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B-2
It is important to keep the acid-base equilibrium NH3(aq) + H2O(l)
NH
4
+
(aq) + OH
-
(aq) in your mind
when doing this experiment. At low concentrations of ammonia, the equilibrium favours the the side with
the most ions (product side in this equilibrium). The OH
-
(aq)
produced by the ammonia-water equilibrium
will react with the Cu
2+
ion to form Cu(OH)2 precipitate. At higher ammonia concentrations, the
equilibrium favours the reactants and hence, the ammonia preferentially reacts, forming strong bonds with
Cu
2+
in solution producing the complex ion, [Cu(NH3)
4
]
2+
(aq)
. In general, any hydrolytically-stable complex
that has a charge associated with it (positive or negative) tends to be quite soluble in polar solvents, such
as water and tends to be less soluble as the polarity of the solvent decreases (i.e. less soluble in solvents
such as ethanol). Thus, ethyl alcohol will be used to precipitate the tetraamminecopper(II) sulfate
monohydrate. The general properties of complex ions will be further studied later in the course and will
not concern you here.
By following Scheme B, you will first prepare copper(II) hydroxide, Cu(OH)2, from copper(II) sulfate by
addition of NaOH. Since you will be adding a strong base to a solution that contains some sulfuric acid,
the addition must be done slowly. A lot of heat will be evolved in the neutralization reaction if H2SO
4
is
present, and some splattering may occur. To avoid this, add NaOH drop wise and slowly.
Copper(II) oxide (CuO) is prepared by heating copper(II) hydroxide. Copper(II) hydroxide loses water
very easily at high temperature to form copper(II) oxide, even in the presence of a large excess of water.
Once the copper(II) oxide is formed, it is difficult to get back the Cu(OH)2. However, copper(II) oxide
may be treated with H2SO
4
to reform copper(II) sulfate. Again, to avoid excess heat generation through
the neutralization reaction, you must use care in adding H2SO
4
as the CuO solution may contain some
excess NaOH.
The recovery of the copper metal is accomplished by adding a more reactive metal than copper metal to
the CuSO
4
solution. One such metal is zinc. An oxidation-reduction reaction will take place, which
converts zinc metal to Zn
2+
(aq)
, while Cu
2+
(aq)
is converted to copper metal.
Zn
(s)
+ Cu
2+
(aq)
Zn
2+
(aq)
+ Cu
(s)
The net effect is to precipitate out the metallic copper, which is then recovered by vacuum filtration.
Since Cu
2+
(aq)
is blue in aqueous solution and Zn
2+
is colorless, the color of the solution changes from blue
to colorless as the reaction proceeds. When all of the Cu
2+
(aq)
is reduced to Cu
(s)
, the solution will be
colorless. Note that zinc will also react with H2SO
4 (aq)
to form hydrogen gas. Therefore, in the reaction
solution, you should observe the formation of copper solid and hydrogen gas.
