Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
hydrochloric acid : HCl
hydroquinone : HO C6H4 OH
sodium sulfite : Na2SO3
sodium hydroxide : NaOH
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
coordination
redox
Summary:
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Hydroquinone is capable of reducing copper (II) to copper (I) in alkaline
environments. The copper is not reduced to its metallic form.
Description:
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Sequence 1
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Add a dilute solution of copper sulfate to a solution of NaOH, Na2SO3 and
hydroquinone (stabilized hydroquinone in alkaline environment): The liquid
turns yellow/green and contains a precipitate in the form of little flakes
(solid copper hydroxide).
Heat the liquid, containing the precipitate to appr. 60 C: The liquid turns
orange and becomes turbid. After appr. 1 day the liquid still is orange and
an orange solid layer has settled on the bottom of the test tube.
Decant the orange liquid and add some dilute HCl (appr. 3% by weight) to the
remaining solid precipitate: The precipitate turns white, the liquid remains
turbid. The orange precipitate most likely was Cu2O.
Add some H2O2 (3% by weight): Liquid immediately becomes clear. The color of
the liquid becomes pale yellow/green. Probably this color is the result of
a mix of pale blue of copper (II) and remnants of the orange oxidation
product of hydroquinone.
Sequence 2
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Add a dilute solution of copper sulfate to a solution, containing sodium
hydroxide and sodium sulfite: The liquid becomes pale blue and a little
turbid. Many little flakes float around through the liquid.
Heat to appr. 60 C: No visible changes.
Add some solid hydroquinone: The solid dissolves. The liquid immediately
turns yellow/green. Around the crystals of hydroquinone a dark green compound
is formed.
Reheat again (liquid cooled down in the meantime): The liquid turns orange
and an orange precipitate is formed.
Add a mixture of HCl and H2O2 (both appr. 3% by weight before mixing): The
orange precipitate quickly dissolves and the liquid becomes clear.
Remark: Hydroquinone builds a yellow/green complex with copper (II) in the
cold. When heated moderately, the copper (II) is reduced to copper (I),
resulting in the formation of an orange precipitate of Cu2O.