Chemistry Reference
In-Depth Information
8.4
Acids and Bases
There is another common way to classify chemical reactions: acid-base reac-
tions, oxidation-reduction reactions, and reactions of more complicated types
(beyond the scope of this topic). Acid-base reactions are considered to involve
the reactions of hydrogen ions with hydroxide ions. The reactions of acids and
bases will be taken up in this section, and a more sophisticated view of these
reactions is presented in Chapter 19. Oxidation-reduction reactions involve the
transfer of electrons from one substance to another. Many combination reac-
tions, many decomposition reactions, all single substitution reactions, and all
combustion reactions are of this type, but more complex examples are presented
in Chapters 16 and 17.
The reactions of acids with active metals fit into the single substitution class
discussed in Section 8.3. Reactions of acids with bases are double substitution
reactions, also discussed in Section 8.3. However, the reactions of acids and
bases are so important that they have a special terminology that we need to
know.
According to the most fundamental theory concerning acids and bases—
the
Arrhenius theory
—an
acid
is a compound that furnishes hydrogen ions,
to an aqueous solution, and a
base
is a compound that furnishes hydrox-
ide ions,
H
,
OH
,
to an aqueous solution. The hydrogen ion does not exist alone,
H
,
H
3
O
,
as
but is stable in aqueous solution in the form
which is frequently
H
(aq).
represented as
In beginning courses, formulas for acids (and no other compounds except
water and hydrogen peroxide) are written with the ionizable hydrogen atoms
first, as in HCl.
H
2
O
HCl(g)
⎯→
H
(aq)
Cl
(aq)
Methane, ammonia, and sucrose (table sugar), are exam-
ples of compounds that are not acids because they do not provide hydrogen ions
to aqueous solutions. Their hydrogen atoms are therefore not written first in their
formulas. For certain acids, such as acetic acid, and citric acid,
only the hydrogen atom(s) written first is (are) capable of being
ionized; the other hydrogen atoms do not yield
CH
4
,
NH
3
,
C
12
H
22
O
11
,
HC
2
H
3
O
2
,
H
3
C
6
H
5
O
7
,
H
in solution.
Properties of Acids and Bases
Acids in general have a sour taste, turn indicators (Section 11.3) certain colors, and
react with bases to form salts. For example, the sour taste of lemon is the taste of
citric acid, and the sour taste of vinegar is due mainly to acetic acid, its principal
acid component. Simple acids have one or more hydrogen atoms per molecule.
Bases feel slippery, turn indicators certain colors that differ from those acids
produce, and react with acids to form salts. We may experience the slipperiness
of a base by putting our fingertips in some dilute ammonia water. Simple bases
contain one or more hydroxide ions or are able to react with water to some
extent to form hydroxide ions. For example, ammonia is a base because of the
following reaction:
Caution: Do not taste chemicals
unless specifically directed to do
so by your instructor.
Caution: Never touch concen-
trated solutions of strong bases,
such as liquid Drano, because
they are capable of dissolving
the fat in the skin.
H
2
O(
/
) £ NH
4
(aq)
OH
(aq)
NH
3
(aq)
(0.1% to 2%)
