Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
potassium bromide : KBr
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu,Br
coordination
Summary:
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Copper (II) ions form a brown complex with bromide ions, but only when the
product of concentration of copper and bromide is high.
In strongly acidic environments, bromide gives a red/brown/purple complex
with cupric ions.
Description:
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Sequence 1:
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Add some solid potassium bromide to a 0.5 M solution of copper sulfate: When
a small amount of KBr is added, then no visible changes can be observed, but
when the amount of KBr is gradually increased then the color of the solution
shifts from sky-blue to green. Around the crystals of KBr the liquid is
red/brown. When more KBr is added, such that it just does not dissolve
anymore, then the resulting liquid is brown.
Add some water to the brown liquid, such that the total volume is increased
by a factor of approximately 4: The liquid becomes green/blue, cyan.
Add more water, such that the volume is doubled another time: The liquid
becomes pale blue, not much different from the colour of a plain dilute
solution, containing aquated copper (II) ions.
Sequence 2:
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Add some water to solid potassium bromide, such that not all of the bromide
dissolves and then add a small amount of copper sulfate: The copper sulfate
only partly dissolves. The liquid becomes dark brown.
Add a lot of water: The liquid becomes light blue.
Sequence 3:
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Dissolve some potassium bromide in a small volume of water and add some
concentrated sulphuric acid: The acid mixes with the liquid and some heat
is formed. The liquid becomes light yellow, almost colorless. The yellow
color probably is due to formation of a small amount of bromine.
Add some solid copper sulfate: The solid partly dissolves and a very dark
red/brown/purple compound is formed. Apparently with strong acids, copper
(II) does not build a pure brown coordination complex with bromide, but
a very dark red/purple compound. Is this a coordination complex of the form
[HCuBr4]- or a similar acid bromo-cuprate?