Chemistry Reference
In-Depth Information
Metal carbonates contain carbonate ions, CO
3
2−
.
In this reaction the carbonate ions react with the
hydrogen ions in the acid.
carbonate + hydrogen→ carbon + water
ions ions dioxide
CO
3
2−
(
aq
) + 2H
+
(
aq
) → CO
2
(
g
) + H
2
O(
l
)
Acid
+
alkali (soluble base)
This method is generally used for preparing the
salts of very reactive metals, such as potassium or
sodium. It would certainly be too dangerous to
add the metal directly to the acid. In this case, we
solve the problem indirectly and use an alkali which
contains the particular reactive metal whose salt we
wish to prepare.
Metal oxides are
basic
. Metal oxides and
hydroxides that dissolve in water to produce OH
-
(
aq
)
ions are known as
alkalis
, or
soluble bases
. If the
metal oxide or hydroxide does not dissolve in water it
is known as an
insoluble base
.
A
base
is a substance which neutralises an acid,
producing a salt and water as the only products. If
the base is soluble the term alkali can be used, but
there are several bases which are insoluble. It is also a
substance which accepts a hydrogen ion (see p. 119).
In general, most metal oxides and hydroxides
(as well as ammonia solution) are bases. Some
examples of soluble and insoluble bases are shown in
Table 8.4. Salts can be formed by this method only if
the base is soluble.
Figure 8.13
The acid is added to the alkali until the indicator just
changes colour.
The solution which is produced can then be
evaporated slowly to obtain the salt. For example,
hydrochloric + sodium → sodium
+ water
acid hydroxide chloride
HCl(
aq
) + NaOH(
aq
) → NaCl(
aq
) + H
2
O(
l
)
As previously discussed on p. 116, this reaction can
best be described by the ionic equation:
H
+
(
aq
) + OH
−
(
aq
) → H
2
O(
l
)
Acid
+
insoluble base
This method can be used to prepare a salt of an
unreactive metal, such as lead or copper. In these
cases it is not possible to use a direct reaction of the
metal with an acid so the acid is neutralised using the
particular metal oxide (Figure 8.14).
Table 8.4
Examples of soluble and insoluble bases.
Soluble bases (alkalis)
Insoluble bases
Sodium hydroxide (NaOH)
Iron(
iii
) oxide (Fe
2
O
3
)
Potassium hydroxide (KOH)
Copper(
ii
) oxide (CuO)
Calcium hydroxide (Ca(OH)
2
)
Lead(
ii
) oxide (PbO)
Ammonia solution (NH
3
(
aq
))
Magnesium oxide (MgO)
Because in this neutralisation reaction both
reactants are in solution, a special technique called
titration
is required. Acid is slowly and carefully
added to a measured volume of alkali using a
burette (Figure 8.13) until the indicator, usually
phenolphthalein, changes colour.
An indicator is used to show when the alkali has
been neutralised completely by the acid. This is called
the
end-point
. Once you know where the end-
point is, you can add the same volume of acid to the
measured volume of alkali but this time without the
indicator.
Figure 8.14
Citric acid has been used on the right-hand side of this piece
of copper metal to remove the oxide coating on its surface, giving it a
