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Common Ion Effect

Common Ion Effect. CH 3 COOH H + (aq) + CH 3 COO  (aq) pH of 0.1 M soln = Add 0.1 M CH 3 COONa: CH 3 COONa  Na + + CH 3 COO  (aq) pH = What happened to [CH 3 COO  ]? [CH 3 COOH]?. Buffer Solutions. A buffer is a solution that “resists” a change in pH

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Common Ion Effect

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  1. 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]?

  2. 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

  3. Buffers HX(aq) H+(aq) + X(aq) Henderson-Hasselbalch equation Note: If [HX] = [X], pH = pKa Buffers work best near pH = pKa

  4. Buffer Capacity CH3COOH H+(aq) + CH3COO(aq) • [CH3COOH] = [CH3COO] = 1.0 M • 1 L solution, pH = pKa = 4.74 • Add a dropper (~20ml) of 1M HCl • Add a dropper (~20ml) of NaOH • Repeat calculation starting from a 1.8 x 10-5 M HCl solution (pH = 4.74)

  5. Buffer Capacity ACH3COOH + CH3COONa (both 1 M) pH = B dilute solution A 10x pH = Repeat with 1 M HCl + 1 M NaCl solution

  6. 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 sparinglysoluble (< 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)

  7. 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

  8. SOLUBILITY Solubility Product: Ksp AgCl 1.8 x 10-10 AgBr 5.0 x 10-13 AgI 8.3 x 10-17 CdS 8.0 x 10-27 ZnS 1.1 x 10-21 Mg(OH)2 1.8 x 10-11 Ca(OH)2 5.5 x 10-6 CaF2 3.9 x 10-11 BaCO3 5.1 x 10-9 BaSO4 1.1 x 10-10 Ksp is constant for a given solid at a given temp.

  9. 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 1011 at 25oC What is the solubility? solubility = amount of CaF2 dissociated

  10. COMMON ION EFFECT precipitation saturation dissolution 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)

  11. SOLUBILITY CALCULATION Common Ion Effect What about CaF2 in 0.01 M NaF solution? [F] = 0.01 M Ksp = 3.9 x 1011 CaF2(s)  Ca2+(aq) + 2F(aq)

  12. Water Chemistry (Ch. 18.5-6) Water in State College/UP Campus Predominantly well water 23 wells + 1 open reservoir [Ca2+ ] 165-185 ppm10-15 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

  13. Remediation of Water Ca2+ (hard water), Pb2+ (toxic) are precipitated by CO32- Ksp 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 ?

  14. 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

  15. 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?

  16. 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 solubilitye.g., Mg(OH)2 • Saltsof weakly basic anions Low pH increases solubility examples (write out equilibria for practice) Mg(HCO3)2 ZnCO3 Ca3(PO4)2 NaF Contrast with NaCl, Ca(NO3)2

  17. 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)4(OH2)2 Amphoteric hydroxides: Both low and high pH increases solubility

  18. FORMATION OF COMPLEX IONS Hydration of metal ions Cu2+(aq) + 4 H2O(l)  [Cu(OH2)4]2+(aq) Lewis + Lewis  Lewis Acid/Base Acid 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)

  19. METAL COMPLEX STABILITY Cu(NH3)42+ + 4H2O  Cu(OH2)42+ + 4NH3 Cu2+(aq) [H2O] = constant Dissociation constant Cu(OH2)42+ + 4NH3 Cu(NH3)42+ + 4H2O Formation constant

  20. 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

  21. 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

  22. Complex Ion Formation CuCO3 is a sparingly soluble salt? KspCuCO3 = 2.3 x 1010 How can I get it to dissolve? What is theequilibrium constant for the following reaction? CuCO3(s)+ 4CN(aq) CO32(aq) + Cu(CN)42(aq) KspCuCO3 =2.3 x 1010 Kf Cu(CN)42 = 1 x 1025

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