Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sodium metabisulfite : Na2S2O5
sodium thiosulfate : Na2S2O3 . 5H2O
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr
coordination
Summary:
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Chromium (III) builds a green coordination complex with metabisulfite,
not with thiosulfate.
When hydrogen peroxide is added, then the complex with metabisulfite is
destroyed.
Description:
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Sequence 1:
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Add a large excess amount of sodium thiosulfate to a solution of chrome alum:
The sodium thiosulfate dissolves, the liquid remains violet. After appr.
half a minute the liquid becomes slightly turbid (probably due to decomposition
of the thiosulfate, with finely dispersed sulphur in the liquid). The color
of the liquid remains violet. The violet color seems to become a little more
intense, but this may also be due to darkening of the liquid, because of its
turbidity.
Sequence 2:
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Add a little amount of sodium metabisulfite to a solution of sodium
thiosulfate: The sodium metabisulfite dissolves, the liquid remains clear.
Add some solid chrome alum: The first chrome alum, which dissolves, causes
a green coloration of the liquid. Further chrome alum, which dissolves, seems
to cause a violet coloration of the liquid. Finally, when all chrome alum is
dissolved, the liquid has a greyish green color, not a bright convincing
green. The liquid remains clear now. The metabisulfite has a stabilizing
effect on the thiosulfate.
Sequence 3:
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Add a large amount of solid sodium metabisulfite and some chrome alum to
water: Both solids dissolve, the metabisulfite much faster than the chrome
alum. The chrome alum dissolves as a bright bluish green compound.
Add an excess amount of hydrogen peroxide (30% by weight): Almost immediately
the liquid becomes violet again, just as a plain solution of chrome alum.
A noticeable amount of heat is generated, due to the reaction between the
hydrogen peroxide and the metabisulfite.
Sequence 4:
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Add an excess amount of Na2S2O5 to a solution of chrome alum: The violet
liquid turns green as the metabisulfite dissolves.
Conclusions:
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Hexaaqua chrome (III) forms a new coordination compound with metabisulfite
in aqueous solution. This compound is destroyed on oxidation of the sulfite.
Thiosulfate does not form a new coordination compound with chrome (III).