CHAPTER 9: DISSOLUTION AND PRECIPITATION EQUILIBRIADissolution and PrecipitationSaturationSolubility EquilibriaExampleExample, cont’dPredicting PrecipitationExample“Common Ion” EffectEffect of pH on solubilityComplex Ion EquilibriaComplexes of metals with waterCHAPTER 9: DISSOLUTION AND PRECIPITATION EQUILIBRIA• Equilibrium constants for dissolution and precipitation reactions• Saturated, unsaturated, and supersaturated solutions• Reaction quotients and solubility• Common Ion effect• Effect of pH on solubility• Complex Ion EquilibriaCHEM 1310 A/B Fall 2006Dissolution and Precipitation• Dissolution: substance dissolves, e.g,– NaCl(s) → Na+(aq) +Cl-(aq)– sugar(s) → sugar(aq)• Precipitation: Reverse of the above,e.g.,–Na+(aq) + Cl-(aq) → NaCl(s)– sugar(aq) → sugar(s)• For completely soluble/insoluble substances, reaction goes only one way. For partially soluble substances, an equilibrium is established, ala chapter 7CHEM 1310 A/B Fall 2006Saturation• Saturated solution: enough solute that some remains as undissolved solid. Equilibrium between solid & solvated solute• Unsaturated solution: Not enough solute to precipitate• Supersaturated solution: concentration of dissolved solute exceeds its equilibrium value (achieving equilibrium can be slow, particularly for dissolution/precipitation)CHEM 1310 A/B Fall 2006Solubility Equilibria• Equilibrium expressions for solutions ---just like chapter 7. Here, “activities” are concentrations in mol/L.• AgCl(s) ⇔ Ag+(aq) + Cl-(aq)•K = Ksp= [Ag+][Cl-] “solubility product”CHEM 1310 A/B Fall 2006ExampleIf Kspof Pb(IO3)2at 25oC is 2.6x10-13, what mass of Pb(IO3)2is dissolved in a saturated aqueous solution?(Saturated: the max possible amount allowed by Ksp is dissolved.)CHEM 1310 A/B Fall 2006Example, cont’dCHEM 1310 A/B Fall 2006Predicting Precipitation• For given concentrations, how do we know if a solute will precipitate?• If we know Ksp, we can make these predictions• Use “Q” concept (chapter 7)• Q>K : Reaction goes backward• Q<K: Reaction goes forwardCHEM 1310 A/B Fall 2006Example1.22x10-3mol Tl+is added to 9.79x10-4mol IO3-. Total final volume is 1L. Does TlIO3(s) precipitate at equilibrium? Ksp= 3.1x10-6@ 25oC.CHEM 1310 A/B Fall 2006“Common Ion” Effect• Can use a different source of cations or anions to drive reactions forwards or backwards• An application of LeChatelier’s principle!• AgCl(s) ⇔ Ag+(aq) + Cl-(aq)• What does the LeChatelier principle say will happen to [Ag+] if we dump in some NaCl into solution?•Cl-is the “common ion”CHEM 1310 A/B Fall 2006Effect of pH on solubility• Similar to common ion effect. Obviously an equlibrium likeZn(OH)2(s) ⇔ Zn2+(aq) + 2 OH-(aq)is affected by pH, since[H3O+][OH-] = Kw = 10-14,so [OH-] is linked to [H3O+] and hence pH.• What happens to [Zn2+] as pH goes up??CHEM 1310 A/B Fall 2006Complex Ion Equilibria• Often transition metals “coordinate” other molecules to form metal complexes, e.g.,Ag+(aq) + NH3(aq) ⇔ Ag(NH3)+(aq)K1= [Ag(NH3)+] / [Ag+][NH3]Ag(NH3)+(aq) + NH3(aq) ⇔ Ag(NH3)2+(aq)K2= [Ag(NH3)2+] / [Ag(NH3)+][NH3]•If K1, K2both large, most Ag exists as Ag(NH3)2+, asuming [NH3] > 2 [Ag+]• In this case, can work equilibrium as 2 separate steps, starting from the last equation written backwards; solve to get [Ag(NH3)+] and then do other equilibrium to get [Ag+] (see example in book)CHEM 1310 A/B Fall 2006Complexes of metals with water• Similar to last example, although for water it’s also common to give up a proton, e.g.,Fe(H2O)63+(aq) + H2O(l) ⇔ H3O+(aq) + Fe(H2O)5OH2+(aq)• Can also doZn2+(aq) + OH-(aq) ⇔ ZnOH+(aq)ZnOH+(aq) + OH-(aq) ⇔ Zn(OH)2(s) Zn(OH)2(s) + OH-(aq) ⇔ Zn(OH)3-(aq)… pH dependent again!CHEM 1310 A/B Fall
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