University Chemistry Chapter 11: Acids and Bases Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 Arrhenius acid is a substance that produces H + in water Arrhenius base is a substance that produces OH - in water Arrhenius Definition of Acids and Bases

3 Properties of Acids Have a sour taste. Vinegar owes its taste to acetic acid. Citrus fruits contain citric acid. React with certain metals to produce hydrogen gas. React with carbonates and bicarbonates to produce carbon dioxide gas Acids cause color changes in plant dyes, for example, they change the color of litmus from blue to red. Aqueous acid solutions conduct electricity. 2HCl (aq) + Mg (s) MgCl 2 (aq) + H 2 (g) 2HCl (aq) + CaCO 3 (s) CaCl 2 (aq) + CO 2 (g) + H 2 O (l)

4 Properties of Bases Have a bitter taste. Feel slippery. Many soaps contain bases. Bases cause color changes in plant dyes, for example, they change the color of litmus from red to blue. Aqueous base solutions conduct electricity.

5 Brønsted Definition of Acids and Bases Base is a proton acceptor Acid is a proton* donor * In solution exists as a hydrated proton (H 3 O + ) or hydronium ion.

6 Hydronium ion, hydrated proton, H 3 O + 4.3

7 Strong acids for all practical purposes are completely ionized in water. Weak acids ionize only to a limited extent in water. Strong bases for all practical purposes are completely ionized in water. Weak bases ionize only to a limited extent in water.

8 Monoprotic acids HCl H + + Cl - HNO 3 H + + NO 3 - CH 3 COOH H + + CH 3 COO - Strong electrolyte, strong acid Weak electrolyte, weak acid Diprotic acids H 2 SO 4 H + + HSO 4 - HSO 4 - H + + SO 4 2- Strong electrolyte, strong acid Weak electrolyte, weak acid Triprotic acids H 3 PO 4 H + + H 2 PO 4 - H 2 PO 4 - H + + HPO 4 2- HPO 4 2- H + + PO 4 3- Weak electrolyte, weak acid

9 Amphoteric: refers to a substances that can act as both a Brønsted acid and a Brønsted base acid base acid

10 spectator ions Neutralization Reaction acid + base salt + water HCl (aq) + NaOH (aq) NaCl (aq) + H 2 O H + + Cl - + Na + + OH - Na + + Cl - + H 2 O H + + OH - H 2 O net ionic equation molecular equation An ionic compound made up of a cation other than H + and an anion other than OH - or O 2-.

11 acid 2 base 1 acid 1 base 2 acid conjugate base base conjugate acid Conjugate Acid-Base Pairs Defined as an acid and its conjugate base or a base and its conjugate acid. The word conjugate means “joined together”.

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13 Conjugate acid-base pairs: The conjugate base of a strong acid has no measurable strength. H 3 O + is the strongest acid that can exist in aqueous solution. The OH - ion is the strongest base that can exist in aqueous solution.

14 A Lewis acid is a substance that can accept a pair of electrons A Lewis base is a substance that can donate a pair of electrons Lewis Definition of Acids and Bases H+H+ H O H + OH - acidbase N H H H H+H+ + acidbase N H H H H +

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17 O H H+ O H H O H HH O H - + [] + Acid-Base Properties of Water H 2 O + H 2 O H 3 O + + OH - acid conjugate base base conjugate acid autoionization of water hydroxide ion hydronium ion

18 The Ion Product of Water The ion-product constant (K w ) is the product of the molar concentrations of H + and OH - ions at a particular temperature. At 25 0 C K w = [H + ][OH - ] = 1.0 x [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution is neutral acidic basic

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20 pH – A Measure of Acidity [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution Is neutral acidic basic [H + ] = 1 x [H + ] > 1 x [H + ] < 1 x pH = 7 pH < 7 pH > 7 At 25 0 C pH[H + ]

21 [H + ][OH - ] = K w = 1.0 x log [H + ] – log [OH - ] = 14.00

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25 Temperature Dependence of K w and pH Le Châtelier’s principle: the equilibrium of an endothermic process will shift in the direction of products when the temperature is increased, leading to a corresponding increase in the equilibrium constant.

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27 Strong Acid: HCl Weak Acid: HF

28 HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) Weak Acids (HA) and Acid Ionization Constants HA (aq) H + (aq) + A - (aq) K a is the acid ionization constant KaKa weak acid strength

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30 Solvent leveling: All acids with K a >1 will yield identical H 3 O + concentrations in water, independent of their individual K a values The strongest acid that can exist in aqueous solution is the hydronium ion (H 3 O + ). K a = 1 Proton exchange reaction:

31 Weak Bases and Base Ionization Constants K b is the base ionization constant KbKb weak base strength

32 Solvent leveling: All bases with K b >1 will yield identical OH - concentrations in water, independent of their individual K b values The strongest base that can exist in aqueous solution is the hydroxide ion (OH - ). K b = 1 Proton exchange reaction:

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34 Ionization Constants of Conjugate Acid-Base Pairs HA (aq) H + (aq) + A - (aq) A - (aq) + H 2 O (l) OH - (aq) + HA (aq) KaKa KbKb H 2 O (l) H + (aq) + OH - (aq) KwKw K a K b = K w Weak Acid Conjugate Base Ka =Ka = KwKw KbKb Kb =Kb = KwKw KaKa

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39 Ionization Constants for Diprotic and Triprotic Acids

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43 Solving weak acid ionization problems: 1.Identify the major species that can affect the pH. In most cases, you can ignore the autoionization of water. Ignore [OH - ] because it is determined by [H + ]. 2.Use initial concentration and change to express the equilibrium concentrations in terms of single unknown x. 3.Write K a in terms of equilibrium concentrations. Solve for x by the approximation method. 4, If approximation is not valid, solve for x exactly. 5. Calculate concentrations of all species and/or pH of the solution.

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47 Percent Ionization For a monoprotic acid HA Percent ionization = [H + ] [HA] 0 x 100% [HA] 0 = initial concentration

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52 pH of Dilute Acid and Base Solutions Dilute Solutions of Weak Acids mass balance charge balance Use if H + ion concentration is expected to be less than M. If neglect K w Very Dilute Solutions of Strong Acids H 2 O(l) = H + (aq) + OH - (aq) Need two more equations: Only the terms containing K a are significant.

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55 pH of Diprotic and Triprotic Acids Concentration of H + ions is the product of only the first stage of ionization. Concentration of the conjugate base for the second stage of ionization is numerically equal to H 2 A + H 2 O = H 3 O + (aq) + HA - (aq) K a1 HA - (aq) + H 2 O = H 3 O + (aq) + A 2- (aq) K a2

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59 Molecular Structure and Acid Strength H X H + + X - The stronger the bond The weaker the acid HF << HCl < HBr < HI The polar the bond The stronger the acid Two competing effects: Hydrohalic Acids

60 Z O HZ O-O- + H + -- ++ The O-H bond will be more polar and easier to break if: Z is very electronegative or Z is in a high oxidation state Oxoacids

61 1. Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z H O Cl O O H O Br O O Cl is more electronegative than Br HClO 3 > HBrO 3

62 2. Oxoacids having the same central atom (Z) but different numbers of attached groups. Acid strength increases as the oxidation number of Z increases. HClO 4 > HClO 3 > HClO 2 > HClO

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64 Lewis structures can be generalized as: Carboxyl group (-COOH). R is part of the acid molecule Electronegative groups in the R group polarize the –O—H bond thereby increasing acidity Strengths of carboxylic acids depend on the nature of the R group. Carboxylic Acids How about benzoic acid?

65 Acid-Base Properties of Salts Salts producing neutral solutions: Salts containing an alkali metal or alkaline earth metal ion (except Be 2+ ) and the conjugate base of a strong acid (e.g. Cl -, Br -, and NO 3 - ). NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Salts producing basic solutions: Salts derived from a strong base and a weak acid. CH 3 COONa (s) Na + (aq) + CH 3 COO - (aq) H2OH2O CH 3 COO - (aq) + H 2 O (l) CH 3 COOH (aq) + OH - (aq)

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69 Salts producing acidic solutions: Salts derived from a strong acid and a weak base. NH 4 Cl (s) NH 4 + (aq) + Cl - (aq) H2OH2O NH 4 + (aq) NH 3 (aq) + H + (aq) Salts with small, highly charged metal cations (e.g. Al 3+, Cr 3+, and Be 2+ ) and the conjugate base of a strong acid. Al(H 2 O) 6 (aq) Al(OH)(H 2 O) 5 (aq) + H + (aq) 3+2+

70 Acid hydrolysis of Al 3+

71 K b for the anion > K a for the cation, solution will be basic K b for the anion < K a for the cation, solution will be acidic K b for the anion  K a for the cation, solution will be neutral Salts in which both the cation and the anion hydrolyze:

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73 Salts of amphoteric ions:

74 Oxides of the Representative Elements In Their Highest Oxidation States

75 Antacids and the Stomach pH Balance NaHCO 3 (aq) + HCl (aq) NaCl (aq) + H 2 O (l) + CO 2 (g) Mg(OH) 2 (s) + 2HCl (aq) MgCl 2 (aq) + 2H 2 O (l)

76 Amphoteric hydroxides: Basic hydroxides: Alkali metal hydroxides Alkaline earth metal hydroxides except Be(OH) 2 Be(OH) 2, Al(OH) 3, Sn, Pd, Cr, Cu, Zn & Cd hydroxides Al(OH) 3 (s) + 3H +  Al 3+ (aq) + 2H 2 O(l) Al(OH) 3 (s) + OH - = Al(OH) 4 - (aq)