Transcript Slide 1
Chapter 14 Aqueous Equilibria: Acids and Bases Polyprotic Acids Acids that contains more than one dissociable proton Dissociate in a stepwise manner Each dissociation step has its own Ka Stepwise dissociation constants decreases in the order Ka1 > Ka2 > Ka3 More difficult to remove a positively charge proton from negative ion Polyprotic Acids Diprotic acid solutions contain a mixture of acids: H2A, HA, H2O Strongest acid – HA Principle reaction – dissociation of H2A All of H3O+ come from the first ionization Polyprotic Acids Equilibria in Solutions of Weak Bases B(aq) + H2O(l) Base Acid BH1+(aq) + OH1-(aq) Acid Base [BH1+][OH1-] Base-Dissociation Constant: NH3(aq) + H2O(l) [NH41+][OH1-] Kb = [NH3] Kb = NH41+(aq) + OH1-(aq) [B] Equilibria in Solutions of Weak Bases Equilibria in Solutions of Weak Bases Calculate the [-OH] and pH of a 0.40 M NH3 solution. At 25 °C, Kb = 1.8 x 10-5. Example Morphine (C17H19NO3), a narcotic used in painkillers, is a weak organic base. If the pH of a 7.0 x 10-4 M solution of morphine is 9.5, what is the value of Kb? Relation Between Ka and Kb NH41+(aq) + H2O(l) H3O1+(aq) + NH3(aq) Ka NH3(aq) + H2O(l) NH41+(aq) + OH1-(aq) Kb H3O1+(aq) + OH1-(aq) Kw 2H2O(l) Ka x Kb = [H3O1+][NH3] [NH41+] x [NH41+][OH1-] [NH3] = (5.6 x 10-10)(1.8 x 10-5) = 1.0 x 10-14 = [H3O1+][OH1-] = Kw Relation Between Ka and Kb Ka x Kb = Kw conjugate acid-base pair Ka = Kw Kb Kb = pKa + pKb = pKw = 14.00 Kw Ka Example Find the pH of a 0.100 M NaCHO2 solution. The salt completely dissociate into Na+(aq) and CHO2-(aq) and Na+ ion has no acid or base properties. Ka (HCHO2)= 1.8 x 10-4 Acid-Base Properties of Salts pH of a salt solution is determined by the acid-base properties of the consistuent cations and anions In an acid-base reaction, the influence of the stronger partner is predominant Strong acid + Strong Base Neutral solution Strong acid + Weak Base Basis solution Weak acid + Strong Base Acidic solution Acid-Base Properties of Salts Acidic cation + neutral anion Acidic salt Neutral cation + neutral anion neutral salt Neutral cation + basic anion basic salt Acid-Base Properties of Salts Acidic cation + basic anion (50 :50 mixture) must compare Ka and Kb • • • Ka > Kb: The solution will contain an excess of H3O1+ ions (pH < 7). Ka < Kb: The solution will contain an excess of OH1ions (pH > 7). Ka ≈ Kb: The solution will contain approximately equal concentrations of H3O1+ and OH1- ions (pH ≈ 7). Acid-Base Properties of Salts Salts That Yield Acidic Solutions Hydrated cations of small, highly charged metal ions, such as Al3+. Acid-Base Properties of Salts Copyright © 2008 Pearson Prentice Hall, Inc. Chapter 14/17 Examples Calculate Ka for the cation, and Kb for the anion in an aqueous NH4CN solution. Is the solution acidic, basic or neutral? Write the hydrolysis reaction of the salt (Kb for NH3 = 1.8 x 10-5, Ka for HCN = 4.9 x 10-10) Predict whether 0.35 M NH4Br solution is neutral, basic or acid. Write the hydrolysis equation and calculate the pH Lewis Acids and Bases Lewis Acid: An electron-pair acceptor. • Include cations and neutral molecule having vacant valence orbitals that can accept a share in a pair of electrons from a Lewis Base Lewis Base: An electron-pair donor. • All Lewis bases are Bronsted-Lowry bases Lewis Acids and Bases Lewis Acids and Bases Lewis Acids and Bases Examples For each of the following reactions, identify the Lewis acid and the Lewis base CO2(g) + -OH(aq) HCO3-(aq) AlCl3(aq) + Cl-(aq) AlCl4-(aq)