Description of experiment
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Needed compounds:
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copper : Cu
potassium bromide : KBr
sulphuric acid : H2SO4
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu,Br
coordination
redox
Summary:
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Copper (II) forms a very dark coordination compound with concentrated
bromide in highly acidic environments. When metallic copper is added,
then this complex quickly disappears and the liquid almost becomes
colorless.
Description:
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Dissolve quite a lot of KBr in dilute H2SO4 (20% by weight), such that the
solution is close to saturated in KBr. To this solution add some solid
CuSO4.5H2O: The blue solid quickly dissolves and the liquid becomes very
dark purple/brown. An intensely dark colored compound is formed,
Add an excess amount of copper wire (relative to the copper sulfate): The
liquid quickly becomes lighter and after approximately 15 minutes it is
almost colorless. When the liquid is allowed to come in contact with air,
then surprisingly, it does not become dark again.
Add a single drop of H2O2 (30%) to the liquid, while the solid copper still
is present as well: The liquid becomes dark purple/brown again at once. A
very slow evolution of gas can be observed. After the initial formation of
the dark brown compound, the liquid becomes lighter again on shaking. After
one day, the liquid is colorless again. The copper wire is covered by a thin
dull white layer. At the bottom, some light blue crystalline solid has
formed.
Remarks:
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The dark purple/brown compound is expected to be a complex of copper (II),
bromide and possibly H+. The colorless compound probably is a copper (I)
complex with bromide. This complex is not very sensitive to oxidation by
air.
The behavior with bromide is quite different from the behavior with chloride.
With chloride, a colorless [CuCl2]- complex anion can be formed, but this
is very sensitive to air-oxidation. On contact with air, such solutions
become very dark quickly, probably forming a complex of copper (I), copper (II)
and chloride.