Lec26 2007acidbasefour by fDQ7Zq

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									                            Fred J. Grieman




Lec. 26 Indicators
        Polyprotic Acids
        Lewis Acids/Bases

HW: 8: 64, 68, 69, 72
          Indication of end point  equivalence point
• pH Meter: Measure pH as a function of Volume of Titrant
          Plot titration curve and graphical determine endpoint

• Use an indicator: weak organic acid that changes color
                       over small pH range
      HIn(aq) + H2O(ℓ) ⇄ H3O+(aq) + In-(aq)
• Where it changes color depends on Ka(HIn):
                    [H3O+] [HIn]
                    ——— = ———
                     Ka     [In-]

     Ratio dictates dominate form
Phenophthalein used in Chem 1a Lab




              The color change in phenolpthalein is due
               to a change in structure of the molecule.

         In acid, the molecule is in its HIn form containing a
        central 5 membered ring which is somewhat strained.
                     Clear (absorbs in ultraviolet)

        In base the In- structure opens up and becomes flatter.
                     Pink (absorbs in the visible)

                            Ka = 8 x 10-10
Phenolphthalein Ka = 8 x 10-10  10-9; at pH = 9
[H3O+] 10-9 [HIn]
——— = ——— = ——— = 1; Pink begins to show
  Ka   10-9  [In-]
                                        Why did we select this Indicator?
                                  pH at endpoint of our rxn? Hydrogen Phthalate
                                        [HP-][OH-]
                                  Kb = ─────── = 2.56 x 10-9
                                          [P2-]
                                    P2- +    H2O   →      HP- + OH-
                                  [P2-]-x                 x      x at equil.

                                  [P2-] = .030 M; Calculate pH at endpoint.

                                     x2
                                  ───── = 2.56 x 10-9 ;   x = 8.76 x 10-6 = [OH-]
                                  .030 – x

                                       pOH = 5.063 ; pH = 14 - 5.063 = 8.937


      Polyprotic acids on board
          Lewis acid / base interaction

    Ligand → L            M ← positive center


     Lewis base                    Lewis acid
(electron pair donor)           (electron pair acceptor)

     Coordinate covalent bond (e- pair from ligand only)

     H3N: +       H+    → NH4+

     But also,         H3N: +     Co2+    → Co(NH3)2+

       6 times:    6 H3N: +        Co2+    → Co(NH3 )62+

								
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