Description of experiment
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Needed compounds:
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potassium dichromate : K2Cr2O7
hydrogen peroxide : H2O2
hydrochloric acid : HCl
hydroquinone : HO C6H4 OH
Class:
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elem=C
redox
Summary:
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When hydroquinone is oxidized by hydrogen peroxide in acidic environments,
then a pale yellow compound is formed. This is in strong contrast with
oxidation by oxygen from air in alkaline environment, where a dark brown/
black compound is formed.
Description:
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Add some solid hydroquinone to a mix of HCl (10% by weight) and H2O2 (3% by
weight): The hydroquinone dissolves, the liquid becomes colorless and clear.
Heat to appr. 60 C: The liquid turns pale yellow and remains clear. The
pale yellow color may be due to formation of p-benzoquinone.
Add a little solid K2Cr2O7: The solid dissolves, while a dark blue/indigo
compound is formed. This means some hydrogen peroxide was left and all
hydroquinone was oxidized. The final color of the liquid is yellow/green.
Remark:
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Probably the pale yellow compound is quinone, dissolved in water. The structure
of quinone is as follows:
H H
C == C
/ \
O == C C == O
\ /
C == C
H H