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Common Ion Effect CH3COOH H+(aq) + CH3COO(aq) pH of 0.1 M soln = Add 0.1 M CH3COONa: CH3COONa Na+ + CH3COO(aq) pH = What happened to [CH3COO]? [CH3COOH]? Buffer Solutions A buffer is a solution that “resists” a change in pH E.g. blood contains substances that keep its pH fixed at 7.3 - important for life functions Buffer solutions consist of either: A weak acid + salt of its conjugate base or A weak base + salt of its conjugate acid Buffers H+(aq) + X(aq) HX(aq) H X X H HX HX Ka X pKa pH log HX X pH pKa log HX HendersonHasselbalch equation Note: If [HX] = [X], pH = pKa Buffers work best near pH = pKa Buffer Capacity CH3COOH H+(aq) + CH3COO(aq) [CH3COOH] = [CH3COO] = 1.0 M 1 L solution, pH = pKa = 4.74 1. Add a dropper (~20ml) of 1M HCl 2. Add a dropper (~20ml) of NaOH Repeat calculation starting from a 1.8 x 10-5 M HCl solution (pH = 4.74) Buffer Capacity A CH3COOH + CH3COONa (both 1 M) pH = B dilute solution A 10x pH = Repeat with 1 M HCl + 1 M NaCl solution SOLUBILITY Solubility: quantity of a substance that dissolves to form a saturated solution Solubility: g/L Molar solubility: mole/L Some salts are very soluble (> 0.1 M). Recall solubility rules. Some salts are sparingly soluble (< 0.1 M) sometimes referred to as ‘insoluble’. Precipitation and Solubility of ionic salts and their equilibrium in water MX(s) M+(aq) + X(aq) SOLUBILITY EQUILIBRIA • Precipitation Ag+(aq) + Cl(aq) AgCl(s) • Dissolution AgCl(s) Ag+(aq) + Cl(aq) At equilibrium when forward rate = backward rate AgCl(s) Ag+(aq) + Cl(aq) Keq = [Ag+][Cl]/ [AgCl(s)] Keq [AgCl(s)] = [Ag+][Cl] Ksp = [Ag+][Cl] The concentration of solid does not change at equilibrium SOLUBILITY Solubility Product: Ksp AgCl AgBr AgI CdS ZnS Mg(OH)2 Ca(OH)2 CaF2 BaCO3 BaSO4 1.8 x 10-10 5.0 x 10-13 8.3 x 10-17 8.0 x 10-27 1.1 x 10-21 1.8 x 10-11 5.5 x 10-6 3.9 x 10-11 5.1 x 10-9 1.1 x 10-10 Ksp is constant for a given solid at a given temp. SOLUBILITY CALCULATION Calculate [Ca2+] and [F-] for a saturated CaF2 solution. CaF2 (s) Ca+2 (aq) +2F (aq) Ksp = [Ca2+][F]2 =3.9 x 1011 at 25oC What is the solubility? solubility = amount of CaF2 dissociated COMMON ION EFFECT [Ag+] precipitation saturation dissolution [Cl-] If Q = [Ag+][Cl-] > Ksp, AgCl precipitates (Ion product > solubility product) If [Ag+][Cl-] < Ksp; AgCl dissolves When [Ag+][Cl-] = Ksp, the solution is saturated Adding either [Ag+] or [Cl-] will precipitate AgCl(s) SOLUBILITY CALCULATION Common Ion Effect What about CaF2 in 0.01 M NaF solution? [F] = 0.01 M Ksp = 3.9 x 1011 CaF2(s) Ca2+(aq) + 2F(aq) Water Chemistry (Ch. 18.5-6) Water in State College/UP Campus Predominantly well water 23 wells + 1 open reservoir [Ca2+ ] 165-185 ppm 10-15 ppm Mixed to reduce water hardness • Cl2 injected to kill bacteria • F- added • Alum (K2SO4.Al2(SO4)3.24H2O) added to improve clarity • NaOH added to neutralize pH Remediation of Water Ca2+ (hard water), Pb2+ (toxic) are precipitated by CO32Ksp CaCO3 8.7 x 10-9 PbCO3 3.3 x 10-14 Which compound(s) could we use to supply CO32-? How much do we need to add ? If we add a stoichiometric amount of Na2CO3, [Ca2+] = [CO32-] = Ksp1/2 [Ca2+] = 3 x 10-3 M 2.9 x 10-3 mol/L x 40 g/mol x 1000 mg/g = 116 mg/L (116 ppm) If [CO32-] = 3 mM, what is [Pb2+]? [Pb2+] = Ksp/[CO32-] = 3.3 x 10-14/3 x 10-3 = 1 x 10-11 M = 2 parts per trillion Effect of pH What is the solubility of Mg(OH)2 in pure water? Ksp = 1.8 x 10-11 What is the solubility of Mg(OH)2 in a solution with a pH of 9? Effect of pH on common ions If either the anion or the cation is involved in an acid base equilibrium, then it is a common ion problem. •Basic metal hydroxides Low pH increases solubility e.g., Mg(OH)2 •Salts of weakly basic anions Low pH increases solubility examples (write out equilibria for practice) Mg(HCO3)2 ZnCO3 Ca3(PO4)2 Contrast with NaCl, Ca(NO3)2 NaF AMPHOTERIC METAL HYDROXIDES There are amphoteric hydroxides of Al3+ Cr3+ Zn2+ Sn2+ and many transition metal ions Al(OH)3 , Cr(OH)3 , Zn(OH)2 , Sn(OH)2 , … Dissolution involves formation of complex ions: Al(OH)3(OH2)3 (s) + H+ Al(OH)2(OH2)4+ + OH Al(OH) (OH ) 4 2 2 Amphoteric hydroxides: Both low and high pH increases solubility FORMATION OF COMPLEX IONS Hydration of metal ions Cu2+(aq) + 4 H2O(l) [Cu(OH2)4]2+(aq) Lewis Acid + Lewis Lewis Acid/Base Base Adduct = Metal Complex Other Lewis bases react with metal ions to form complexes Cu2+(aq) + 4 NH3(aq) [Cu(NH3)4]2+(aq) Cu2+(aq) + 4 CN(aq) [Cu(CN)4]2(aq) Cu2+(aq) + 4 Cl(aq) [Cu(Cl)4]2(aq) METAL COMPLEX STABILITY Cu(NH3)42+ + 4H2O Cu(OH2)42+ + 4NH3 Cu(OH ) NH K Cu(NH ) H O [H O] = constant Cu NH K Dissociation constant Cu(NH ) 4 2 2 4 Cu2+(aq) 3 4 2 3 4 2 2 4 2 3 D 2 3 4 Cu(OH2)42+ + 4NH3 Cu(NH3)42+ + 4H2O KF Cu(NH ) Cu NH 2 3 4 4 2 3 Formation constant KF 1 KD Kf VALUES OF SOME COMPLEXES Ag(NH3)2+ 2 x 107 Cu(NH3)42+ 5 x 1012 Cu(CN)42- 1 x 1025 Ag(CN)2- 1 x 1021 Ag(S2O3)23- 3 x 1013 Complex Ion Formation What is the conc of free Cu2+ ions in a 1 L solution that contains 1 x10-3 moles total Cu2+ and is 0.1 M in NH3? Cu2+(aq) + 4 NH3(aq) Cu(NH3)42+(aq) Kf = 5 x 1012 Complex Ion Formation CuCO3 is a sparingly soluble salt? Ksp CuCO3 = 2.3 x 1010 How can I get it to dissolve? What is the equilibrium constant for the following reaction? CuCO3(s) + 4CN(aq) CO32(aq) + Cu(CN)42(aq) Ksp CuCO3 = 2.3 x 1010 Kf Cu(CN)42 = 1 x 1025