2/6/2004 OFB Chapter 8 1 Chapter 8 Acid-Base Equilibria • 8-1 Brønsted-Lowry Acids and Bases • 8-2 Water and the pH Scale • 8-3 The Strengths of Acids and Bases • 8-4 Equilibria Involving Weak Acids and Bases • 8-5 Buffer Solutions • 8-6 Acid-Base Titration Curves • 8-7 Polyprotic Acids • 8-8 Lewis Acids and Bases
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2/6/2004 OFB Chapter 8 1
Chapter 8Acid-Base Equilibria
• 8-1 Brønsted-Lowry Acids and Bases
• 8-2 Water and the pH Scale• 8-3 The Strengths of Acids and
Bases• 8-4 Equilibria Involving Weak
Acids and Bases• 8-5 Buffer Solutions• 8-6 Acid-Base Titration Curves• 8-7 Polyprotic Acids• 8-8 Lewis Acids and Bases
2/6/2004 OFB Chapter 8 2
Acid and Base Definitions
1) Arrhenius (Section 4.3)• Acids are H+ donors• Bases are OH- donors
2) Broadened Definition (Section 4.3)
• Acids are substances that increase [H+]
• Bases are substances that increase [OH-]
3) Brønsted-Lowry (Section 8.1)••
4) Lewis (Section 8.8)••
2/6/2004 OFB Chapter 8 3
Chapter 8Acid-Base Equilibria
• Brønsted-Lowry– Acids
– Bases
– Conjugate Base -
– Conjugate Acid
2/6/2004 OFB Chapter 8 4
Acid-Base Equilibria
Brønsted-Lowry Acids and Bases
A Brønsted-Lowry acid is a substance that can donate a hydrogen ion.
A Brønsted-Lowry base is a substance that can accept a hydrogen ion.
In the Brønsted-Lowry Acid and Base concept, acids and bases occur as conjugate acid-base pairs.
2/6/2004 OFB Chapter 8 5
Conjugate Base - subtract an H+ from the acidConjugate Acid add H+ to the base
Examples
1. is the conjugate base of H2O
2. is the conjugated base of H3O+ (called the hydronium ion)
3. is the conjugated acid of OH-
4. (or shown as H+) is the conjugate acid of H2O
2/6/2004 OFB Chapter 8 6
H2O
- H+ + H+
OH-conjugate
base of H2O
conjugate acid of H2O
H3O+
+ H+- H+
H2OWater is the conjugate acid
of OH-
Water is the conjugate base
of H3O+
Hydroxide ion
Hydronium ion
Conjugate Base - subtract an H+ from the acidConjugate Acid add H+ to the base
2/6/2004 OFB Chapter 8 7
CH3CO2H + H2O ↔ H3O+ + CH3CO2-
Pairs 1 2 2 1
Acetic Acid
Point of View #1
acid base Conjugate acid of H2O
Conjugate base of
CH3CO2H
CH3CO2H + H2O ↔ H3O+ + CH3CO2-
Point of View #2
acid baseConjugate base of H3O+
Conjugate acid of
CH3CO2-
CH3CO2H + H2O ↔ H3O+ + CH3CO2-
basebaseacid acid
Acetate Ion
2/6/2004 OFB Chapter 8 8
H2O + H2O ↔ H3O+ + OH-
Pairs 1 2 2 1basebaseacid acid
Autoionization of H2O
Point of View #2
acid baseConjugate base of H3O+
Conjugate acid of OH-
H2O + H2O ↔ H3O+ + OH-
Point of View #1
acid base Conjugate acid of H2O
#2
Conjugate base of H2O #1
H2O + H2O ↔ H3O+ + OH-# 1 # 2
2/6/2004 OFB Chapter 8 9
Exercise 8-1:
Trimethylamine (C3H9N) is a soluble weak base with a foul odor (it contributes to the smell of rotten fish). Write the formula of its conjugate acid.
“… the formula of a conjugate acid is obtained by adding H+ to the formula of the base.”
2/6/2004 OFB Chapter 8 10
Nomenclature
When H+ is hydrated it is H3O+
and called a hydronium ion.
Often H3O+ is written in a simpler notation H+
111.7°
+
2/6/2004 OFB Chapter 8 11
Amphoterism - an ion or molecule can act as an acid or base depending upon the reaction conditions
H2O + NH3 ↔ NH4+ + OH-
1.) Water in NH3 serves as an acid
2.) Water in acetic acid serves as a base
H2O + CH3CO2H ↔ H3O+ + CH3CO2-
2/6/2004 OFB Chapter 8 12
3.) Acetic Acid is also amphoteric, if in the presence of a strong acid serves as a base
H2SO4 + CH3CO2H ↔ CH3CO2H2+ + HSO4
-
Amphoterism - an ion or molecule can act as an acid or base depending upon the reaction conditions
2/6/2004 OFB Chapter 8 13
Water
[H3O+][OH-]
[H2O]2
Autoionization of water:
2 H2O(l) ⇔ H3O+(aq) + OH-(aq)
KW = 1.0 × 10-14
(at 25oC)
2/6/2004 OFB Chapter 8 14
Strong Acids and Bases
A strong acid is one that reacts essentially completely with water to produce H3O+(aq).
The strength of a base is inversely related to the strength of its conjugate acid; the weaker the acid, the stronger its conjugate base, and vice versa
wba KKK =
2/6/2004 OFB Chapter 8 28
A soluble compound, generally an organic dye, that changes its color noticeably over a fairly short range of pH.Typically, a weak organic acid that has a different color than its conjugate base.
HIn(aq) + H2O(l) ⇔ H3O+(aq) + In-(aq)
[H3O+][In-]
[HIn]= Ka
Indicators (denoted by In)
2/6/2004 OFB Chapter 8 29
2/6/2004 OFB Chapter 8 30
Methyl Red
Bromothymol blue
Phenolphtalein
2/6/2004 OFB Chapter 8 31
2/6/2004 OFB Chapter 8 32
Equilibria Involving Weak
Acids and Bases
Weak acids Ka < 1
i.e., pKa > 0
H3O+ (hydronium ion)
Ka = 1
pKa =0
HA + H2O↔ H3O+ + A-
HA is a weak acid,
Ka < 1 or pKa > 1
2/6/2004 OFB Chapter 8 33
2/6/2004 OFB Chapter 8 34
2/6/2004 OFB Chapter 8 35
CH3CO2H + H2O ↔ H3O+ + CH3CO2-
Init. conc.
∆ conc.
Equil. conc.
==+
O]H][HCO[CH]CO][CHO[H K
223
-233
a
Problem: (a) Calculate pH and (b) the fraction of CH3CO2H ionized at equilibrium. Assume 1.0M CH3CO2H initially
2/6/2004 OFB Chapter 8 36
2-8 Table from 1.8x10K 5a
−=
A trick to solving:
Assume that y is small (less than 5% of the initial conc.)
y)(1.0y
O]H][HCO[CH]CO][CHO[H K
2
223
-233
a −==
+
2.37 )(4.24x10 log- pH
]O[H10 x 4.24 y 1.8x10y
3-3
3-
52
==
==
=+
−
2/6/2004 OFB Chapter 8 37
(b) Fraction CH3CO2H ionized at equilibrium
Problem: (a) Calculate pH and (b) the fraction of CH3CO2H ionized at equilibrium. Assume 1.0M CH3CO2H initially
CH3CO2H + H2O ↔ H3O+ + CH3CO2-
2/6/2004 OFB Chapter 8 38
Weak Bases
Similarly NH3 acts as a weak base in H2O
Kb = 1.8 x 10-5
H20 + NH3 ↔ NH4+ + OH-
acid1 base2 acid2 base1
O]][H[NH]][OH[NH K
23
-4
b
+
= 1
2/6/2004 OFB Chapter 8 39
Init. conc.
∆ conc.
Equil. conc.
As before, you can calculate a pH at equilibrium. Assume 0.01M NH3initially. Calculate the pH of the resulting solution
H20 + NH3 ↔ NH4+ + OH-
53 b
3
-4
b
1.8x10NHfor K
y)(0.01y2
][1][NH]][OH[NH K
−
+
=
−==
2/6/2004 OFB Chapter 8 40
10.62 )(2.4x10log- pH
2.4x104.2x101x10
][OHKw]O[H
11-10
114
14
3
==
=== −−
−
−+
As before, you can calculate a pH at equilibrium. Assume 0.01M NH3initially. Calculate the pH of the resulting solution
4-4
-
52
b
2
3
-4
b
4.2x10][NH ][OH y
1.8x10 01.0
yK
y0.01y
][1][NH]][OH[NH K
===
=≈
=
−==
+
−
+
2/6/2004 OFB Chapter 8 41
• Hydrolysis is a term applied to reactions of aquated ions that change the pH from 7
• When NaCl is placed in water, the resulting solution is observed to be neutral (pH = 7)• However when sodium acetate (NaC2H3O2) is dissolved in water the resulting solution is basic• Other salts behave similarly, NH4Cl and AlCl3 give acid solutions.• These interactions between salts and water are called hydrolysis
2/6/2004 OFB Chapter 8 42
Despite the special term, hydrolysis, there is no reason to treat hydrolysis in a special manner.
It is still a Brønsted-LowryAcid and Base Reaction
2/6/2004 OFB Chapter 8 43
Example problem:
Suppose a 0.1 mole solution sodium acetate is dissolved in 1 liter of water. What is the pH of the solution?
Init. conc.
∆ conc.
Equil. conc.
1.Find Kb
2.Find [OH-]
3.Find [H+]
4.Find pH
2/6/2004 OFB Chapter 8 44
Example problem:
What is the pH of the solution?CH3CO2
- + H2O ↔ CH3CO2H + OH-
Init. conc. 0.1M 0 ~0
∆ conc. - y + y + y
Equil. conc. 0.1– y y y
1. Find Kb
2. Find [OH-]
3. Find [H+]
4. Find pH
6
45
14
a
wb
2
-23
-23
b
7.5x10][OHy
5.6x10 table1.8x10
1.0x10KK K
y0.1y
][1]CO[CH]H][OHCO[CH K
−−
−−
−
==
===
−==
Ka x Kb = Kw
8.89 )(1.39x10log- pH
x103.17.5x101x10
][OHKw]O[H
9-10
96
14
3
==
=== −−
−
−+
2/6/2004 OFB Chapter 8 45
Raise pH
Lower pH
Anions
Cations
ResultHydrolysis of
Non-Hyrolyzed Ions (a few)7 Anions, not hydrolyzed