Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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copper sulfate penta hydrate : CuSO4 . 5H2O
copper : Cu
hydrochloric acid : HCl
hydrogen peroxide : H2O2
Class:
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elem=Cu
coordination
redox
Summary:
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Copper (II) reacts with metallic copper, in the presence of a large amount
of hydrochloric acid, forming an intensely colored compound. This compound
contains copper (I). If copper (II) is used alone, with hydrochloric acid
and peroxide, then the strong coloration does not occur.
Description:
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Sequence 1:
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Add some hydrochloric acid (30% by weight) to hydrogen peroxide (appr. 27%)
at a ratio of 3 : 1: The liquid becomes pale green and has a strong smell
of chlorine.
Add some copper metal: The metal dissolves, while a lot of gas is evolved.
The liquid turns dark green.
Add much more copper: Again the metal dissolves, while a lot of gas is
evolved. As long as a gas is evolved, the liquid remains dark green. At
a certain point in time, the evolution of gas stops and dissolving of the
copper goes much slower, but it still dissolves. From this point in time,
the color of the liquid shifts from dark green to dark brown.
Sequence 2:
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Add some solid copper sulfate to a mixture of HCl (30%) and H2O2 (27%): The
solid dissolves. The liquid becomes yellow/green. A gas is evolved slowly.
No brown compound is formed now.
Sequence 3:
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Add some solid copper sulfate to HCl (30% by weight): The copper sulfate
is covered by a green solid and dissolves slowly (much slower than in
plain water). If all copper sulfate is dissolved, then the liquid has a
fairly intense green color (much more intense than the blue of a similir
amount of copper sulfate dissolved in a similar amount of water).
Add some copper metal: At first sight, nothing special seems to happen.
However, after 5 minutes the liquid has become brown/green. After a much
longer time, the liquid has become entirely brown.
Sequence 4:
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Add some HCl (30% by weight) to a fairly large amount of copper: The copper
metal looses its shiny appearance and becomes dull orange. No further
changes.
Add some H2O2 (27%), but do this carefully, in small amounts and let react
after every small amount, before a new amount is added: The first time, when
H2O2 is added, the copper dissolves, while a gas is evolved. The copper
metal dissolves as a green compound. As soon as evolution of gas ceases,
the dissolving of copper continues, but now the liquid becomes brown. If
a little amount of H2O2 is again added to the brown liquid, it immediately
becomes green again, and copper dissolves as a green compound and a gas is
evolved again. After a few times of adding more H2O2, the dissolving of
copper stops and adding more H2O2 only results in a lot of fizzling of
the liquid (decomposition of H2O2), without dissolving copper metal.
If some more HCl (30%) is added, then the experiment can be repeated again
with small amounts of H2O2.
Add a 20-fold volume of water to the remaining dark brown liquid: A large
amount of a white crystalline solid precipitates, the liquid above the
crystalline precipitate becomes blue/green.
Decant the blue/green liquid and remove the remaining copper metal and add
a large amount of water to the white precipitate: The white solid does not
dissolve in the water. The liquid above the white solid is almost colorless,
just a very light blue can be observed.
Decant the water above the liquid again and add new water: The white solid
becomes pale yellow.
Decant the water again and add some HCl (30%) to the yellow solid: The solid
dissolves and the liquid becomes yellow/brown. In the course of time, the
liquid becomes darker and darker (it is in contact with air).
Add an excess amount of NH3 (15% by weight) to the brown liquid: The liquid
becomes deep blue, no precipitate is formed. Above the liquid a white smoke
of NH4Cl is formed.
Add a solution of NaOH to a part of the white crystalline precipitate: The
solid becomes bright yellow, it does not dissolve.
Remarks:
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Copper (II) builds a complex with chloride in very concentrated solutions.
This complex is green [CuCl4]2-, which may look yellowish at lower
concentrations. When copper metal dissolves in a liquid containing copper (II)
in the presence of large concentrations of chloride, then a dark brown
complex is formed, which contains copper (I).
When copper is added to a concentrated HCl/H2O2 mixture, then the H2O2 oxidizes
copper metal to copper (II), resulting in a green liquid. When all H2O2 is
used up, then the copper (II) in the liquid further oxidizes any remaining
copper metal. This third step is shown clearly by sequence 3. Sequence 2
shows that the brown compound is not formed, due to the peroxide.
Does the brown complex also contain copper (II)? What is the exact
constitution of the brown complex?
The white crystalline solid, which precipitates on addition of water, is
copper (I) chloride, CuCl. This white compound does not dissolve in water,
but it dissolves in concentrated HCl as a colorless complex. On standing,
this colorless liquid absorbs oxygen from the air, causing the formation
of a brown complex with copper (II). This sensitivity to air makes it very
hard to obtain a colorless copper (I) solution in hydrochloric acid.
The yellow solid is hydrous copper (I) oxide or copper (I) hydroxide.