CHM 320 Lecture 12 Chap 9Chapter 9 - Systematic Treatment of EquilibriumHomework: Due Wednesday, February 22Problems 9-2, 9-5, 9-8, 9-12, 9-14, 9-17Extra Credit: 9-25 (5 pts)CHM 320 Lecture 12 Chap 9Common reactions in aqueous solutions• Precipitation• Acid-base• Complex formation• Oxidation-reduction• Partitioning between phasesOne, a few, or all of these reactions may occur in any given sample.CHM 320 Lecture 12 Chap 9Systematic approach to solving equilibrium problemsStep 1. Write all pertinent chemical reactions.Step 2. Write charge balance equation.Step 3. Write mass balance equations.Step 4. Write all equilibrium expressions. Use activities if known.Step 5. Count equations and unknowns. There must be as many or more equations than unknowns or else the problem cannot be solved.Step 6. Use algebraic expressions to solve for species concentrations. http://www.chem.usu.edu/faculty/sbialkow/Classes/3600/Overheads/systematic.htmlhttp://mason.gmu.edu/~gfoster/Chem_321/systematic_ho/systematic_treatment.htmlCHM 320 Lecture 12 Chap 9Step 1 – Write all pertinent chemical reactions.What are “pertinent chemical reactions”?Example: Dissolving CO2in waterCO2+ H2O ⇔ H2CO3K = 2 x 10-3H2CO3⇔ HCO3-+ H+ Ka1= 4.45 x 10-7HCO3-⇔ CO3-2+ H+Ka2= 4.69 x 10-11H2O ⇔ H++ OH-Kw= 1 x 10-14CHM 320 Lecture 12 Chap 9Step 2 - Write charge balance equation.Pertinent Reactions:CO2+ H2O ⇔ H2CO3H2CO3⇔ HCO3-+ H+ HCO3-⇔ CO3-2+ H+H2O ⇔ H++ OH-Charge balance equation:[H+] = [HCO3-] + 2[CO3-2] + [OH-]Note: coefficient in front of the species is equal to the magnitude of the charge (so –2 has a coeff. of 2)CHM 320 Lecture 12 Chap 9Step 3. Write mass balance equations.Pertinent Reactions:CO2+ H2O ⇔ H2CO3H2CO3⇔ HCO3-+ H+ HCO3-⇔ CO3-2+ H+H2O ⇔ H++ OH-Mass balance equation:[CO2] = [H2CO3] + [HCO3-] + [CO3-2] + [CO2] Note: Mass balance is really a conservation of atoms (i.e., account for all the atoms put into solution).CHM 320 Lecture 12 Chap 9Step 4. Write all equilibrium expressions. Use activities if known.Pertinent Reactions:CO2+ H2O ⇔ H2CO3HCO3-⇔ CO3-2+ H+H2CO3⇔ HCO3-+ H+ H2O ⇔ H++ OH-K = [H2CO3] / [CO2] = 2 x 10-3Ka1= [HCO3-] [H+] / [H2CO3] = 4.45 x 10-7Ka2= [CO3-2] [H+] / [HCO3-] = 4.69 x 10-11Kw= [H+] [OH-] = 1 x 10-14CHM 320 Lecture 12 Chap 9Step 5. Count equations and unknowns.Pertinent Reactions:CO2+ H2O ⇔ H2CO3HCO3-⇔ CO3-2+ H+H2CO3⇔ HCO3-+ H+ H2O ⇔ H++ OH-Unknowns: [CO2], [H2CO3], [HCO3-], [CO3-2], [H+], [OH-] Equations: [H+] = [HCO3-] + 2[CO3-2] + [OH-][CO2] = [H2CO3] + [HCO3-] + [CO3-2] + [CO2]K = [H2CO3] / [CO2] = 2 x 10-3Ka1= [HCO3-] [H+] / [H2CO3] = 4.45 x 10-7Ka2= [CO3-2] [H+] / [HCO3-] = 4.69 x 10-11Kw= [H+] [OH-] = 1 x 10-146 unknownsand6 equationsCHM 320 Lecture 12 Chap 9Step 6. Use algebraic expressions to solve for species concentrations. This step is many times easier said than done. It is very useful to use a spreadsheet.If there are as many or more equations than unknowns, then we can move to Step 6. If there are more unknowns than equations, then some assumptions must be made to simplify the system. (You cannot solve for more unknowns than there are equations.CHM 320 Lecture 12 Chap 9Problem 9-A Write the charge balance for a solution prepared by dissolving CaF2in H2O. Consider that the CaF2can give Ca2+, F-, and CaF+.Problem 9-B a) Write the mass balance for CaCl2in H2O if the aqueous species are Ca2+ and Cl-. b) Write the mass balance if the species are Ca2+, Cl-, and CaCl+.CHM 320 Lecture 12 Chap 9Problem 9-C a) Write the mass balance for CaF2in H2O if the Rx’s are: CaF2(s)⇔ Ca2+ + 2F-F-+ H+⇔ HF(aq)b) Write a mass balance for CaF2in water if, in addition to the previous reactions, the following reaction occurs: HF(aq)+ F-⇔ HF2-CHM 320 Lecture 12 Chap 9Problem 9-F Calculate the solubility of ZnC2O4(g/L) in a solution held at pH 3.00. Consider the equilibria:ZnC2O4(s)⇔ Zn2+ + C2O42-Ksp= 7.5 x 10-9C2O42-+ H2O ⇔ H C2O4-+ OH-Kb1= 1.8 x 10-10HC2O4-+ H2O ⇔ H2C2O4 + OH-Kb2= 1.8 x 10-13Problem 9-E Find the concentrations of Ag+, CN-, and HCN in a saturated solution of AgCN whose pH is somehow fixed at 9.00. Consider the following equilibria:AgCN(s)⇔ Ag+ + CN-Ksp= 2.2 x 10-16CN-+ H2O ⇔ HCN(aq)+ OH-Kb= 1.6 x