Chapter 15: Acid-Base Equilibria

1. Chapter 15: Acid-Base Equilibria

2. Chapter 15 Outline Acids-Base Concepts BrØnsted-Lowry Acid and Base Strength The Autoionization of Water The pH Scale Strong Acids and Bases Weak Acids and Bases Equilibria in solutions of weak acids % Dissociation Relationship Between Ka and Kb Acid-Base Properties of Salt Solutions Acid-Base Behavior and Chemical Structure Lewis Acids and Bases

3. Why Study Acids and Bases? Acids and bases are common in the everyday world as well as in the lab. Some common acidic products vinegar (dilute acetic acid), soft drinks (carbonic acid), aspirin (acetylsalicylic acid), vitamin C (ascorbic acid), lemonade (citric acid + ascorbic acid), muriatic acid (hydrochloric acid) Some common basic products Antacids such as milk of magnesia (magnesium hydroxide) and tums (calcium carbonate), household ammonia, oven and drain cleaners (sodium hydroxide), some bleaches (sodium hydroxide)

4. Why study acids and bases? Many biological and geological processes involve acid-base chemistry. Gastric juice contains hydrochloric acid Lactic acid builds up in muscles during strenuous exercise Basicity of blood must be maintained within a certain narrow range or death can result Acidity/basicity of soil and water are of importance to animals and plants living there Cave formation and weathering of rocks is affected by acidity of water

6. Why study acids and bases? 15 of the top 50 chemicals produced in the largest quantities annually in the U.S. are acids or bases. (#1 = sulfuric acid) Used as reactants and catalysts in manufacture of various consumer and industrial products Plastics, synthetic fibers, detergents, pharmaceuticals, agricultural fertilizers, explosives, etc. Understanding acid-base chemistry is necessary for chemistry, biology, geology, and other related scientific disciplines

7. Arrhenius Concept Arrhenius Concept (S. Arrhenius, 1887) Acids produce hydrogen ions, H+, in water H+ ions attach to H2O molecules forming hydronium ions, H3O+ Bases produce hydroxide ions, OH-, in water Base contains OH group in its formula. Neutralization reaction: acid + base ? salt + water Problems Some basic substances do not have OH- Confined to H2O solutions

9. Bronsted-Lowry Concept Bronsted-Lowry Concept (J. Bronsted, T. Lowry, 1923) Acid-base reaction involves proton transfer Acid: proton donor Base: proton acceptor does not have to have OH- in formula Dissociation Animation

10. Bronsted-Lowry Concept

11. Bronsted-Lowry Concept

12. Bronsted-Lowry Concept Bronsted-Lowry Concept (J. Bronsted, T. Lowry, 1923) Acid-base reaction involves proton transfer Acid: proton donor Base: proton acceptor does not have to have OH- in formula Water is amphoteric

13. Bronsted-Lowry Concept

14. Bronsted-Lowry Concept Bronsted-Lowry Concept (J. Bronsted, T. Lowry, 1923) Acid-base reaction involves proton transfer Acid: proton donor Base: proton acceptor does not have to have OH- in formula Water is amphoteric Acids & bases can be molecules or ions

15. Bronsted-Lowry Concept

16. Bronsted-Lowry Concept Bronsted-Lowry Concept (J. Bronsted, T. Lowry, 1923) Acid-base reaction involves proton transfer Acid: proton donor Base: proton acceptor does not have to have OH- in formula Water is amphoteric Acids & bases can be molecules or ions Back reaction is also a proton transfer.

17. Bronsted-Lowry Concept

18. Bronsted-Lowry Concept

20. The H+ Ion in Water The H+(aq) ion is simply a proton with no electrons. In water, the H+ form clusters. The simplest cluster is H3O+(aq). Larger clusters are H5O2+ and H9O4+. Generally we use H+(aq) and H3O+(aq) interchangeably.

21. Acid-Base Strength Strong acids Ionize completely in water HX + H2O ? H3O+ + X- Weak acids Ionize only partially in water HX + H2O ? H3O+ + X- Common weak acids: Numerous molecules and certain cations (we’ll learn more about these later)

22. Acid and Base Strength Strong: Completely Dissociated Weak: Partially Dissociated Acids:Animation Base: Animation

23. Common Strong Acids (Know these!) HCl HBr HI HNO3 HClO4 H2SO4 Acid Strength

24. Acid-Base Strength Strong bases Ionize completely in water NaOH ? Na+ + OH- Common strong bases: MOH and M(OH)2, M2O and MO (M = Grp IA and IIA metals) Weak bases Ionize only partially in water B + H2O ? BH+ + OH- Common weak bases: NR3 (R = H or other chemical species) and certain anions

25. Conjugate Acid-Base Pairs The stronger the acid the weaker is its conjugate base The stronger the base, the weaker is its conjugate acid

28. Ka’s for Some Weak Acids

29. The Autoionization of Water

30. The Autoionization of Water The Ion Product of Water

31. The Autoionization of Water The Ion Product of Water

32. H+/OH- “See-Saw”

34. pH and pOH Scales Convenient ways to express acidity and basicity We can represent the concentration of [H3O+] as: pH = -log [H3O+] We can represent the concentration of [OH-] as: pOH = -log [OH-] The p function simply means -log. For a number X: pX = -log X

35. pH Scale is Logarithmic

36. pH and pOH Scales In neutral water at 25 ?C, pH = pOH = 7.00. In acidic solutions, [H+] > 1.0 ? 10-7, so pH < 7.00. In basic solutions, [H+] < 1.0 ? 10-7, so pH > 7.00. The higher the pH, the lower the pOH, the more basic the solution. Most pH and pOH values fall between 0 and 14.

38. pH and pOH Scales A relationship between pH and pOH may be derived for all aqueous solutions: Kw = [H3O+][OH-] Take the -log of each side: -logKw = -log([H3O+][OH-]) -logKw = -log[H3O+] - log[OH-] pKw = pH + pOH 14.00 = pH + pOH ( at 25 °C)

39. Example  Calculate the pH and pOH of a solution in which [H3O+] = 0.00028 M at 25 °C.

43. Weak Acid and Base Ionizations How do we calculate the [H3O+] and pH of weak acid or base solutions if these do not ionize completely?

45. Ka’s for Some Weak Acids

46. Calculations with Ka Ka can be used to calculate: equilibrium concentrations pH percent ionization

47. Calculations Using Ka Basic Steps for Weak Acid Calculations Using Ka Write balanced chemical equation and the expression for Ka Look up value for Ka For each chemical species involved in the equilibrium (except H2O), write: Initial concentration Equilibrium concentration Let the change in the [H3O+] be the variable “x” Substitute the equilibrium concentrations into Ka and solve for x using either quadratic approach simplified approach Calculate pH, equilibrium concentrations, % ionization, etc., as specified in the problem.

52. Weak Bases and Base Ionization Constants Write the chemical equation and the Kb expression for the ionization of each of the following bases in aqueous solution. (CH3)2NH CO32-

53. Weak Bases and Base Ionization Constants What is the pH of a 0.075M ethylamine (C2H5NH2) solution? (Kb = 6.4 x 10-4)

54. Polyprotic Acids Some acids have more than one ionizable proton. Called polyprotic acids Ionize in steps Consider the ionization of a weak diprotic acid: H2A + H2O <=> H3O+ + HA- Ka1 = [H3O+][HA-]/[H2A] HA- + H2O <=> H3O+ + A2- Ka2 = [H3O+][A2-]/[HA-] The 1st ionization effectively controls solution pH for most polyprotic acids: Ka1 >> Ka2 > Ka3, etc. When calculating pH, ignore any ionization after the first ionization if Ka1 >> Ka2.

55. Polyprotic Acids Write the chemical equations and the Ka expressions for the complete dissociation of the following acids: H2SO4 H3PO4

61. Conceptual Question Predict whether the dipotassium salt of citirc acid (K2HC6H5O7) will form an acidic or basic solution in water.

62. Acid-Base Behavior and Molecular Structure When a substance is dissolved in water, it may behave as an acid, a base or exhibit no acid-base properties. How does Molecular Structure determine which of these behaviors is exhibited by a substance? Why are some acids stronger than others?

63. Brainstorm Consider the two molecules shown below: Would the following two molecules donate an H+ or OH-? Why? What 3 Factors should we consider when trying to determine acid strength?

64. Acid-Base Behavior and Molecular Structure Factors that affect Acid Strength Recognizing what species typically release OH- Electronegativity differnece Size of conjugate base and stability of resulting anion Oxidation number of S Bond strength

65. Oxyacids – Y-O-H Comparing oxyacids with same central atom, but increasing number of oxygen atoms attached.

66. Oxyacids Comparing oxyacids with different central atoms, but same number of OH groups and same number of O atoms

67. Conceptual Questions Which is the stronger acid HClO4 or HBrO4? Why? Which is the stronger acid HNO2 or HNO3? Why?

68. Binary Acids: H-X Bond strengths and anion size change less as you move across the periodic table, so polarity is the dominating factor.

69. Conceptual Question The atomic radius of carbon is 70 pm and the ionic radius of sulfur is 100 pm. The electronegativity of carbon is 2.55 and the electronegativity of sulfur is 2.58. Given this information, rank these acids in order of increasing acid strength. H2CO3 H2SO4 H2SO3

70. Conceptual Question Which is the stronger acid, acetic acid (CH3COOH) or chloroacetic acid (CH2ClCOOH)? Why?