Description of experiment
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Needed compounds:
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sodium persulfate : Na2S2O8
hydrogen peroxide : H2O2
hydrochloric acid : HCl
copper : Cu
Class:
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elem=Cu
precipitation
coordination
redox
Summary:
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Copper is oxidized by a mix of concentrated hydrochloric acid and
hydrogen peroxide. When the peroxide is used up and there is still an
excess amount of hydrochloric acid, then the copper (II) appears to
oxidize the copper metal, under the formation of an intensely colored
complex. (What is the constitution of this complex??)
When the solution is diluted with water, then the intensely colored
complex is destroyed and a white crystalline precipitate of CuCl is
formed. If too much water is used, then no clear precipitate is formed.
Description:
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Mix approximate equal amounts of H2O2 (27% by weight) with HCl (appr. 30%
by weight) and add a large amount of copper wires: The copper dissolves,
while a gas is evolved. The reaction is fairly vigorous. While the copper
dissolves and the gas is evolved, the liquid turns green. Finally evolution
of gas ceases and at that point the liquid is dark green. After the
evolution of gas has ceased, still more copper dissolves, albeit much
slower. The liquid turns deep brown. After half an hour no more copper
dissolves (or it dissolves extremely slowly).
Sequence 1
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Add a large amount of water to a little quantity of the dark brown liquid:
The liquid becomes light blue with many little flakes of a precipitate
floating around in the liquid.
Add some H2O2 (3% by weight) to the light blue liquid with precipitate: The
liquid turns brown and some gas is evolved. On closer examination it looks
as if the liquid is pale blue, with a finely divided brown precipitate
floating around in it.
Heat the liquid with the brown precipitate: A lot of gas is produced. The
liquid turns pale blue and becomes turbid. After some time a pale
blue/green solid sinks to the bottom.
Let cool down and add some more H2O2 (3% by weight): No visible reaction.
Add a small amount of HCl (10% by weight): The liquid turns pale blue and
becomes clear.
Sequence 2
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Add 4 parts of water to 1 part of the dark brown liquid: Formation of a
green liquid (like moss) which initially is almost clear. It quickly
becomes turbid and a white crystalline precipitate is formed. The precipitate
quickly goes to the bottom of the container. The liquid changes color
from moss-green to blue/green. The white precipitate is solid CuCl.
Add some solid Na2S2O8: This dissolves and the liquid becomes blue. The
white precipitate dissolves. The liquid becomes almost clear.
Sequence 3
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Add some solid Na2S2O8 to a part of the dark brown liquid: The liquid turns
dark green.
Add a large amount of water: The liquid turns sky blue and is almost
clear (just a faint turbidity).