Base Properties of Salts
• Salt- an ionic compound formed by the
reaction between an acid and a base.
They are string electrolytes that
completely dissociate into ions in water.
• Salt Hydrolysis- the reaction of an anion
or a cation of a salt, or both, in water.
Usually affects the pH of a solution.
Salts that produce Neutral Solutions
Neutral Solutions:
Salts containing an alkali metal or alkaline earth metal
ion (except Be2+) and the conjugate base of a strong
acid (e.g. Cl-, Br-, and NO3-).
NaCl (s)
H 2O
Na+ (aq) + Cl- (aq)
Salts that produce Basic Solutions
Basic Solutions:
Salts derived from a strong base and a weak acid.
NaCH3COOH (s)
H 2O
CH3COO- (aq) + H2O (l)
Na+ (aq) + CH3COO- (aq)
CH3COOH (aq) + OH- (aq)
Salts that produce Acidic Solutions
Acid Solutions:
Salts derived from a strong acid and a weak base.
NH4Cl (s)
NH4+ (aq)
H 2O
NH4+ (aq) + Cl- (aq)
NH3 (aq) + H+ (aq)
Salts with small, highly charged metal cations (e.g. Al3+,
Cr3+, and Be2+) and the conjugate base of a strong acid.
Al(H2O)3+
6 (aq)
Al(OH)(H2O)52+(aq) + H+ (aq)
Lewis Acids and Bases
Lewis Acid- a substance that can accept a
pair of electrons.
Lewis Base- a substance that can donate a
pair of electrons.
H
••
H+ + N H
acid
H
••
•• + OH
••
acid base
H+
••
H O H
••
base
H
+
H N H
H
Lewis Acids and Bases
B(OH)3(aq) + H2O(l)
↔ B(OH4)- + H+
(aq)
CO2(g) + H2O(l) ↔ H2CO3(aq)
(aq)