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Chapter 11 PowerPoint Presentation

Chapter 11

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Chapter 11

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  1. Chapter 11 Polyprotic Acid and Bases

  2. Diprotic Acids • Compounds with two acid/base groups • Can be two acids groups • Oxalic Acid • Can be two basic groups • Cadaverine • Or and acid and base group • Amino acids

  3. Oxalic Acid

  4. Cadaverine

  5. Generic Amino Acid

  6. Amino Acids • H2L+ = HL = L2- • Glycine.HCl CAS 6000-43-7 • Glycine CAS 56-40-6 • Sodium Glycinate CAS 6000-44-8

  7. Ionic Structures • Glycine HCl

  8. Sodium Glycinate

  9. Zwitterion form Intermediate Form

  10. Constants • H2L+ = HL = L- • Ka1 Ka2 • Kb2 Kb1 • Ka1 * Kb2 = Kw • or • Ka2 * Kb1 = Kw

  11. How do we then calculate the pH’s • Acid Form H2L+ • Treat was a weak monoprotic acid. • Base Form L- • Treat as a weak monoprotic base

  12. Intermediate Form pH • These compounds can either gain a proton (work as a base) or they can donate a proton (work as an acid). • The magnitude of the K values will determine the pH. • The following equation can be derived …

  13. Intermediate Form

  14. Approximations • In many cases the K1Kw term will be much smaller than the other term in the numerator so the equation becomes: • Then often K1 will be much smaller than [HL] so we end up with:

  15. Intermediate Form • Which when taken to the log form gives us. • pH = (pKa1 + pKa2)/2

  16. Diprotic Buffer Systems • Same treatment as with monoprotic systems but we use the pKa that is between the two major forms that are in the solution.

  17. Buffers • Phosphoric acid system • H3PO4 and H2PO4- use pKa1 • H2PO4- and HPO42- use pKa2 • HPO42- and PO43- use pKa3

  18. Principle Species • Monoprotic

  19. Principle Species • Diprotic • Fumaric acid

  20. Principle Species