CHM 1046
General Chemistry II
Dr. Michael Blaber

Acid-Base Equilibria

Lewis Acids and Bases

Recall the Arrhenius description of acids and bases:

In the Bronstead-Lowry description of acids and bases:

A B-L base, therefore, is a compound with an unshared pair of electrons that can from a bond with a proton:

G.N.Lewis thought about acids and bases in terms of donation and acceptance of unshared pairs of electrons:

The description of an acid and a base by Lewis is consistent with the description by Arrhenius, and with the definition by Bronstead-Lowry. However, the Lewis description, a base is not restricted in donating its electrons to a proton, it can donate them to any molecule that can accept them.

Since we are so used to thinking about aqueous solutions and protons as the electron pair acceptor (i.e. acid), any molecule like BF3 that can act as an "acid" according to Lewis' definition is explicitly referred to as a "Lewis acid" (and not just as an "acid").


Hydrolysis by Metal Ions

The solutions of many metal ions exhibit acidic properties

Fe(H2O)63+(aq) ó Fe(H2O)5(OH)2+(aq) + H+(aq)

Ka = 2 x 10-3

Hydrolysis reactions generally increase with increasing ionic charge and decreasing ionic radius of the metal ion

© 2000 Dr. Michael Blaber