Chapter 16: Acids and Bases Chemistry 140 Fall 2002 CHEMISTRY Ninth Edition GENERAL Principles and Modern Applications Petrucci • Harwood • Herring • Madura Chapter 16: Acids and Bases Juana Mendenhall, Ph.D. Assistant Professor Morehouse College General Chemistry: Chapter 16 Prentice-Hall © 2007

General Chemistry: Chapter 16 Chemistry 140 Fall 2002 Objectives Calculate values of Ka for cations and Kb for anions from ionization constants of their conjugates and Kw of water, and calculate the pH values of salt solutions in which hydrolysis occurs Use the relative strengths of Brønsted-Lowry acids and bases to predict the direction of acid-base reactions. Predict whether certain oxides and hydroxo compounds are acidic, basic, or neutral. Define what an oxoacid is and predict the relative strengths of oxoacids from their molecular structures. General Chemistry: Chapter 16 Prentice-Hall © 2007

16-7 Ions as Acids and Bases [NH3] [H+] Ka= [NH4+] NH4+(aq) NH3(aq) + H+(aq) acid base NH3(aq) + H2O(l) NH4+(aq) + OH-(aq) [NH4+][OH-] Kb base acid = [NH3] H2O(l) H+(aq) + OH-(aq) Ka x Kb = [NH4+][OH-] [NH3][H+] x = [OH-][H+] Kw = [NH4+] [NH3] [NH3] [H+] [OH-] Ka= [NH4+] [OH-] = KW Kb = 1.010-14 1.810-5 = 5.610-10 Ka Kb = Kw pKa + pKb = pKw = 14.00 General Chemistry: Chapter 16 Prentice-Hall © 2007

General Chemistry: Chapter 16 Examples Calculate the a) base-dissociation constant, Kb, for the fluoride ion, F- Ka = 6.8 x 10-4 b) Calculate the acid-dissociation constant, Ka, for the ammonium ion, NH4+ (Kb = 1.8 x 10-5) General Chemistry: Chapter 16 Prentice-Hall © 2007

General Chemistry: Chapter 16 Chemistry 140 Fall 2002 Hydrolysis is the reaction of ions with water to generate H+ or -OH. Na+ + H2O → Na+ + H2O No reaction Cl- + H2O → Cl- + H2O No reaction Hydrolysis NH4+ + H2O → NH3 + H3O+ Thus far we know salt solutions can be acidic or basic. We can also assume that when salts dissolve in water they are completely ionized and nearly all salt solutions are STRONG electrolytes. While the acid-base properties of salt solutions are due to the behavior of their cations and anions. When when ions react with water to generate H+ or -OH hydrlysis often occurs. Water (hydro) causing cleavage (lysis) of a bond. General Chemistry: Chapter 16 Prentice-Hall © 2007

16-8 Molecular Structure and Acid-Base Behavior Chemistry 140 Fall 2002 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. Strength depends on properties of the solvent and temperature. General Chemistry: Chapter 16 Prentice-Hall © 2007

Strengths of Binary Acids Chemistry 140 Fall 2002 Strengths of Binary Acids HI HBr HCl HF Bond length 160.9 > 141.4 > 127.4 > 91.7 pm Bond energy 297 < 368 < 431 < 569 kJ/mol Acid strength 109 > 108 > 1.3106 >> 6.610-4 More energy is required to break the H—F bone, HF is the weak acid. HI has the lowest bond energy, so HI is the strongest acid of the group. The polarity of the bond actually decreases from HF to HI b/c F is the most electronegative of the halogens. HF + H2O → [F-·····H3O+] F- + H3O+ ion pair H-bonding free ions General Chemistry: Chapter 16 Prentice-Hall © 2007

General Chemistry: Chapter 16 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 acid adduct Since BF3 accepts electrons from NH3 it is a lewis acid General Chemistry: Chapter 16 Prentice-Hall © 2007