Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
sulphuric acid : H2SO4
cuprous oxide : Cu2O
Class:
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elem=Cl,Cu
redox
Summary:
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Copper (I) oxide, when added to sulphuric acid, disproportionates. Metallic
copper is formed and a blue solution of copper (II) sulfate. The copper (I)
oxide looses its oxide ion to the acid and that would leave aqueous copper (I)
ions. These are not stable and disproportionate at once.
Copper (I) oxide, added to hydrochloric acid dissolves and forms a solution,
containing a copper (I) complex, [CuCl2]-. This complex is very easily oxidized
by oxygen from the air and then a dark brown mixed valency complex of copper
(I) and copper (II) is formed.
Description:
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Sequence 1:
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Add some copper (I) oxide to dilute sulphuric acid (2 mol/l): The solid seems
not to dissolve, but the liquid becomes sky-blue. This indicates formation of
aqueous copper (II) ions. The solid hardly changes color, it changes from red
copper (I) oxide to dull red/brown copper metal.
Decant the blue liquid, rinse the precipitate once with a lot of water, decant
the water and add some concentrated hydrochloric acid: The solid does not
dissolve and the concentrated hydrochloric acid remains colorless. This
indicates that the precipitate is copper metal. Sequence 2 shows that copper
(I) oxide gives rise to formation of dark compounds, due to aerial oxidation by
oxygen from the air.
Sequence 2:
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Add some copper (I) oxide to concentrated hydrochloric acid: The solid fairly
quickly dissolves and the liquid becomes green/brown. On shaking with contact
with air, the liquid quickly becomes darker and darker. All of the solid
finally dissolves.
Pour the dark brown liquid in a lot of water: A compact white crystalline
precipitate is formed. This settles at the bottom quickly. The liquid above the
precipitate is very light blue/green. The white precipitate is copper (I)
chloride.
Decant the clear very light liquid and allow the white precipitate to loose
most of its water as well and then heat the precipitate to dryness: First the
precipitate becomes very dark brown/black, but on further drying, it becomes
light brown/yellow. On cooling down, it becomes light brown.
Add water to the light brown material: The solid becomes purely white again.
Theory about sequence 2:
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The white precipitate in sequence 2 is copper (I) chloride, probably
contaminated with some copper (II) chloride and acid remains. On heating, the
copper (II) compounds, together with the copper (I) chloride, may form the dark
brown/black material as a mixed valency complex. On heating to total dryness,
the lighter brown color most likely is due to the presence of small amounts of
anhydrous copper (II) chloride, which does not form a very dark mixed valency
copper complex under the totally dry conditions.