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.’

1. Determine the acid-base conjugate pairs for the following 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 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.