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Acids and Bases 227
15-7 Lewis Acids and Bases
Another acid-base theory is the Lewis acid-base theory. According to this theory,
a Lewis acid will accept a pair of electrons and a Lewis base will donate a
pair of electrons. In order to make it easier to see which species is donating
electrons, it is helpful to use Lewis structures for the reactants and if possible
for the products.
The following is an example of a Lewis acid-base reaction.
H (aq) :NH 3(aq) l H–NH 3 (aq)
The hydrogen ion accepts the lone pair of electrons from the ammonia to form
the ammonium ion. The hydrogen ion, because it accepts a pair of electrons, is
the Lewis acid. The ammonia, because it donates a pair of electrons, is the
Lewis base. This reaction is also a Brønsted-Lowry acid-base reaction. This
illustrates that a substance may be an acid or a base by more than one definition.
All Brønsted-Lowry acids are Lewis acids, and all Brønsted-Lowry bases
are Lewis bases. However, the reverse is not necessarily true.
15-8 Utterly Confused About Weak
Acids and Bases
Let’s begin with a series of acid-base equilibria problems.
Calculate the [H ] of a 0.300 M acetic acid solution. K a 1.8 10 5
This problem has many similarities to the last worked example in the Utterly
Confused section in Chapter 14.
We do not have a reaction given, therefore we must write one. The “a” subscript
in the K a tells use the equation must look like this:
HC 2H 3O 2(aq) K H (aq) C 2H 3O 2 (aq)
As always, H (aq) is the same as H 3O (aq).
We need to write the equilibrium constant expression for this reaction:
Ka 1.8 105 [H ][C2H3O2 ]
[HC2H3O2 ]