Chapter 18 “Acids, Bases and Salts”

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.

18-1 Defining Acids and Bases

• What are some properties of acids and bases? (Let’s derive some.)– Taste (Don’t do this at home!)– Touch (Or this!)– Reactions with metals– Electrical conductivity– Reactions with “Indicators”– Neutralization

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-

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)

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.

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

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).

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

-?

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.

The Acid Dissociation Constant

• For the reaction HA (aq) + H2O (l) ↔ H3O+ (aq) + A- (aq) we may write an equilibrium expression:

Keq = [H3O+][A-] / [HA][H2O]or

Ka = [H3O+][A-] / [HA] (Why?)

where Ka is the acid dissociation constant.

The larger the Ka, the stronger the acid.Example

The Base Dissociation Constant

• For the reaction B (aq) + H2O (l) ↔ HB+ (aq) + OH- (aq) we may write an equilibrium expression:

Keq = [HB+][OH-] / [B][H2O]or

Kb = [HB+][OH-] / [B] (Why?)

where Kb is the base dissociation constant.

The larger the Kb, the stronger the base.Example

Calculating Dissociation Constants

• This is a very easy task once the concentrations of ions are known.

• Sample problem (p612).

• LET’S TRY #1 AND #2 ON PAGE 613

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.’

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?

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.

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)

18-3 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)

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.