Chapter 18 “Acids, Bases and Salts”

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? (Let’s derive some.) • Taste (Don’t 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+ • HydroniumIons (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 = [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

12. 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

13. 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

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.