123 16 Acid Base Part III Other Acids Bases Sp 2012 by HC121003183534


									Acids & Bases
Part III:
 Other Types of Acids & Bases
    Jespersen Chapter 16 Sec 4 & 5

              Dr. C. Yau
             Spring 2012
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.     H      F
                     H N:      B   F
                        H      F
                 Lewis Base    Lewis Acid        2
Examples of Lewis Acids and Bases

   OH - + CO                    HCO3 -

                                H ..     _
   ..     ..  ..                  O:
                               ..  ..
 H O:     O C O                O C O:
   ..     ..  ..               ..  ..
e- pair   e- pair
donor     acceptor
 base      acid
 Which is the Lewis acid? Lewis base?
 How are these acids/bases defined??
  Examples of Lewis Acids and Bases
 SO2 (g) + CaO (s)               CaSO3 (s)
 SO2 (g) +   O2-                   SO32-

 ..                                              ..
:O                                              :O :
             .. 2-                                         ..       _
   :S       :O:
             ..                                 :S         O:
                                           ..              ..
 ..                                        O

Do Practice Exercise 18 & 19 on p. 758
Be sure to practice putting in arrows to show how
electron pairs flow in the reaction.
 Brønsted acid-base viewed as Lewis acid-base
  H3O+ + NH3                 H2O       + NH4+
  To figure out which is the Lewis acid & which
  is the Lewis base, draw the Lewis structure
  and put in arrows to show the flow of
  electrons in the rxn.

Note that this is a transfer of a Lewis acid
between two Lewis bases. How?                     5
    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)
        Metals as Lewis Acids
  When metal ions go into solution, they
     become "hydrated", i.e. they are surrounded
     by water molecules.
  The metal ion is acting as a Lewis acid.
  Mn+ + H2O             M(H2O)n+
  The hydrated metal ion is acting as a Brønsted
  Al(H2O)63+ + H2O         Al(H2O)5(OH)2+
(It donated a                          + H 3O +
proton to water.)                             7
      Metals as Lewis Acids
Practice writing the rxn of metal ions as
  Lewis acids:
Fe3+ (aq) + 6 H2O (l)        ?

Hint: As a Lewis acid, it accepts electrons.
Now, write the rxn of the hydrated metal ion
  as a Brønsted acid.

Hint: As an Brønsted acid it is a proton
  Acid Strengths of Metal Ions
The smaller metal ions have a higher
 positive charge density (+ charge is forced
 into a smaller space).
The smaller metal ions are more acidic. Why?

  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
  (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.
     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        (Rxn 1)
Al2O3 + 2OH-            2AlO2- + H2O        (Rxn 2)
Al3+ is amphoteric.
(It is acting as base in Rxn 1, as acid in Rxn 2)
Metals with high oxidation numbers becomes acidic.
CrO3 has an oxidation number of +6 and is acidic.
CrO3 + H2O            H2CrO4 (a strong acid)
      Lewis Acids and Bases
• Lewis acids     What do they do?
  – Molecules & ions with incomplete valence shells
    (e.g. BF3, H+)
  – Molecules & ions with multiple bonds that can be
    shifted to accept electrons (e.g. O=C=O)
  – 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)

Practice Exercise 18 p. 758
Identify the Lewis acids & bases in each
  aqueous rxn.
Hint: Draw Lewis structures of the reactants.

a) NH3 + H+              NH4+

b) (CH3)2O + BCl3             (CH3)2OBCl3

c) Ag+ + 2NH3                     Ag(NH3)22+

Do Practice Exercise 19 as well                 13
 Brønsted vs. Lewis Definitions
Brønsted Acid: H+ donor (proton donor)
Lewis Acid : e- pair acceptor

Brønsted Base: H+ acceptor
               (proton acceptor)
Lewis Base:    e- pair donor
                           +          _
   ..        ..      H         ..
 H N H   H   O H
             ..    H N H       :O H
   H                 H

To top