1. Acid-Base Equilibrium 1
Acid-Base Theory
and
Equilibrium Constants
AP Chemistry

2. Models of Acids and Bases
Arrhenius Concept: Acids produce H+ (or H3O+, hydronium ion) in solution; bases produce OH− ion.
HCl + H2O Cl− + H3O+
acid base
Brønsted-Lowry: Acids are proton (H+ ) donors bases are proton acceptors. 2

3. Conjugate Acid-Base Pairs
HA(aq) + H2O(l) H3O+(aq) + A−(aq)
conj conj conj conj acid base acid base 1
conjugate base: everything that remains of the acid molecule after a proton is lost.
conjugate acid: formed when the proton is transferred to the base. 3

4. Acid Dissociation Constant, Ka
HA(aq) + H2O(l) H3O+(aq) + A−(aq)
Ka =
[H3O+][A-]
[HA] =
[H+][A -]
[HA] 4

5. Acid Strength Strong Acid:
Its equilibrium position lies far to the right. (HNO3)
Large Ka value
Yields a weak conjugate base. (NO3−)
HNO3 + H2O H3O NO3− 5

6. Acid Strength Weak Acid:
Its equilibrium lies far to the left. (CH3COOH)
Small Ka value
Yields a much stronger (it is relatively strong) conjugate base than water. (CH3COO−)
CH3COOH + H2O H3O CH3COO− 6

7. Base Dissociation Constant, Kb
B(aq) + H2O(l) OH-(aq) + HB+(aq)
Kb =
[OH-][HB+]
[B] 7

8. Bases
“Strong” and “weak” are used in the same sense for bases as for acids.
strong = complete dissociation (hydroxide ion supplied to solution)
NaOH(s) → Na+(aq) + OH−(aq) 8

9. Bases (con’t) weak = very little dissociation (or reaction with water)
H3CNH2(aq) + H2O(l) H3CNH3+(aq) + OH−(aq) 9

10. Water is amphoteric. Water can behave either as an acid or a base.
H2O + H2O H3O+ + OH conj conj acid 1 base acid base 1
Kw = [H+] [OH−]
Kw = 1 × 10−14 at 25°C 10

11. Acid-Base Equilibrium 2
The pH Scale
AP Chemistry

12. The pH Scale pH ≈ −log[H+] and pOH = −log[OH-]
pH in water ranges from 0 to 14.
Kw = 1.00 × 10−14 = [H+] [OH−]
pKw = = pH + pOH
As pH rises, pOH falls (sum = ). 12

13. Example Problem
What is the pH, pOH, and [OH-] of a M solution of nitric acid?
HNO3 + H2O H3O NO3−
pH = −log[H+]
pH = −log(0.0020)
pH = 2.70
pH + pOH = pOH = pOH = 11.30
[H+] [OH−] = 1.00 × 10−14 (0.0020) [OH−] = 1.00 × 10−14 [OH−] = 5.0 × 10−12 M

14. Acid-Base Equilibrium 3
Solving Problems
AP Chemistry

15. Solving Weak Acid Equilibrium Problems
List major species in solution.
Choose species that can produce H+ and write reactions.
Based on K values, decide on dominant equilibrium.
Write equilibrium expression for dominant equilibrium.
List initial concentrations in dominant equilibrium. 15

16. (continued) Define change at equilibrium (as “x”).
Write equilibrium concentrations in terms of x.
Substitute equilibrium concentrations into equilibrium expression.
Solve for x the “easy way.”
Calculate [H+] and pH. 16

17. % Dissociation or % Ionization
Amount dissociated (M)
x 100%
Initial concentration (M) 17

18. Acid-Base Equilibrium 4
Ka and Kb Relationships
& Polyprotic Acids
AP Chemistry

19. Ka and Kb Relationship HA(aq) + H2O(l) H3O+(aq) + A−(aq) Ka
A-(aq) + H2O(l) OH-(aq) + HA(aq) Kb
H2O(l) + H2O(l) H3O+(aq) + OH-(aq) Kw
[H3O+][A-]
[HA]
[OH-][HA]
[A] =
[H3O+]
[OH-] Ka Kb Kw x =

20. Polyprotic Acids
. . . can furnish more than one proton (H+) to the solution.
H2CO H+ + HCO Ka1
HCO H+ + CO Ka2
H2CO H+ + CO Ka
Ka1 x Ka2 = Ka(overall) 20

21. Ka and Kb Relationship for Polyprotic Acids
For diprotic acids
Ka1 x Kb2 = Kw
Ka2 x Kb1 = Kw
For triprotic acids
Ka1 x Kb3 = Kw
Ka2 x Kb2 = Kw
Ka3 x Kb1 = Kw

22. Acid-Base Equilibrium 5
Miscellaneous Acid-Base Concepts
AP Chemistry

23. Acidic and Basic Salts 23

24. Molecular Structure and Acidic Properties
Two factors affecting acidity in binary compounds
Bond Polarity (higher is more acidic)
Bond Strength (lower is more acidic) 24

25. Oxides of Metal and Nonmetals
1. Acidic Oxides (Acid Anhydrides):
Nonmetal Oxides (Examples: SO2, CO2, CrO3)
O−X bond is strong and covalent.
2. Basic Oxides (Basic Anhydrides):
Metal Oxides (Examples: K2O, CaO)
O−X bond is ionic. 25

26. Examples of Anhydrides
1. Acidic Oxides (Acid Anhydrides):
H2O + CO H2CO3
N2O H2O HNO3
2. Basic Oxides (Basic Anhydrides):
K2O + H2O KOH
CaO + H2O Ca(OH)2 26

27. Coordinate Covalent Bond
Lewis Acids and Bases
Lewis Acid: electron pair acceptor
Lewis Base: electron pair donor H H F F
B N F B N H F : H :
Lewis Lewis Acid Base F H F H
Coordinate Covalent Bond 27