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Chemistry 1011

Chemistry 1011. TOPIC Acids and Bases TEXT REFERENCE Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589). Note on Topic Sequence. Section 13.4 in the Text includes some Cations as weak acids Section 13.5 in the Text includes some Anions as weak bases

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Chemistry 1011

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  1. Chemistry 1011 TOPIC Acids and Bases TEXT REFERENCE Masterton and Hurley Chapter 4.2 (Review), 13, 14.1, 15.1 (page 427), 21.2 (page589) Chemistry 1011 Slot 5

  2. Note on Topic Sequence • Section 13.4 in the Text includes some Cations as weak acids • Section 13.5 in the Text includes some Anions as weak bases • Section 13.5 also includes material concerning the relation between Ka and Kb • These topics will all be included when we cover Section 13.6 – “Acid-Base Properties of Salt Solutions” Chemistry 1011 Slot 5

  3. 13.5 Weak Bases and Their Ionization Constants YOU ARE EXPECTED TO BE ABLE TO: • Write an expression for the ionization constant, Kb, for a weak base • Place a list of weak bases in order of strength given values for Kb or pKb • Calculate Kb, given the pOH of a solution of a weak base • Given Kb, calculate the percent ionization of a weak base • Given Kb, calculate the [OH-], [H+] pOH and pH of a solution of a weak base of known concentration Chemistry 1011 Slot 5

  4. Ionization Constants of Weak Bases • Weak bases produce hydroxide ions by reacting with water molecules • Ammonia, NH3, is a weak base NH3(aq) + H2O(aq)NH4+(aq) + OH-(aq) cb ca ca cb • In this process, • the NH3 accepts a proton and is a Bronsted-Lowry base • the H2O donates a proton and is a Bronsted-Lowry acid • NH4+ is the conjugate acid of NH3 • OH- is the conjugate base of H2O • Organic amines, R-NH2, are also weak bases Chemistry 1011 Slot 5

  5. Ionization Constants of Weak Bases • NH3(aq) + H2O(aq)NH4+(aq) + OH-(aq) Kb = [NH4+] x[OH-] [NH3] • It is possible to determine • Kb, knowing the pOH and molar concentration of a weak base • pOH and PH, knowing the value of Kb and the molar concentration of a weak base Chemistry 1011 Slot 5

  6. Calculating Kb from the pH or pOH Value • Calculate the value of Kb for methylamine, CH3NH2 given that the pH of a 0.10mol/L solution is 11.8 CH3NH2(aq) + H2O(aq)CH3NH3+(aq) + OH-(aq) Kb = [CH3NH3+] x[OH-] [CH3NH2] • If pH = 11.8, then pOH = 2.2 and [OH-] = 6.3 x 10-3 Kb = (6.3 x 10-3)2 = (6.3 x 10-3)2 = 4.0 x 10-4 (1.0 x 10-1- 6.3 x 10-3) (1.0 x 10-1) Chemistry 1011 Slot 5

  7. Calculating [OH-], pH and pOH from Kb • Calculate the pH of a 5.0 x 10-2 mol/L solution of ammonia, given that Kb = 1.8 x 10-5. NH3(aq) + H2O(aq) NH4+(aq) + OH-(aq) Kb = [NH4+] x[OH-] = 1.8 x 10-5 [NH3] [original] 5.0 x 10-2 0.0 0.0 D[ ] -x +x +x [equilibrium] (5.0 x 10-2 - x) x x Chemistry 1011 Slot 5

  8. Solving for x (Approximate Method) • Assume that x << (5.0 x 10-2) • Then: Ka = 1.8 x 10-5 = x2 (5.0 x 10-2) x2 = (1.8 x 10-5)(5.0 x 10-2) = 9.0 x 10-7 x = 9.5 x 10-4mol/L pOH = 3.0 pH = 11.0 (Check that [OH-] is << [NH3] ) 9.5 x 10-4mol/L<< 5.0 x 10-2 mol/L Chemistry 1011 Slot 5

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