Chapter 18

1
Chapter 18 Acids, Bases and Salts
2
Chapter 18 OBJECTIVES

• State and use the Arrhenius and Brønsted-Lowry
  definitions of acids and bases.
• Identify common physical and chemical properties
  of acids and bases.
• Describe dissociation constants and explain what
  they indicate about acids and bases.
• Explain what most acidic hydrogen atoms have in
  common.
• Explain what most bases have in common.
• Describe nomenclature of acids and bases.

3
18-1 Defining Acids and Bases

• What are some properties of acids and bases?
  (Lets derive some.)
• Taste (Dont do this at home!)
• Touch (Or this!)
• Reactions with metals
• Electrical conductivity
• Reactions with Indicators
• Neutralization

4
18-1 Defining Acids and Bases

• Acids substances that ionize in aqueous
  solution to form hydrogen ions (H)
• Bases substances that accept H ions, producing
  OH-

5
The Arrhenius Definitions

• Acid a substance that dissociates in water to
  produce hydrogen ions (H).
• Base a substance that dissociates in water to
  produce hydroxide ions (OH-).
• See Fig. 18-6 (page 599).
• Arrhenius acids and bases react together
  (neutralize) to form a salt and water.
• HCl(aq) NaOH(aq) ? H2O(l) NaCl(aq)

6
The Brønsted-Lowry Definition

• Arrhenius definition is restrictive
• Applies only to water solutions.
• Does not explain why covalent molecules are acids
  (HCl, HBr)
• Does not explain why certain compounds like NH3
  are bases.
• Brønsted-Lowry Definitions
• Acid a proton (H) donor.
• Base a proton acceptor.

7
The Hydronium Ion

• Protons (H) do not really exist in water
  solutions in this way.
• H H2O ? H3O
• Hydronium Ions (H3O) are a better approximation
  of what occurs.
• Molecules of different acids can ionize to form a
  different of H ions
• HCl(g) H2O(l) ? H3O(aq) Cl-(aq)
• HCl and HNO3 are monoprotic acids 1 H ion per
  mole of acid
• H2SO4 is diprotic acid 2 H ions per mole of
  acid

8
Conjugate Acid-Base Pairs

• The difference between an acid and a base may be
  as simple as one H ion!
• To emphasize this relationship, chemists use the
  terms conjugate acid conjugate base pairs.
• The term conjugate means joined together.
• Conjugate Acid-Base Pair is two compounds that
  differ by only one H ion.
• Examples (Fig. 18-12, page 603).

9
WARM UP

• What is a proton (H) donor?
• What is a proton acceptor?
• What is the conjugate base to HCl?
• What is the conjugate acid to OH-?
• What is the conjugate base to NH4?
• What is the conjugate acid to HSO4-?

10
18-2 Determining The Strengths of Acids and Bases

• Strong and Weak Acids
• Strong acids easily lose H ions, so they are
  strong electrolytes (high degree of
  dissociation).
• Weak acids do not dissociate very much.
• Strong and Weak Bases
• Strong bases (such as compounds with OH-) have
  high affinity for H ions, and they are strong
  electrolytes.
• Weak bases react partially with water to form
  hydroxide ions.
• Use single arrows (?) to signify strong acids
  (100 dissociation). (HCl)
• Use double arrows (?) to signify weak acids (low
  amount of dissociation). (HC2H3O2)
• Strength of Conjugate Acid-Base Pairs
• The stronger the acid, the weaker its conjugate
  base.
• The stronger the base, the weaker its conjugate
  acid.

11
The Acid Dissociation Constant

• For the reaction HA (aq) H2O (l) ? H3O (aq)
  A- (aq) we may write an equilibrium expression
• Keq H3OA- / HAH2O
• or
• Ka H3OA- / HA (Why?)
• where Ka is the acid dissociation constant.
• The larger the Ka, the stronger the acid.
• Example

12
The Base Dissociation Constant

• For the reaction B (aq) H2O (l) ? HB (aq)
  OH- (aq) we may write an equilibrium expression
• Keq HBOH- / BH2O
• or
• Kb HBOH- / B (Why?)
• where Kb is the base dissociation constant.
• The larger the Kb, the stronger the base.
• Example

13
Calculating Dissociation Constants

• This is a very easy task once the concentrations
  of ions are known.
• Sample problem (p612).
• LETS TRY 1 AND 2 ON PAGE 613

14
Acid-Base Properties of Salts

• Salts are strong electrolytes, forming cations
  and anions in water.
• Many of these ions are weak Brønsted-Lowry acids
  or bases, so they produce H or OH-.
• This is called a salt hydrolysis reaction.

15
WARM UP

• 1. Determine the acid-base conjugate pairs for
  the following reaction
• CO32-(aq) H2O(l) ? HCO3-(aq) OH-(aq)
• 2. A weak monoprotic acid of 2.60M is added to
  water. At equilibrium the concentration of H3O
  is 0.34M. What is the Ka for this acid?

16
Types of Salt Hydrolysis Reactions

• Salts of Strong Acids Strong Bases
• Solution is neutral.
• NaOH(aq) HCl(aq) ? NaCl (aq) H2O(l)
• Salts of Strong Acid Weak Bases
• Solution is acidic because the NH4 is a
  Brønsted-Lowry acid.
• NH3 (aq) HCl (aq) ? NH4Cl
• NH4(aq) H2O(l) ? NH3 (aq) H3O (aq)
• Salts of Weak Acids Strong Bases
• Solution is basic (alkaline).
• 2NaOH (aq) H2CO3 (aq) ? Na2CO3 H2O
• Salts of Weak Acids Weak Bases
• Not easily predicted due to the many complex
  equilibrium involved.

17
18-3 Naming and Identifying Acids and Bases

• Acids have acidic hydrogens.
• These have a slight positive charge while still
  part of the molecule.
• Binary Acids Contain hydrogen plus 1 other
  element Hydrochloric Acid (HCl)
• Oxy Acids Contain hydrogen, oxygen and one
  other element. Examples, sulfuric acid (H2SO4)
  nitric acid (HNO3)
• Carboxylic Acids Acids that are organic acids
  and contain the carbon atom. Example acetic acid
  (HC2H3O2)

18
18-3 Naming and Identifying Acids and Bases

• Bases
• These always contain an unshared pair of
  electrons.
• Anions Many negatively charged ions function as
  bases (OH-). Examples sodium hyroxide (NaOH)
  calcium hyroxide (Ca(OH)2).
• Amines Compounds related to ammonia and contain
  a nitrogen atom that has an unpaired share of
  electrons.
• Nomenclature (See p619)

19
Chapter 18 OBJECTIVES

• State and use the Arrhenius and Brønsted-Lowry
  definitions of acids and bases.
• Identify common physical and chemical properties
  of acids and bases.
• Describe dissociation constants and explain what
  they indicate about acids and bases.
• Use experimental data to determine dissociation
  constants.
• Explain what most acidic hydrogen atoms have in
  common.
• Explain what most bases have in common.
• Describe nomenclature of acids and bases.