 |
 |
5 of 19 |  |
 |
 |
B-5
NOTE:
In the presence of high [NH3,], CuSO
4
(aq) forms the complex ion [Cu(NH3)
4
2+
]
(aq)
. This ion is
soluble in water but precipitates as the sulfate salt when ethyl alcohol is added.
Part III. Reaction Scheme B
A. Copper(II) hydroxide from copper(II) sulfate.
Use a Pasteur pipette to transfer 5 drops of the CuSO
4
solution from Part I to an evaporating dish. To this
CuSO
4
solution, add 3 drops of 2.5 M NaOH and swirl gently to mix the reagents. You should observe
the formation of a precipitate. If no precipitate is formed, continue adding NaOH one drop at a time,
with swirling, until a permanent precipitate forms. Let it sit for a minute. Note the color of the
precipitate. Record your observations. Compare these results with those in Part II. What do you
conclude? Now add 10 additional drops of 2.5 M NaOH to the evaporating dish and swirl. The hydroxide
is now in excess. What happens to the precipitate? Compare this observation with your observations in
Part II, when relatively high concentrations of ammonia were added to the copper(II) sulfate.
B. Copper(II) oxide from copper(II) hydroxide.
Place the evaporating dish containing copper(II) hydroxide from Part III A on a hot plate and heat until the
precipitate turns a dark brown or black. Do not boil the mixture. Each copper(II) hydroxide molecule
loses a molecule of water to form the oxide. Record your observations.
NOTE: If the residue remains blue or green after heating, you have not added sufficient NaOH. Add 4-5
more drops of NaOH, 10 drops of deionized water, and continue to heat. Record your
observations.
Remove the evaporating dish from the hot plate and add 10 drops of 6.0 M H2SO
4
. What happens to the
precipitate? Record and explain your observations.
C. Copper metal from copper(II) sulfate.
Pour the rest of the copper(II) sulfate from Part I into the evaporating dish of
Part III B. Wash the beaker with 1 or 2 mL of deionized water and add the washings to the evaporating
dish. The solution should be slightly acidic at this point. Add 4 mL of
6.0 M H2SO
4
to the solution and fill the evaporating dish with deionized water about two thirds full. Then
add some of the metallic zinc provided into the dish and continue to gently mix the solution until the solution
is colorless. If the zinc becomes heavily coated with copper metal, gently stir the solution with a plastic
NH
3(aq)
+ H2O
(l)
?
NH
4
+
(aq)
+ OH
-
(aq)
