1 Acids and Bases Chapter 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

2 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. Have a bitter taste. Feel slippery. Many soaps contain bases. Bases

3 A Brønsted acid is a proton donor A Brønsted base is a proton acceptor acidbaseacidbase acid conjugate base base conjugate acid

Example Identify the conjugate acid-base pairs in the reaction between ammonia and hydrofluoric acid in aqueous solution NH 3 (aq) + HF(aq) (aq) + F - (aq)

Review of Concepts Which of the following does not constitute a conjugate acid-base pair? a)HNO 2 and NO 2 − b)H 2 CO 3 and CO 3 2− c)CH 3 NH 3 + and CH 3 NH 2 5

6 O H H+ O H H O H HH O H - + [] + Acid-Base Properties of Water H 2 O (l) H + (aq) + OH - (aq) H 2 O + H 2 O H 3 O + + OH - acid conjugate base base conjugate acid autoionization of water

7 H 2 O (l) H + (aq) + OH - (aq) The Ion Product of Water K c = [H + ][OH - ] [H 2 O] [H 2 O] = constant K c [H 2 O] = K w = [H + ][OH - ] 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

Example The concentration of OH - ions in a certain household ammonia cleaning solution is M. Calculate the concentration of H + ions.

9 pH – A Measure of Acidity pH = - log [H + ] [H + ] = [OH - ] [H + ] > [OH - ] [H + ] < [OH - ] Solution Is neutral acidic basic [H + ] = 1.0 x [H + ] > 1.0 x [H + ] < 1.0 x pH = 7 pH < 7 pH > 7 At 25 0 C pH[H + ]

10 pOH = −log [OH − ] [H + ][OH − ] = K w = 1.0 x 10 −14 [H + ] = 10 −pH [OH - ] = 10 −pOH pH + pOH = Other important relationships pH Meter

Example The concentration of H + ions in a bottle of table wine was 3.2 x M right after the cork was removed. Only half of the wine was consumed. The other half, after it had been standing open to the air for a month, was found to have a hydrogen ion concentration equal to 1.0 x M. Calculate the pH of the wine on these two occasions.

Example The pH of rainwater collected in a certain region of the northeastern United States on a particular day was Calculate the H + ion concentration of the rainwater.

Example In a NaOH solution [OH - ] is 2.9 x M. Calculate the pH of the solution.

14 Strong Electrolyte – 100% dissociation NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Weak Electrolyte – not completely dissociated CH 3 COOH CH 3 COO - (aq) + H + (aq) Strong Acids are strong electrolytes HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) HNO 3 (aq) + H 2 O (l) H 3 O + (aq) + NO 3 - (aq) HClO 4 (aq) + H 2 O (l) H 3 O + (aq) + ClO 4 - (aq) H 2 SO 4 (aq) + H 2 O (l) H 3 O + (aq) + HSO 4 - (aq)

15 HF (aq) + H 2 O (l) H 3 O + (aq) + F - (aq) Weak Acids are weak electrolytes HNO 2 (aq) + H 2 O (l) H 3 O + (aq) + NO 2 - (aq) HSO 4 - (aq) + H 2 O (l) H 3 O + (aq) + SO 4 2- (aq) H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) Strong Bases are strong electrolytes NaOH (s) Na + (aq) + OH - (aq) H2OH2O KOH (s) K + (aq) + OH - (aq) H2OH2O Ba(OH) 2 (s) Ba 2+ (aq) + 2OH - (aq) H2OH2O

16 F - (aq) + H 2 O (l) OH - (aq) + HF (aq) Weak Bases are weak electrolytes NO 2 - (aq) + H 2 O (l) OH - (aq) + HNO 2 (aq) 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.

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18 Strong Acid (HCl)Weak Acid (HF)

Example Calculate the pH of a (a) 1.0 x M HCl solution (b) M Ba(OH) 2 solution

Example 20 Predict the direction of the following reaction in aqueous solution: HNO 2 (aq) + CN - (aq) HCN(aq) + (aq) 15.7

21 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 = [H + ][A - ] [HA] K a is the acid ionization constant KaKa weak acid strength

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23 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 ICE 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. If approximation is not valid, solve for x exactly. 4.Calculate concentrations of all species and/or pH of the solution.

Example Calculate the pH of a M nitrous acid (HNO 2 ) solution: HNO 2 (aq) H + (aq) + (aq)

Example The pH of a 0.10 M solution of formic acid (HCOOH) is What is the K a of the acid?

26 percent ionization = Ionized acid concentration at equilibrium Initial concentration of acid x 100% For a monoprotic acid HA, Percent ionization = [H + ] [HA] 0 x 100% [HA] 0 = initial concentration

27 NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq) Weak Bases and Base Ionization Constants K b = [NH 4 + ][OH - ] [NH 3 ] K b is the base ionization constant KbKb weak base strength Solve weak base problems like weak acids except solve for [OH-] instead of [H + ].

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Example 29 What is the pH of a 0.40 M ammonia solution? 15.10

30 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 and Its Conjugate Base Ka =Ka = KwKw KbKb Kb =Kb = KwKw KaKa

31 Diprotic and Triprotic Acids May yield more than one hydrogen ion per molecule. Ionize in a stepwise manner; that is, they lose one proton at a time. An ionization constant expression can be written for each ionization stage. Consequently, two or more equilibrium constant expressions must often be used to calculate the concentrations of species in the acid solution.

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Example Oxalic acid (H 2 C 2 O 4 ) is a poisonous substance used chiefly as a bleaching and cleansing agent (for example, to remove bathtub rings). Calculate the concentrations of all the species present at equilibrium in a 0.10 M solution.

34 Molecular Structure and Acid Strength H X H + + X - The stronger the bond The weaker the acid HF << HCl < HBr < HI acidity increases

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36 Molecular Structure and Oxoacid Strength 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

37 Molecular Structure and Oxoacid Strength 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 acidity increases

38 Molecular Structure and Acid Strength 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

Example Predict the relative strengths of the oxoacids in each of the following groups: (a) HClO, HBrO, and HIO (b) HNO 3 and HNO 2

40 Acid-Base Properties of Salts 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 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)

Example 41 Calculate the pH of a 0.15 M solution of sodium acetate (CH 3 COONa). What is the percent hydrolysis? 15.13

42 Acid-Base Properties of Salts Acid 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+

43 Acid Hydrolysis of Al 3+

44 Acid-Base Properties of Salts Solutions in which both the cation and the anion hydrolyze: 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

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Example Predict whether the following solutions will be acidic, basic, or nearly neutral: (a)NH 4 I (b) NaNO 2 (c) FeCl 3 (d) NH 4 F

47 Oxides of the Representative Elements In Their Highest Oxidation States CO 2 (g) + H 2 O (l) H 2 CO 3 (aq) N 2 O 5 (g) + H 2 O (l) 2HNO 3 (aq) Na 2 O (s) + H 2 O (l) 2NaOH (aq)

48 Arrhenius acid is a substance that produces H + (H 3 O + ) in water A Brønsted acid is a proton donor 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 Definition of An Acid H+H+ H O H + OH - acidbase N H H H H+H+ + acidbase N H H H H +

49 Lewis Acids and Bases N H H H acidbase F B F F + F F N H H H No protons donated or accepted!

Example 50 Identify the Lewis acid and Lewis base in each of the following reactions: (a) C 2 H 5 OC 2 H 5 + AlCl 3 (C 2 H 5 ) 2 OAlCl 3 (b) Hg 2+ (aq) + 4CN - (aq) (aq) 15.15