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Common Ion Effect

common-ion effect - ionization of electrolyte decreases if common ion added  

  • shifts equilibrium against a certain side
  • Ka = [H+][common ion] / [acid]
  • Kb = [OH-][common ion] / [base]
  • can decrease the solubility of certain slightly soluble salts

Find the pH in a solution of 0.1 M lactic acid and 0.1 M sodium lactate  

  • Given:
    • Ka of lactic acid = 1.4 x 10-4
    • Ka = [H+][common ion] / [acid]
  • 1.4 x 10-4 = [H+] x 0.1 / 0.1
  • 1.4 x 10-4 = [H+]
  • pH = -log [H+] = 3.85

Find the pH of a 500mL solution containing 1.00 g propionic acid and 1.0 g sodium propionate  

  • Given:
    • Ka of propionic acid = 1.3 x 10-5
    • propionic acid = HC3H5O2
    • sodium propionate = NaC3H5O2
    • Ka = [H+][common ion] / [acid]
  • [propionic acid] = 1 / (1+36+5+32) / 0.5 = 0.027
  • [sodium propionate] = 1 / (23+36+5+32) / 0.5 = 0.0208
  • 1.3 x 10-5 = [H+] [sodium propionate] / [propionic acid]
  • [H+] = 1.68 x 10-5
  • pH = -log[H+] = 4.77

complex ions - made up of metal ion and Lewis base  

  • stability depends on equilibrium constant for its formation
  • metal ions act as electron-pair acceptors, Lewis acids
  • metal salts can dissolve more easily when metal ion can react w/ the solvent

amphoterism - able to act as acid or base  

  • includes hydroxides, oxides of Al3+, Cr3+, Zn2+, Sn2+
  • contains basic anions >> dissolves in acidic solutions
  • can form complex ions >> dissolves in basic solutions

precipitation/separation of ions - occurs when product of ion concentrations > Ksp  

  • equilibrium = saturation
  • selective precipitation - uses reagent to form a precipitate w/ 1 of the ions
  • insoluble chlorides - formed from Ag+, Hg22+, Pb2+
  • acid-insoluble sulfides - CuS, Bi2S3, CdS, PbS, HgS, As2S3, Sb2S3, SnS2
  • base-insoluble sulfides/hydroxides - formed from Al3+, Cr3+, Fe3+, Zn2+, Ni2+, Co2+, Mn2+
  • insoluble phosphates - formed from Mg2+, Ca2+, Sr2+, Ba2+
  • alkali meta ions - remainder of possible metal ions, can be tested for individually
Subject: 
Chemistry
Subject X2: 
Chemistry

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