Lewis acid = a substance that accepts an electron pair

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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.

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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.

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Lewis Acid/Base Reaction
5

• Other good examples involve metal ions.

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• The combination of metal ions (Lewis acids) with
  Lewis bases such as H2O and NH3
• ------gt COMPLEX IONS

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Reaction of NH3 with Cu2(aq)
8

• Ni(H2O)62 6 NH3 ---gt Ni(NH3)62

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

• Many complex ions containing water undergo
  HYDROLYSIS to give acidic solutions.
• Cu(H2O)42 H2O ---gt Cu(H2O)3(OH) H3O

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• 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

• This explains AMPHOTERIC nature of some metal
  hydroxides.
• Al(OH)3(s) 3 H --gt Al3 3 H2O
• Here Al(OH)3 is a Brønsted base.
• Al(OH)3(s) OH- --gt Al(OH)4-
• Here Al(OH)3 is a Lewis acid.

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• Formation of complex ions explains why you can
  dissolve a ppt. by forming a complex ion.
• AgCl(s) e Ag Cl- Ksp 1.8 x 10-10
• Ag 2 NH3 --gt Ag(NH3)2 Kform 1.6 x
  107
• -------------------------------------
• AgCl(s) 2 NH3 e Ag(NH3)2 Cl-
• Knet __________________

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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?

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Why is CH3CO2H an Acid?
Figure 17.9

• 1. The electro-negativity of the O atoms causes
  the H attached to O to be highly positive.
• 2. The OH bond is highly polar.
• 3. The H atom of OH is readily attracted to
  polar H2O.

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Acetic acid
Trichloroacetic acid
Ka 1.8 x 10-5
Ka 0.3

• 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 OH bond highly polar.
  The H of OH is very positive.

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Basicity of Oxoanions
NO3-
CO32-
PO43-

• 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.