Chapter 18: Acids and Bases

1. CHEMISTRY Matter and Change Chapter 18: Acids and Bases

2. Table Of Contents CHAPTER18 Section 18.1 Introduction to Acids and Bases Section 18.2 Strengths of Acids and Bases Section 18.3 Hydrogen Ions and pH Section 18.4 Neutralization Click a hyperlink to view the corresponding slides. Exit

3. Introduction to Acids and Bases SECTION18.1 • Identify the physical and chemical properties of acids and bases. • Classify solutions as acidic, basic, or neutral. • Compare the Arrhenius, Brønsted-Lowry, and Lewis models of acids and bases. Lewis structure: a model that uses electron-dot structures to show how electrons are arranged in molecules

4. Introduction to Acids and Bases SECTION18.1 acidic solution basic solution Arrhenius model Brønsted-Lowry model conjugate acid conjugate base conjugate acid-base pair amphoteric Lewis model Different models help describe the behavior of acids and bases.

5. Introduction to Acids and Bases SECTION18.1 Properties of Acids and Bases • Acids taste sour. Bases taste bitter and feel slippery. • Acids and bases are conductors of electricity. • Acids and bases can be identified by their reactions with some metals and metal carbonates.

6. Introduction to Acids and Bases SECTION18.1 Properties of Acids and Bases(cont.) • Acids turn blue litmus red. • Bases turn red litmus blue. • Magnesium and zinc react with acids to produce hydrogen gas. • Geologists identify limestone because it produces bubbles of carbon dioxide when exposed to hydrochloric acid.

7. Introduction to Acids and Bases SECTION18.1 Properties of Acids and Bases(cont.) • All water solutions contain hydrogen ions (H+) and hydroxide ions (OH–). • An acidic solutioncontains more hydrogen ions than hydroxide ions. • A basic solutioncontains more hydroxide ions than hydrogen ions.

8. Introduction to Acids and Bases H2O(l) + H2O(l) H3O+(aq) + OH–(aq) SECTION18.1 Properties of Acids and Bases(cont.) • The usual solvent for acids and bases is water—water produces equal numbers of hydrogen and hydroxide ions in a process called self-ionization. • The hydronium ion is H3O+.

9. Introduction to Acids and Bases SECTION18.1 The Arrhenius Model • The Arrhenius model states that an acid is a substance that contains hydrogen and ionizes to produce hydrogen ions in aqueous solution, and a base is a substance that contains a hydroxide group and dissociates to produce a hydroxide ion in solution.

10. Introduction to Acids and Bases SECTION18.1 The Arrhenius Model(cont.) • Arrhenius acids and bases • HCl ionizes to produce H+ ions. • HCl(g) → H+(aq) + Cl–(aq) • NaOH dissociates to produce OH– ions. • NaOH(s) → Na+(aq) + OH–(aq) • Some solutions produce hydroxide ions even though they do not contain a hydroxide group.

11. Introduction to Acids and Bases SECTION18.1 The Brønsted-Lowry Model • The Brønsted-Lowry Model of acids and bases states that an acid is a hydrogen ion donor, and a base is a hydrogen ion acceptor. • The Brønsted-Lowry Model is a more inclusive model of acids and bases.

12. Introduction to Acids and Bases SECTION18.1 The Brønsted-Lowry Model(cont.) • A conjugate acidis the species produced when a base accepts a hydrogen ion. • A conjugate baseis the species produced when an acid donates a hydrogen ion. • A conjugate acid-base pair consists of two substances related to each other by donating and accepting a single hydrogen ion.

13. Introduction to Acids and Bases SECTION18.1 The Brønsted-Lowry Model(cont.) • Hydrogen fluoride—a Brønsted-Lowry acid • HF(aq) + H2O(l) ↔ H3O+(aq) + F–(aq) • HF = acid, H2O = base, H3O+ = conjugate acid, F– = conjugate base

14. Introduction to Acids and Bases SECTION18.1 The Brønsted-Lowry Model(cont.) • Ammonia— Brønsted-Lowry base • NH3(aq) + H2O(l) ↔ NH4+(aq) + OH–(aq) • NH3 = base, H2O(l) = acid, NH4+ = conjugate acid, OH– = conjugate base • Water and other substances that can act as acids or bases are called amphoteric.

15. Introduction to Acids and Bases SECTION18.1 Monoprotic and Polyprotic Acids • An acid that can donate only one hydrogen ion is a monoprotic acid. • Only ionizable hydrogen atoms can be donated.

16. Introduction to Acids and Bases SECTION18.1 Monoprotic and Polyprotic Acids • Acids that can donate more than one hydrogen ion are polyprotic acids.

17. Introduction to Acids and Bases SECTION18.1 The Lewis Model • According to the Lewis model, a Lewis acid is an electron-pair acceptor and a Lewis base is an electron pair donor. • The Lewis model includes all the substances classified as Brønsted-Lowry acids and bases and many more.

18. Section Check SECTION18.1 A Lewis acid is a(n) ____. A.electron pair donor B.hydrogen ion donor C.electron pair acceptor D.substance that contains an hydroxide group

19. Section Check SECTION18.1 A conjugate acid is formed when: A.a base accepts a hydrogen ion B.an acid accepts a hydrogen ion C.an acid donates a hydrogen ion D.a base donates a hydrogen ion

21. Strengths of Acids and Bases SECTION18.2 • Relate the strength of an acid or base to its degree of ionization. electrolyte: an ionic compound whose aqueous solution conducts an electric current • Compare the strength of a weak acid with the strength of its conjugate base. • Explain the relationship between the strengths of acids and bases and the values of their ionization constants.

22. Strengths of Acids and Bases SECTION18.2 strong acid weak acid acid ionization constant strong base weak base base ionization constant In solution, strong acids and bases ionize completely, but weak acids and bases ionize only partially.

23. Strengths of Acids and Bases SECTION18.2 Strengths of Acids • Acids that ionize completely are strong acids. • Because they produce the maximum number of hydrogen ions, strong acids are good conductors of electricity.

24. Strengths of Acids and Bases SECTION18.2 Strengths of Acids(cont.) • Acids that ionize only partially in dilute aqueous solutions are called weak acids.

25. Strengths of Acids and Bases SECTION18.2 Strengths of Acids(cont.) • With a strong acid, the conjugate base is a weak base. • Equilibrium lies almost completely to the right in the equation because the conjugate base has a weaker attraction for the H+ ion than does the base in the forward reaction. • In a weak acid, the ionization equilibrium lies to the far left in the ionization equation because the conjugate base has a greater attraction for H+ ions than does the base in the forward reaction.

26. Strengths of Acids and Bases SECTION18.2 Strengths of Acids(cont.) • The equilibrium constant, Keq, provides a quantitative measure of the degree of ionization of an acid. • The acid ionization constant is the value of the equilibrium constant expression for the ionization of a weak acid, Ka. • Ka indicates whether products or reactants are favored at equilibrium.

27. Strengths of Acids and Bases SECTION18.2 Strengths of Acids(cont.) • For weak acids, the products tend to be smaller compared to the un-ionized molecules (reactant). • Weaker acids have a smaller Ka.

28. Strengths of Acids and Bases SECTION18.2 Strengths of Bases • A base that dissociates completely into metal ions and hydroxide ions is known as a strong base. • A weak base ionizes only partially in dilute aqueous solution.

29. Strengths of Acids and Bases SECTION18.2 Strengths of Bases(cont.) • The base ionization constant, Kb, is the value of the equilibrium constant expression for the ionization of a base.

30. Section Check SECTION18.2 A solution with a small Kb is a ____. A.weak acid B.weak base C.strong acid D.strong base

31. Section Check SECTION18.2 Where is the equilibrium point in the ionization equation for a strong acid? A.far right B.far left C.slightly right D.slightly left

33. Hydrogen Ions and pH SECTION18.3 • Explain pH and pOH. Le Châtelier’s principle: states that if a stress is applied to a system at equilibrium, the system shifts in the direction that relieves the stress • Relate pH and pOH to the ion product constant for water. • Calculate the pH and pOH of aqueous solutions. pH and pOH are logarithmic scales that express the concentrations of hydrogen ions and hydroxide ions in aqueous solutions. ion product constant for water pH pOH

34. Hydrogen Ions and pH SECTION18.3 Ion Product Constant for Water • Pure water contains equal concentrations of H+ and OH– ions. • The ion product constant of water, Kw = [H+][OH–]. • The ion product constant for wateris the value of the equilibrium constant expression for the self-ionization of water.

35. Hydrogen Ions and pH SECTION18.3 Ion Product Constant for Water(cont.) • With pure water at 298 K, both [H+] and [OH–] are equal to 1.0 × 10–7M. Kw at 298 K = 1.0 × 10–14 • Kw and LeChâtelier’s Principle proves [H+] × [OH–] must equal 1.0 × 10–14 at 298 K, and as [H+] goes up, [OH–] must go down.

36. Hydrogen Ions and pH SECTION18.3 pH and pOH • Concentrations of H+ ions are often small numbers expressed in scientific notation. • pH is the negative logarithm of the hydrogen ion concentration of a solution.pH = –log [H+]

37. Hydrogen Ions and pH SECTION18.3 pH and pOH(cont.) • pOH of a solution is the negative logarithm of the hydroxide ion concentration. • pOH = –log [OH–] • The sum of pH and pOH equals 14.

38. Hydrogen Ions and pH SECTION18.3 pH and pOH(cont.) • For all strong monoprotic acids, the concentration of the acid is the concentration of H+ ions. • For all strong bases, the concentration of the OH– ions available is the concentration of the base. • Weak acids and weak bases only partially ionize and Ka and Kb values must be used.

39. Hydrogen Ions and pH SECTION18.3 pH and pOH(cont.) • Litmus paper and a pH meter with electrodes can determine the pH of a solution.

40. Section Check SECTION18.3 In dilute aqueous solution, as [H+] increases ____. A.pH decreases B.pOH increases C.[OH–] decreases D.all of the above

41. Section Check SECTION18.3 What is the pH of a neutral solution such as pure water? A.0 B.7 C.14 D.1.0 × 10–14

43. Neutralization SECTION18.4 • Write chemical equations for neutralization reactions. stoichiometry: the study of quantitative relationships between the amounts of reactants used and products formed by a chemical reaction; is based on the law of conservation of mass • Explain how neutralization reactions are used in acid-base titrations. • Compare the properties of buffered and unbuffered solutions.

44. Neutralization SECTION18.4 neutralization reaction salt titration titrant equivalence point acid-base indicator end point salt hydrolysis buffer buffer capacity In a neutralization reaction, an acid reacts with a base to produce a salt and water.

45. Neutralization SECTION18.4 Reactions Between Acids and Bases • A neutralization reactionis a reaction in which an acid and a base in an aqueous solution react to produce a salt and water. • A salt is an ionic compound made up of a cation from a base and an anion from an acid. • Neutralization is a double-replacement reaction.

46. Neutralization SECTION18.4 Reactions Between Acids and Bases(cont.)

47. Neutralization SECTION18.4 Reactions Between Acids and Bases(cont.) • Titrationis a method for determining the concentration of a solution by reacting a known volume of that solution with a solution of known concentration.

48. Neutralization SECTION18.4 Reactions Between Acids and Bases(cont.) • In a titration procedure, a measured volume of an acid or base of unknown concentration is placed in a beaker, and initial pH recorded. • A buret is filled with the titrating solution of known concentration, called a titrant.

49. Neutralization SECTION18.4 Reactions Between Acids and Bases(cont.) • Measured volumes of the standard solution are added slowly and mixed into the solution in the beaker, and the pH is read and recorded after each addition. The process continues until the reaction reaches the equivalence point, which is the point at which moles of H+ ion from the acid equals moles of OH– ion from the base. • An abrupt change in pH occurs at the equivalence point.

50. Neutralization SECTION18.4 Reactions Between Acids and Bases(cont.)