1. 1 Acids and Bases

2. 2 Acid-Base Concepts Antoine Lavoisier was one of the first chemists to try to explain what makes a substance acidic.

3. 3 Acid-Base Concepts In the first part of this chapter we will look at several concepts of acid-base theory including:

4. Common Characteristics Acids Bases

5. SOLUTION NO Current Nonelectrolytic solution Molecular Solution Molecule = no net charge EXCEPTIONS: Covalent / Hydrohalic Acids Conducts Current Electrolytic Solution Acid, Base or Salt Solution Ion = charged particle

6. 6 Arrhenius Concept of Acids and Bases According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H 3 O + ). –Remember, however, that the aqueous hydrogen ion is actually chemically bonded to water, that is, H 3 O +.

7. 7 Arrhenius Concept of Acids and Bases The H 3 O + is shown here hydrogen bonded to three water molecules. According to the Arrhenius concept of acids and bases, an acid is a substance that, when dissolved in water, increases the concentration of hydronium ion (H 3 O + ).

8. 8 Arrhenius Concept of Acids and Bases A base, in the Arrhenius concept, is a substance that, when dissolved in water, increases the concentration of hydroxide ion, OH - (aq).

9. 9 Theory of Ionization Svente Arrhenius 1884 Water  Acid  Base Neutralization

10. 10 Arrhenius Concept of Acids and Bases In the Arrhenius concept, a strong acid is a substance that ionizes completely in aqueous solution to give H 3 O + (aq) and an anion. (See Animation: Acid Ionization Equilibirum)(See Animation: Acid Ionization Equilibirum) –Other strong acids include –An example is perchloric acid, HClO 4.

11. 11 Arrhenius Concept of Acids and Bases In the Arrhenius concept, a strong base is a substance that ionizes completely in aqueous solution to give OH - (aq) and a cation. See Base Ionization animation –Other strong bases include –An example is sodium hydroxide,.

12. 12 Arrhenius Concept of Acids and Bases Most other acids and bases that you encounter are weak. They are not completely ionized and exist in reversible reaction with the corresponding ions. –An example is acetic acid,

13. 13 Arrhenius Concept of Acids and Bases The Arrhenius concept is limited in that it looks at acids and bases in aqueous solutions only. –Broader definitions of acids and bases are discussed in the next sections.

14. 14 Consider… What is the acid? What is the base? Production of NH 4

15. 15 Brønsted-Lowry Concept of Acids and Bases A base is –In any reversible acid-base reaction, According to the Brønsted-Lowry concept, an acid is

16. 16 Brønsted-Lowry Concept of Acids and Bases Consider the reaction of NH 3 and H 2 O. H+H+ baseacid

17. 17 Brønsted-Lowry Concept of Acids and Bases Consider the reaction of NH 3 and H 2 O. H+H+ baseacid

18. 18 Brønsted-Lowry Concept of Acids and Bases Consider the reaction of NH 3 and H 2 O. baseacid

19. 19 Brønsted-Lowry Concept of Acids and Bases Consider the reaction of NH 3 and H 2 O. baseacid

20. 20 Bronsted – Lowry Theory Acid Base When an acid loses a proton, its conjugate base is formed. When a base accepts a proton, its conjugate acid is formed

21. 21 Conjugate Acids and Bases H 2 O + NH 3  NH 4 + + OH - acid base CAN donate H +  conjugate acid CAN donate OH -  conjugate base Conjugate Acid – Base Pairs

22. 22 What’s the Deal With Water? H 2 O + NH 3  NH 4 + + OH - acid base H 2 O + HCl  H 3 O + + Cl - base acid Amphoteric Autoionization Protolysis

23. 23 Brønsted-Lowry Concept of Acids and Bases –HCO 3 - acts as a proton donor (an acid) in the presence of OH - –H+–H+

24. 24 Brønsted-Lowry Concept of Acids and Bases –HCO 3 can act as a proton acceptor (a base) in the presence of HF. H+H+

25. 25 Brønsted-Lowry Concept of Acids and Bases In the Brønsted-Lowry concept:

26. 26 Lewis Concept of Acids and Bases The Lewis concept defines an acid as an electron pair acceptor and a base as an electron pair donor. –The Lewis concept embraces many reactions that we might not think of as acid-base reactions.

27. 27 Lewis Theory 1923 Extended the acid–base theory electron pair donor = electron pair receiver =

28. 28 4 ammonia molecules forming a complex ion with cupric ion

29. 29 Characteristics of Lewis… Acids Bases Lone e- pair

30. 30 Neutralization = Coordinate covalent bond Both shared e - donated by the same atom / ion

31. 31 Nesting TheoriesLewis Bronsted - Lowry Arrhenius

32. 32 Strength in Water Depends on degree of ionization  Ease of bond breakage  Stability of resulting ions Higher the [H + ] or [OH - ]

33. 33 And the Winner is… Hydronium H 3 O + is the Hydroxide OH - is the

34. 34 Water is a leveling solvent;.  HCl, HBr, HI  NaOH, Ca(OH) 2

35. 35 ACIDS & BASES H 2 SO 4 v H 2 SO 2 NaOH v S(OH) 6

36. 36 H 3 PO 4 weaker than HNO 3 +5 +5 HNO 2 weaker than H 3 PO 3 +3 +3 N OO H P OO O HH H Trigonal planar Tetrahedral

37. 37 Relative Strength of Acids and Bases The Brønsted-Lowry concept introduced the idea of conjugate acid-base pairs and proton- transfer reactions. –We consider such acid-base reactions to be a competition between species for hydrogen ions.

38. 38 Relative Strength of Acids and Bases The Brønsted-Lowry concept introduced the idea of conjugate acid-base pairs and proton- transfer reactions.

39. 39 Relative Strength of Acids and Bases The Brønsted-Lowry concept introduced the idea of conjugate acid-base pairs and proton- transfer reactions.

40. 40 Relative Strength of Acids and Bases Consider the equilibrium below. acid base conjugate acid-base pairs

41. 41 Relative Strength of Acids and Bases Consider the equilibrium below. acid base conjugate acid-base pairs

42. 42 Molecular Structure and Acid Strength Two factors are important in determining the relative acid strengths. –The H atom should have a partial positive charge: ++ --

43. 43 Molecular Structure and Acid Strength Two factors are important in determining the relative acid strengths. d+d-

44. 44 Molecular Structure and Acid Strength Consider a series of binary acids from a given column of elements. –You can predict the following order of acidic strength.

45. 45 Molecular Structure and Acid Strength As you go across a row of elements, the polarity of the H-X bond becomes the dominant factor. –You can predict the following order of acidic strength.

46. 46 Molecular Structure and Acid Strength Consider the oxyacids. An oxyacid has the structure:

47. 47 Molecular Structure and Acid Strength –You can predict the following order of acidic strength.

48. 48 Molecular Structure and Acid Strength Consider the oxyacids. An oxyacid has the structure:

49. 49 Molecular Structure and Acid Strength Consider the oxyacids. An oxyacid has the structure:

50. 50 Molecular Structure and Acid Strength Consider polyprotic acids and their corresponding anions. –Therefore the acid strength of a polyprotic acid and its anions decreases with increasing negative charge.

51. 51 Self-ionization of Water Self-ionization is a reaction in which two like molecules react to give ions. (See Animation: Self-ionization of Water to Form H + and OH - in Equilibrium)(See Animation: Self-ionization of Water to Form H + and OH - in Equilibrium) –In the case of water, the following equilibrium is established. –The equilibrium-constant expression for this system is:

52. 52 Self-ionization of Water –The concentration of ions is extremely small, so the concentration of H 2 O remains essentially constant. This gives: constant Self-ionization is a reaction in which two like molecules react to give ions. [i.e. H 2 O]

53. 53 Self-ionization of Water –.–. –At 25 o C, the value of K w is 1.0 x 10 -14. Self-ionization is a reaction in which two like molecules react to give ions.

54. 54 Self-ionization of Water –Because we often write H 3 O + as H +, the ion- product constant expression for water can be written: –Using K w you can calculate the concentrations of H + and OH - ions in pure water. Self-ionization is a reaction in which two like molecules react to give ions.

55. 55 Self-ionization of Water These ions are produced in equal numbers in pure water, so if we let x = [H + ] = [OH - ]

56. 56 Solutions of Strong Acid or Base In a solution of a strong acid you can normally ignore the self- ionization of water as a source of H + (aq).

57. 57 Solutions of Strong Acid or Base As an example, calculate the concentration of OH - ion in 0.10 M HCl. Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H + (aq). –Substituting [H + ]=0.10 into the ion-product expression, we get:

58. 58 Solutions of Strong Acid or Base As an example, calculate the concentration of OH - ion in 0.10 M HCl. Because you started with 0.10 M HCl (a strong acid) the reaction will produce 0.10 M H + (aq). –Substituting [H + ]=0.10 into the ion-product expression, we get:

59. 59 Solutions of Strong Acid or Base Similarly, in a solution of a strong base you can normally ignore the self-ionization of water as a source of OH - (aq).

60. 60 Solutions of Strong Acid or Base As an example, calculate the concentration of H + ion in 0.010 M NaOH. Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH - (aq). –Substituting [OH - ]=0.010 into the ion-product expression, we get:

61. 61 Solutions of Strong Acid or Base As an example, calculate the concentration of H + ion in 0.010 M NaOH. Because you started with 0.010 M NaOH (a strong base) the reaction will produce 0.010 M OH - (aq). –Substituting [OH - ]=0.010 into the ion-product expression, we get:

62. 62 Solutions of Strong Acid or Base By dissolving substances in water, you can alter the concentrations of H + (aq) and OH - (aq).

63. 63 Solutions of Strong Acid or Base At 25°C, you observe the following conditions. –In an acidic solution, [H + ] –In a neutral solution, [H + ] –In a basic solution, [H + ]

64. 64 The pH of a Solution Although you can quantitatively describe the acidity of a solution by its [H + ], it is often more convenient to give acidity in terms of pH.

65. 65 The pH of a Solution For a solution in which the hydrogen-ion concentration is 1.0 x 10 -3, the pH is:

66. 66 The pH of a Solution In a neutral solution, whose hydrogen-ion concentration is 1.0 x 10 -7, the pH = 7.00. For acidic solutions, Similarly, a basic Figure 16.6 shows a diagram of the pH scale and the pH values of some common solutions.

67. Figure 16.8: The pH Scale

68. 68 A Problem to Consider A sample of orange juice has a hydrogen-ion concentration of 2.9 x 10 -4 M. What is the pH?

69. 69 A Problem to Consider The pH of human arterial blood is 7.40. What is the hydrogen-ion concentration?

70. 70 The pH of a Solution A measurement of the hydroxide ion concentration, similar to pH, is the pOH.

71. 71 The pH of a Solution A measurement of the hydroxide ion concentration, similar to pH, is the pOH.

72. 72 What is the [H 3 O + ] of a.050 M Sr(OH) 2 solution? Sr(OH) 2 ↔ Sr 2+ + 2OH -.050 M 0 0 (strong base) 0.050 M.100 M [OH - ] =.100 M 2H 2 O ↔ H 3 O + + OH –

73. 73 Negligibility Applies only to addition and subtraction calculations Variable very small  10 -3, 10 -4, 10 -5 and  Less than 5% of the number to which it is added / subtracted

74. 74 pH and pOH p = -log pK w = -logK w pHpOH

75. 75 A Problem to Consider An ammonia solution has a hydroxide-ion concentration of 1.9 x 10 -3 M. What is the pH of the solution? You first calculate the pOH: Then the pH is:

76. 76 If and acid solution has a [H 3 O + ] of.05 M, what are its pH and pOH ? pH = -log [H 3 O + ] = = K w = = = = pOH =

77. 77 pH + pOH =

78. 78 Calculate the pH and pOH of a household ammonia solution that contains 2.5 mol of NH 3 per liter of solution. Assume 10% ionization. NH 3 + H 2 O ↔ NH 4 + + OH - Initial Change @equilib

79. 79 The pH of a Solution The pH of a solution can accurately be measured using a pH meter (see Figure 16.9).(see Figure 16.9).

80. 80 Net Ionic Equation Real physical state of every component of the reaction  Strong acids and strong bases in ionic form  Soluble salts in ionic form  Pure substances, oxides, gases and solids in molecular form CaCO3(s) + HCl  ? Molecular Equation: Total Ionic Equation: Net Ionic Equation:

81. 81 Salts Product of acid – base rxn. Types 1. Normal

82. 82 2. Acidic 3. Basic

83. 83 Indicators Organic dyes Change color over pH range Acidic NeutralBasic pH 7 Red [Litmus] Blue Yellow Yellow [Bromothymol Blue] Blue Colorless Colorless [Phenolphthalein] Deep Pink Yellow Red [Methyl Orange] Yellow

84. 84 Animation: Acid Ionization Equilibrium Return to Slide 7 (Click here to open QuickTime animation)

85. 85 Animation: Self-Ionization of Water to Form H + and OH - in Equilibrium Return to Slide 38 (Click here to open QuickTime animation)

86. 86 Figure 16.9: A digital pH meter. Photo courtesy of American Color. Return to Slide 60