Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sulphuric acid : H2SO4
chrome alum : KCr(SO4)2 . 12H2O
sodium sulfite : Na2SO3
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
coordination
redox
Summary:
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When chromium (III) is created by reduction from dichromate with acidified
sulfite, then a green ion is formed. When acidified sulfite is added to
violet chromium (III), then the ions remain violet.
Apparently the way of creating chromium (III) determines its color (and
hence to what it is coordinated).
Description:
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Sequence 1:
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Add a slight excess amount of a solution of sodium sulfite to a solution of
potassium dichromate, acidified with an excess amount of sulfuric acid: The
resulting liquid becomes bluish/green. A slight odour of SO2 shows that an
excess amount of sulfite was used. This liquid is called liquid I.
Sequence 2:
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Add a fairly large amount of sodium sulfite to a solution of chrome alum,
which was acidified with some sulfuric acid: The color of the liquid does
not change. It remains violet. A strong smell of SO2 is present. This violet
liquid is called liquid II.
Sequence 3:
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Add a fairly large amount of sodium sulfite to a solution of chrome alum,
which is not acidified with sulfuric acid: The liquid turns green and
remains clear. The green color of this liquid differs from that of liquid I.
The bluish shade is not present here. No smell of SO2 is observed.
Add some dilute sulfuric acid (appr. 10% by weight): The liquid remains green,
its color becomes a little bit less pronounced, it becomes a little bit more
greyish. But still, the color is a convincing green. A strong smell of SO2
is observed. This greyish green liquid is called liquid III.
One day later:
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No visible changes to the three liquids. Liquids I and III are treated with
small excess amounts of H2O2 (30%) in order to get rid of the SO2 (which may
influence coordination to chromium (III)).
Two more days later:
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Color of liquids I and III has slightly shifted towards blue, the green color
is less convincing. Comparison was done by preparing fresh solutions I and III.