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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. 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
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
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.
lewis acids bases1
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.
slide5

Lewis Acids & Bases

Other good examples involve metal ions.

slide6

Lewis Acids & Bases

The combination of metal ions (Lewis acids) with Lewis bases such as H2O and NH3

------> COMPLEX IONS

slide8

Lewis Acids & Bases

[Ni(H2O)6]2+ + 6 NH3 ---> [Ni(NH3)6]2+

+ DMG

See page 924

lewis acid base interactions in biology
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

slide10

Lewis Acids & Bases

Many complex ions containing water undergo HYDROLYSIS to give acidic solutions.

[Cu(H2O)4]2+ + H2O ---> [Cu(H2O)3(OH)]+ + H3O+

slide11

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+

slide13

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.

slide14

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 = __________________

slide15
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?
why is ch 3 co 2 h an acid

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.

slide17
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

basicity of oxoanions
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-