Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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copper : Cu
hydrochloric acid : HCl
cupric chloride dihydrate : CuCl2 . 2H2O
Class:
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elem=Cu,Cl
coordination
redox
Summary:
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Cupric chloride forms a coordination complex when a large excess of chloride
is present and this chloro cuprate complex easily oxidizes metallic copper.
This oxidation causes formation of copper (I), which forms a very dark
brown complex in the presence of copper (II) and hydrochloric acid (what
is this dark brown compound?).
Description:
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Dissolve some copper (II) chloride in HCl (appr. 30% by weight): The solid
dissolves and the liquid becomes brown/green and fairly dark.
Add an excess amount of copper wire: Around the wire a dark compound is formed.
The wire slowly dissolves and the liquid becomes very dark brown (almost
black) and appr. 1 cm of the liquid looks completely opaque. The liquid,
although very dark, remains clear.
Heat the liquid with copper and wait a while while shaking and keeping air
out: The metallic copper dissolves at a higher rate, due to the heat. The
liquid at first remains very dark, but from a certain point in time, the
liquid becomes lighter again. Finally, the liquid becomes fairly light green/
yellow/brown.
Wait a while (appr. 1 hour): The bottom part of the liquid (where the copper
wire is), is completely colorless, the upper part of the liquid is dark brown/
green. When the liquid is shaken, then it becomes fairly light green/brown.
This experiment can be repeated.
In this experiment, the test tube was capped with a rubber stopper all the
time in order to keep air out.
Remark: Copper (II) reacts with copper metal to form copper (I). At places
where there is only copper (I), a colorless complex is formed with the
hydrochloric acid (probably with the chloride of this acid). At places where
there are both copper (I) and copper (II) a very dark compound is formed.
The copper (I) complex is very susceptible to oxidation by oxygen from air.
When the liquid is left in contact with air, then it darkens at its surface
within a few seconds.