1. Chapter 16: Acids and Bases
Chemistry 140 Fall 2002
CHEMISTRY
Ninth
Edition
GENERAL
Principles and Modern Applications
Petrucci • Harwood • Herring • Madura
Chapter 16: Acids and Bases
Juana Mendenhall, Ph.D.
Assistant Professor
Morehouse College
General Chemistry: Chapter 16
Prentice-Hall © 2007

2. General Chemistry: Chapter 16
Chemistry 140 Fall 2002
Objectives
Calculate values of Ka for cations and Kb for anions from ionization constants of their conjugates and Kw of water, and calculate the pH values of salt solutions in which hydrolysis occurs
Use the relative strengths of Brønsted-Lowry acids and bases to predict the direction of acid-base reactions.
Predict whether certain oxides and hydroxo compounds are acidic, basic, or neutral.
Define what an oxoacid is and predict the relative strengths of oxoacids from their molecular structures.
General Chemistry: Chapter 16
Prentice-Hall © 2007

3. 16-7 Ions as Acids and Bases
[NH3] [H+]
Ka=
[NH4+]
NH4+(aq) NH3(aq) + H+(aq)
acid
base
NH3(aq) + H2O(l) NH4+(aq) + OH-(aq)
[NH4+][OH-] Kb
base
acid =
[NH3]
H2O(l)
H+(aq) + OH-(aq)
Ka x Kb =
[NH4+][OH-]
[NH3][H+] x =
[OH-][H+] Kw =
[NH4+]
[NH3]
[NH3] [H+] [OH-]
Ka=
[NH4+] [OH-] = KW Kb =
1.010-14
1.810-5
= 5.610-10
Ka Kb = Kw
pKa + pKb = pKw = 14.00
General Chemistry: Chapter 16
Prentice-Hall © 2007

4. General Chemistry: Chapter 16
Examples
Calculate the a) base-dissociation constant, Kb, for the fluoride ion, F-
Ka = 6.8 x 10-4
b) Calculate the acid-dissociation constant, Ka, for the ammonium ion,
NH4+ (Kb = 1.8 x 10-5)
General Chemistry: Chapter 16
Prentice-Hall © 2007

5. General Chemistry: Chapter 16
Chemistry 140 Fall 2002
Hydrolysis
is the reaction of ions with water to generate H+ or -OH.
Na+ + H2O → Na+ + H2O
No reaction
Cl- + H2O → Cl- + H2O
No reaction
Hydrolysis
NH4+ + H2O → NH3 + H3O+
Thus far we know salt solutions can be acidic or basic. We can also assume that when salts dissolve in water they are completely ionized and nearly all salt solutions are STRONG electrolytes. While the acid-base properties of salt solutions are due to the behavior of their cations and anions. When when ions react with water to generate H+ or -OH hydrlysis often occurs.
Water (hydro) causing cleavage (lysis) of a bond.
General Chemistry: Chapter 16
Prentice-Hall © 2007

6. 16-8 Molecular Structure and Acid-Base Behavior
Chemistry 140 Fall 2002
16-8 Molecular Structure and Acid-Base Behavior
Why is HCl a strong acid, but HF is a weak one?
Why is CH3CO2H a stronger acid than CH3CH2OH?
There is a relationship between molecular structure and acid strength.
Bond dissociation energies are measured in the gas phase and not in solution.
Strength depends on properties of the solvent and temperature.
General Chemistry: Chapter 16
Prentice-Hall © 2007

7. Strengths of Binary Acids
Chemistry 140 Fall 2002
Strengths of Binary Acids
HI HBr HCl HF
Bond length
160.9 > > > pm
Bond energy
297 < 368 < 431 < kJ/mol
Acid strength
109 > 108 > 1.3106 >> 6.610-4
More energy is required to break the H—F bone, HF is the weak acid. HI has the lowest bond energy, so HI is the strongest acid of the group.
The polarity of the bond actually decreases from HF to HI b/c F is the most electronegative of the halogens.
HF + H2O → [F-·····H3O+] F- + H3O+
ion pair
H-bonding
free ions
General Chemistry: Chapter 16
Prentice-Hall © 2007

8. General Chemistry: Chapter 16
16-9 Lewis Acids and Bases
Lewis Acid
A species (atom, ion or molecule) that is an electron pair acceptor.
Lewis Base
A species that is an electron pair donor.
base
acid
adduct
Since BF3 accepts electrons from NH3 it is a lewis acid
General Chemistry: Chapter 16
Prentice-Hall © 2007