Chemistry Reference
In-Depth Information
Lewis Acid-Base Theory
The final acid-base theory that we shall consider was proposed by chem-
ist Gilbert Lewis in the early 1920s. The Lewis Theory is the most general,
including more substances under its definitions than the other theories of
acids and bases. A Lewis acid is a substance that accepts a pair of electrons
to form a covalent bond. A Lewis base is a substance that provides a pair of
electrons to form a covalent bond. In order for a substance to act as a
Lewis base, it must have a pair of unshared electrons in its valence shell.
An example of this is seen when a hydrogen ion attaches to the unpaired
electrons of oxygen in a water molecule, as shown here:
H
+
Acting as a Lewis Acid and H
2
O Acting as a Lewis Base
H
+
+ H
2
O
H
3
O
+
The bond formed between the hydrogen ion and the water, in this ex-
ample, is a coordinate covalent bond. A coordinate covalent bond is formed
between two atoms, where one atom provides both of the electrons that
become shared. In the Lewis Acid-Base Theory, all interactions between
acids and bases will involve coordinate covalent bonding.
Lewis Acids and Bases
Lewis Acid
A substance that accepts a pair of electrons to form
a covalent bond. Examples include H
+
and BF
3
.
Lewis Base
A substance that donates a pair of electrons to form
a covalent bond. Examples include Cl
-
, NH
3
, and
H
2
O.
Lesson 9-4 Review
1.
A(n) _______________ acid is a substance that donates a proton to
another substance.
2.
A(n) _______________ substance acts as an acid in some situations
and a base in other situations.
3.
A _______________ base is the substance that is left over, after an
acid donates a proton.
4.
Write the conjugate base of each of the following Brønsted-Lowry
acids.
a) HI
b) H
3
PO
4
c) H
3
d) H
2
O
