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Lewis Acids & Bases

Lewis Acids & Bases. Lewis acid = a substance that accepts an electron pair. Lewis base = a substance that donates an electron pair. Reaction of a Lewis Acid and Lewis Base. New bond formed using electron pair from the Lewis base. Coordinate covalent bond Notice geometry change on reaction.

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Lewis Acids & Bases

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  1. Lewis Acids & Bases Lewis acid = a substance that accepts an electron pair Lewis base = a substance that donates an electron pair

  2. Reaction of a Lewis Acid and Lewis Base • New bond formed using electron pair from the Lewis base. • Coordinate covalent bond • Notice geometry change on reaction.

  3. Lewis Acids & Bases Formation of hydronium ion is also an excellent example. • Electron pair of the new O-H bond originates on the Lewis base.

  4. Lewis Acid/Base Reaction

  5. Lewis Acids & Bases Other good examples involve metal ions.

  6. Lewis Acids & Bases The combination of metal ions (Lewis acids) with Lewis bases such as H2O and NH3 ------> COMPLEX IONS

  7. Reaction of NH3 with Cu2+(aq)

  8. Lewis Acids & Bases [Ni(H2O)6]2+ + 6 NH3 ---> [Ni(NH3)6]2+ + DMG See page 924

  9. Lewis Acid-Base Interactions in Biology • The heme group in hemoglobin can interact with O2 and CO. • The Fe ion in hemoglobin is a Lewis acid • O2 and CO can act as Lewis bases Heme group

  10. Lewis Acids & Bases Many complex ions containing water undergo HYDROLYSIS to give acidic solutions. [Cu(H2O)4]2+ + H2O ---> [Cu(H2O)3(OH)]+ + H3O+

  11. Lewis Acids & Bases This explains why water solutions of Fe3+, Al3+, Cu2+, Pb2+, etc. are acidic. This interaction weakens this bond Another H2O pulls this H away as H+

  12. Amphoterism of Al(OH)3

  13. Lewis Acids & Bases This explains AMPHOTERICnature of some metal hydroxides. Al(OH)3(s) + 3 H+ --> Al3+ + 3 H2O Here Al(OH)3 is a Brønsted base. Al(OH)3(s) + OH- --> Al(OH)4- Here Al(OH)3 is a Lewis acid.

  14. Lewis Acids & Bases Formation of complex ions explains why you can dissolve a ppt. by forming a complex ion. AgCl(s) eAg+ + Cl- Ksp = 1.8 x 10-10 Ag+ + 2 NH3 --> Ag(NH3)2+ Kform = 1.6 x 107 ------------------------------------- AgCl(s) + 2 NH3eAg(NH3)2+ + Cl- Knet = __________________

  15. Why? • Why are some compounds acids? • Why are some compounds bases? • Why do acids and bases vary in strength? • Can we predict variations in acidity or basicity?

  16. Figure 17.9 Why is CH3CO2H an Acid? 1. The electro-negativity of the O atoms causes the H attached to O to be highly positive. 2. The O—H bond is highly polar. 3. The H atom of O—H is readily attracted to polar H2O.

  17. Trichloroacetic acid is much stronger acid owing to the high electronegativity of Cl, which withdraws electrons from the rest of the molecule. This makes the O—H bond highly polar. The H of O—H is very positive. Acetic acid Trichloroacetic acid Ka = 1.8 x 10-5 Ka = 0.3

  18. Basicity of Oxoanions NO3- These ions are BASES. They become more and more basic as the negative charge increases. As the charge goes up, they interact more strongly with polar water molecules. CO32- PO43-

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