Chapter 1: Matter and Measurement

Chapter 1: Matter and Measurement

General Chemistry Principles and Modern Applications Petrucci Harwood Herring 8th Edition Chapter 16: Acids and Bases Philip Dutton University of Windsor, Canada N9B 3P4 Prentice-Hall 2002 Contents 16-1 16-2 16-3 16-4 16-5 16-6 16-7 16-8 16-8 The Arrhenius Theory: A Brief Review Brnsted-Lowry Theory of Acids and Bases The Self-Ionization of Water and the pH Scale Strong Acids and Strong Bases Weak Acids and Weak Bases

Polyprotic Acids Ions as Acids and Bases Molecular Structure and Acid-Base Behavior Lewis Acids and Bases Focus On Acid Rain. Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 2 of 47 16-1 The Arrhenius Theory: A Brief Review H2O HCl(g) H+(aq) + Cl-(aq) H2O NaOH(s) Na+(aq) + OH-(aq) Na+(aq) + OH-(aq) + H+(aq) + Cl-(aq) H2O(l) + Na+(aq) + Cl-(aq) H+(aq) + OH-(aq) H2O(l) Arrhenius theory did not handle non OHbases such as ammonia very well. Prentice-Hall 2002 General Chemistry: Chapter 16

Slide 3 of 47 16-2 Brnsted-Lowry Theory of Acids and Bases An acid is a proton donor. A base is a proton acceptor. base acid id se c a a b e ate g at g u u j j n co

con NH3 + H2O NH4+ + OHNH4+ + OH- NH3 + H2O acid Prentice-Hall 2002 base General Chemistry: Chapter 16 Slide 4 of 47 Base Ionization Constant base acid conjugate conjugate acid base- NH3 + H2O NH4 + OH + [NH4+][OH-]

Kc= [NH3][H2O] [NH4+][OH-] Kb= Kc[H2O] = = 1.810-5 [NH3] Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 5 of 47 Acid Ionization Constant acid base conjugate base conjugate acid CH3CO2H + H2O CH3CO2 + H3O+ -

[CH3CO2-][H3O+] Kc= [CH3CO2H][H2O] Ka= Kc[H2O] = Prentice-Hall 2002 [CH3CO2-][H3O+] = 1.810-5 [CH3CO2H] General Chemistry: Chapter 16 Slide 6 of 47 Table 16.1 Relative Strengths of Some Brnsted-Lowry Acids and Bases HCl + OH- Cl- + H2O HClO4 + H2O ClO4- + H3O+ Prentice-Hall 2002 NH4+ + CO32- NH3 + HCO3H2O + I- OH- + HI General Chemistry: Chapter 16 Slide 7 of 47

16-3 The Self-Ionization of Water and the pH Scale Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 8 of 47 Ion Product of Water base acid conjugate conjugate acid base- H2O + H2O H3O + OH + [H3O+][OH-] Kc= [H2O][H2O] KW= Kc[H2O][H2O] = [H3O+][OH-] = 1.010-14

Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 9 of 47 pH and pOH The potential of the hydrogen ion was defined in 1909 as the negative of the logarithm of [H+]. pH = -log[H3O+] pOH = -log[OH-] KW = [H3O+][OH-]= 1.010-14 -logKW = -log[H3O+]-log[OH-]= -log(1.010-14) pKW = pH + pOH= -(-14) pKW = pH + pOH = 14 Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 10 of 47 pH and pOH Scales Prentice-Hall 2002

General Chemistry: Chapter 16 Slide 11 of 47 16-4 Strong Acids and Bases HCl CH3CO2H Thymol Blue Indicator pH < 1.2 < pH < 2.8 < pH Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 12 of 47 16-5 Weak Acids and Bases Acetic Acid Prentice-Hall 2002 HC2H3O2 or CH3CO2H General Chemistry: Chapter 16

Slide 13 of 47 Weak Acids Ka= [CH3CO2-][H3O+] = 1.810-5 [CH3CO2H] pKa= -log(1.810-5) = 4.74 O lactic acid CH3CH(OH) CO2H R glycine H2NCH2CO2H Prentice-Hall 2002 General Chemistry: Chapter 16 C OH Slide 14 of 47 Table 16.3 Ionization Constants of Weak Acids and Bases Prentice-Hall 2002 General Chemistry: Chapter 16

Slide 15 of 47 Example 16-5 Determining a Value of KA from the pH of a Solution of a Weak Acid. Butyric acid, HC4H7O2 (or CH3CH2CH2CO2H) is used to make compounds employed in artificial flavorings and syrups. A 0.250 M aqueous solution of HC4H7O2 is found to have a pH of 2.72. Determine KA for butyric acid. C4H7O2H + H2O C4H7O2-+ H3O+ Ka = ? Solution: For C4H7O2H KA is likely to be much larger than KW. Therefore assume self-ionization of water is unimportant. Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 16 of 47 Example 16-5 HC4H7O2 + H2O C4H7O2- + H3O+ Initial conc.

0.250 M 0 0 Changes -x M +x M +x M Eqlbrm conc. (0.250-x) M xM xM Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 17 of 47

Example 16-5 C4H7O2H + H2O C4H7O2- + H3O+ Log[H3O+] = -pH = -2.72 [H3O+] = 10-2.72 = 1.910-3 = x Ka= [H3O+] [C4H7O2-] [HC4H7O2] Ka= 1.46310-5 Prentice-Hall 2002 = 1.910-3 1.910-3 (0.250 1.910-3) Check assumption: Ka >> KW. General Chemistry: Chapter 16 Slide 18 of 47 Percent Ionization

HA + H2O H3O+ + ADegree of ionization = Percent ionization = Prentice-Hall 2002 [H3O+] from HA [HA] originally [H3O+] from HA [HA] originally General Chemistry: Chapter 16 100% Slide 19 of 47 Percent Ionization Ka = [H3O+][A-] [HA] n H O nA 1 Ka = nHA V

3 Prentice-Hall 2002 General Chemistry: Chapter 16 + - Slide 20 of 47 16-6 Polyprotic Acids Phosphoric acid: A triprotic acid. H3PO4 + H2O H3O+ + H2PO4- Ka1 = 7.110-3 H2PO4- + H2O H3O+ + HPO42- Ka2 = 6.310-8 HPO42- + H2O H3O+ + PO43- Ka3 = 4.210-13

Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 21 of 47 Phosphoric Acid Ka1 >> Ka2 All H3O+ is formed in the first ionization step. H2PO4- essentially does not ionize further. Assume [H2PO4-] = [H3O+]. [HPO42-] Ka2 regardless of solution molarity. Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 22 of 47 Example 16-9 Calculating Ion Concentrations in a Polyprotic Acid Solution. For a 3.0 M H3PO4 solution, calculate: (a) [H3O+];

(b) [H2PO4-]; (c) [HPO42-] (d) [PO43-] H3PO4 + H2O H2PO4- + H3O+ Initial conc. 3.0 M 0 0 Changes -x M +x M +x M Eqlbrm conc. (3.0-x) M

xM xM Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 23 of 47 Example 16-9 H3PO4 + Ka1= H2O H2PO4- + H3O+ [H3O+] [H2PO4-] [H3PO4] = xx (3.0 x) = 7.110-3

Assume that x << 3.0 x2 = (3.0)(7.110-3) x = 0.14 M [H2PO4-] = [H3O+] = 0.14 M Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 24 of 47 Example 16-9 H2PO4- + H2O HPO42- + H3O+ Initial conc. 0.14 M 0 0.14 M

Changes -y M +y M +y M Eqlbrm conc. (0.14 - y) M yM (0.14 +y) M Ka2= [H3O+] [HPO42-] [H2PO4-] y << 0.14 M Prentice-Hall 2002 =

y (0.14 + y) (0.14 - y) = 6.310-8 y = [HPO42-] = 6.310-8 General Chemistry: Chapter 16 Slide 25 of 47 Example 16-9 HPO4- + Ka3= H2O PO43- + H3O+ [H3O+] [PO43-] [HPO42-] = (0.14)[PO43-] 6.310-8 = 4.210-13 M

[PO43-] = 1.910-19 M Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 26 of 47 Table 16.4 Ionization Constants of Some Polyprotic Acids Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 27 of 47 Sulfuric Acid Sulfuric acid: A diprotic acid. H2SO4 + H2O H3O+ + HSO4- Ka = very large HSO4- + H2O H3O+ + SO42- pKa = 1.96

Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 28 of 47 General Approach to Solution Equilibrium Calculations Identify species present in any significant amounts in solution (excluding H2O). Write equations that include these species. Number of equations = number of unknowns. Equilibrium constant expressions. Material balance equations. Electroneutrality condition. Solve the system of equations for the unknowns. Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 29 of 47 16-7 Ions as Acids and Bases CH3CO2- + H2O CH3CO2H + OHbase

acid Ka= NH4+ + H2O NH3 + H3O+ acid Ka= base [NH3] [H3O+] [OH-] [NH4+] [OH-] = KW Kb = [NH3] [H3O+] [NH4+] 1.010-14 1.810-5 =?

= 5.610-10 Ka Kb = Kw Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 30 of 47 Hydrolysis Water (hydro) causing cleavage (lysis) of a bond. Na+ + H2O Na+ + H2O No reaction Cl- + H2O Cl- + H2O No reaction NH4+ + H2O NH3 + H3O+ Hydrolysis Prentice-Hall 2002 General Chemistry: Chapter 16

Slide 31 of 47 16-8 Molecular Structure and Acid-Base Behavior Why is HCl a strong acid, but HF is a weak one? Why is CH3CO2H a stronger acid than CH3CH2OH? There is a relationship between molecular structure and acid strength. Bond dissociation energies are measured in the gas phase and not in solution. Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 32 of 47 Strengths of Binary Acids HI HBr HCl HF

Bond length 160.9 > 141.4 > 127.4 > 91.7 pm Bond energy 297 < 368 kJ/mol Acid strength 109 > 108 > 1.3106 >> 6.610-4 < 431 < 569 HF + H2O [F-H3O+] F- + H3O+ ion pair H-bonding

Prentice-Hall 2002 free ions General Chemistry: Chapter 16 Slide 33 of 47 Strengths of Oxoacids Factors promoting electron withdrawal from the OH bond to the oxygen atom: High electronegativity (EN) of the central atom. A large number of terminal O atoms in the molecule. H-O-Cl H-O-Br ENCl = 3.0 ENBr= 2.8 Ka = 2.910-8 Prentice-Hall 2002 Ka = 2.110-9

General Chemistry: Chapter 16 Slide 34 of 47 The strength of oxoacids: HOCl, HOClO Electron rich Csonka Gbor Electron poor ltalnos Kmia: 7. Savak s bzisok Dia 35 /43 HClO3, HClO4 Electron rich Csonka Gbor ltalnos Kmia: 7. Savak s bzisok Dia 36 /43 O

O H O O S H H O

O S H O Ka 103 Ka =1.310-2 O O

H O S O H H O S + O

H - O 2+ Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 37 of 47 Strengths of Organic Acids H O

H C C O H acetic acid Ka = 1.810-5 Prentice-Hall 2002 H H H H C C

H H O H ethanol Ka =1.310-16 General Chemistry: Chapter 16 Slide 38 of 47 Focus on the Anions Formed H C H

C H O General Chemistry: Chapter 16 H C O C O - O

H Prentice-Hall 2002 H H C H C O

H H H Slide 39 of 47 Structural Effects H C O H

H Ka = 1.310-5 H H H H H H H C C C C C

C C Prentice-Hall 2002 H H H H General Chemistry: Chapter 16 H O H C

H O C Ka = 1.810-5 H O Slide 40 of 47 Structural Effects O

H C Cl H C C O Prentice-Hall 2002 O H Ka = 1.410-3 C Ka = 1.810-5

H O H General Chemistry: Chapter 16 Slide 41 of 47 Strengths of Amines as Bases H Br H N N

H H H ammonia bromamine pKb = 4.74 pKb = 7.61 Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 42 of 47 Strengths of Amines as Bases H H

C NH2 H H H H C C H H NH2 H H H H C

C C H H H methylamine ethylamine propylamine pKb = 4.74 pKb = 3.38 pKb = 3.37 Prentice-Hall 2002 General Chemistry: Chapter 16

NH2 Slide 43 of 47 Resonance Effects Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 44 of 47 Inductive Effects Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 45 of 47 16-9 Lewis Acids and Bases Lewis Acid A species (atom, ion or molecule) that is an electron pair acceptor. Lewis Base A species that is an electron pair donor.

base Prentice-Hall 2002 acid General Chemistry: Chapter 16 adduct Slide 46 of 47 Showing Electron Movement Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 47 of 47 Focus On Acid Rain CO2 + H2O H2CO3 3 NO2 + H2O 2 HNO3 + NO H2CO3 + H2O HCO3- + H3O+ Prentice-Hall 2002

General Chemistry: Chapter 16 Slide 48 of 47 Chapter 16 Questions Develop problem solving skills and base your strategy not on solutions to specific problems but on understanding. Choose a variety of problems from the text as examples. Practice good techniques and get coaching from people who have been here before. Prentice-Hall 2002 General Chemistry: Chapter 16 Slide 49 of 47

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