Acids

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Acids BasesPart III Other Types of Acids
Bases

• C. Yau
• Spring 2009

Based on Brady Senese 5/eChap. 15 Sec 3 4
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Yet a Different Definitions of Acid/Base
A Lewis acid is any ionic or molecular species
that can accept a pair of electrons to form a
coordinate covalent bond. A Lewis base is any
ionic or molecular species that can donate a pair
of electrons in the formation of a coordinate
covalent bond. Neutralization is the formation of
a coordinate covalent bond between e- donor and
the e- acceptor.
Lewis Base Lewis Acid
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Examples of Lewis Acids and Bases

• OH- CO2 HCO3-

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Brønsted acid-base viewed as Lewis acid-base

• H3O NH3 H2O NH4

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Oxides as Acids Bases

• This is something you learned back in Gen Chem I
• Metal oxides water basic solutions
• Na2O (s) H2O (l) 2NaOH (aq)
• Nonmetal oxides water acid solutions
• SO3 (g) H2O (l) H2SO4 (aq)
• N2O5 (g) H2O (l) 2 HNO3 (aq)
• CO2 (g) H2O (l) H2CO3 (aq)

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Metals as Lewis Acids

• When metal ions go into solution, they become
  "hydrated", i.e. they are surrounded by water
  molecules.
• The metal is acting as a Lewis acid.
• M(H2O) H2O M(OH) H3O
• The hydrated metal ion is acting as a Brønsted
  acid.
• Al(H2O)63 H2O Al(H2O)5(OH)2
• 
  H3O

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Acid Strengths of Metal Ions

• The smaller metal ions have a higher electron
  positive charge density ( charge is forced into
  a smaller space).
• The smaller metal ions are more acidic.

ACID STRENGTH
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Acid Strengths of Metal Ions

• Group IA metal ions have only a charge 1 and do
  not act as acids.
• Group IIA metal ions have a higher charge (2)
  but only Be2 is small enough (e- positive charge
  density is high enough) to act as an acid.
• Transition metals, especially ones with 3 (such
  as Fe3 and Cr3) have a tendency to act as an
  acid.

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Effect of Oxidation Number

• We had said that metal oxides in water becomes a
  base. However
• for metal oxides, as the oxidation number
  increases, the tendency of the metal to act as an
  acid increases.
• Al2O3 6H 2Al3 3H2O
• Al2O3 2OH- 2AlO2- H2O
• Al3 is amphoteric.
• Metals with high oxidation numbers becomes
  acidic.
• CrO3 has an oxidation number of 6 and is acidic.
• CrO3 H2O H2CrO4 (a strong acid)

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Lewis Acids and Bases

• Lewis acids
• Molecules ions with incomplete valence shells
• (e.g. BF3, H)
• Molecules ions with multiple bonds that can be
  shifted to accept electrons (e.g. OCO)
• Molecules or ions with central atoms that can
  accommodate additional electrons (SO2 SO32-)
• Lewis bases
• Molecules ions that have complete valence
  shells with unshared electrons (e.g. OH-, NH3)

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Brønsted vs. Lewis Definitions

• Brønsted Acid H donor
• Lewis Acid e-pair acceptor
• Brønsted Base H acceptor
• Lewis Base e-pair donor