Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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zinc : Zn
hydrochloric acid : HCl
sulphuric acid : H2SO4
chrome alum : KCr(SO4)2 . 12H2O
Class:
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elem=Cr
coordination
redox
Summary:
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Cr(III) can be reduced to Cr(II) by a zinc nail. On reoxidation to Cr(III)
a coordination complex appears to be created, both with sulfate and with
chloride.
Description:
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Sequence 1:
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Dissolve some chrome alum in dilute H2SO4: The resulting liquid is violet.
Add a zinc nail: Evolution of a gas (H2). The liquid turns green/blue.
After removal of the nail and shaking, the liquid remains green/blue, even
if left in contact with air for a long time.
Remark: It looks as if the zinc nail reduces the chrome (III) to chrome (II),
but that the chrome (II) is oxidized again to chrome (III) by oxygen from
the air. The resulting chrome (III) now is green instead of violet, because
of formation of a coordination complex with the sulphate ions in the liquid.
Sequence 2:
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Dissolve some chrome alum in dilute HCl: The resulting liquid is violet.
Add a zinc nail: Evolution of a gas (H2). The liquid turns dark green.
After removal of the nail and shaking, the liquid remains dark green, even
if left in contact with air for a long time.
Remark: It looks as if the zinc nail reduces the chrome (III) to chrome (II),
but that the chrome (II) is oxidized again to chrome (III) by oxygen from
the air. The resulting chrome (III) now is green instead of violet, because
of formation of a coordination complex with the chloride ions in the liquid.