Lecture 7 -- Chemical Equilibrium

Calculating titration curvesSelf-study sectionA nice application of acid-base equilibrium calculations79Self-studyAgendaFormal vs equilibrium concentrationAcid equilibriaBase equilibriaKa, Kb and KwCommon ion effectBuffersAcid-Base Titrations2Note to StudentsThis chapter is considered the hardest in CHY102. Many students struggle with this chapter and the average mark on Test #2 is typically lower than on Test #1.To prepare for the test, and for your future careers as engineers, please practice with lots of questions in your textbook and Mastering Chemistry. Come to our office hours with questions well before the test!The last section of these notes is a self-study section that provides two worked examples calculating a titration curve. We will not go over the self-study section during class.3CoverageSections you should know:16.2 Buffers: Solutions That Resist pH Change16.3 Buffer Effectiveness: Buffer Range and Buffer Capacity16.4 Titrations and pH Curves

Things to exclude:16.1 The Danger of Antifreeze16.5 Solubility Equilibria and the Solubility Product Constant16.6 Precipitation16.7 Qualitative Chemical Analysis16.8 Complex-Ion Equilibria4Formal and equilibrium concentrations5Formal vs. Equilibrium Conc.Formal concentration cX = moles of X, regardless of chemical form, in 1 L solnEquilibrium concentration [X] = actual moles of X in 1 L soln at equilibrium66Acid examplese.g. Strong acid: 1.0 molL-1 HCl (formal) is really1.0 molL-1 H+ (aq)1.0 molL-1 Cl-(aq)cHCl = [H+] = [Cl-]e.g. Weak acid: 0.100 M acetic acid (formal conc.) is[HOAc] = 0.096 molL-1 [OAc-] = 4.210-3 molL-1 cHOAc = [HOAc] + [OAc-]77Base examplese.g. Strong base: 1.0 molL-1 NaOH (formal) is really1.0 molL-1 Na+ (aq)1.0 molL-1 OH-(aq)cNaOH = [Na+] = [OH-]e.g. Weak base: 0.100 molL-1 NH3 (formal conc.) is[NH3] = 0.096 molL-1 [NH4+] = 4.210-3 molL-1 cNH3 = [NH3] + [NH4+]88LC: Formal ConcentrationWhat is true about [HF] in a solution that is formally 1.0 M HF?

[HF] > 1.0 molL-1 [HF] = 1.0 molL-1 [HF] < 1.0 molL-1 Not enough information given.9

9Acid equilibria10Acid dissociation constant, Ka1111Note on abbreviationsGeneric weak acid: HAHA = acidA- = conjugate base (anion)Acetic acid: CH3COOHHOAc = CH3COOH acetic acidOAc- = CH3COO- acetate (an anionic base)Ac = CH3CO- acetyl group (a radical)12Getting Ka from pHe.g. The pH of a soln of acetic acid with a formal conc of 0.020 molL-1 is 3.23. Find Ka.

[H+] = 10-pH = 10-3.23 = 5.8910-4 molL-1 13

13Regarding H+Actually two sources of H+ Water dissociation: H2O(l) H+(aq) + OH-(aq)HOAc dissociation: HOAc(l) H+(aq) + OAc-(aq)Total [H+] = [OAc-] + [OH-] (by charge balance)BUT: [OH-]