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B-6
Now add 10 drops of 6.0 M NH3 to the mixture on the watch glass. This creates a relatively high
concentration of NH3 in solution
.
What happens to the precipitate? Record your observations. Now add
2-3 mL of ethyl alcohol from a squeeze bottle to the solution on the watch glass, swirl, and record your
observations. The solid compound formed in alcohol is tetraamminecopper(II) sulfate monohydrate.
Dispose of the precipitate into the toxic waste container in the fumehood.
NOTE:
In the presence of high concentrations of
NH3, copper(II) sulfate forms the complex ion
[Cu(NH3)
4
2+
]
(aq)
. This ion is soluble in water but will be precipitated as the sulfate salt in this
experiment by adding ethyl alcohol to the aqueous solution.
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 5 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 5 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.
