Description of experiment
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Needed compounds:
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hydrogen peroxide : H2O2
ferric chloride : FeCl3 . 6H2O
hydroquinone : HO C6H4 OH
Class:
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elem=C,Fe
coordination
redox
Summary:
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Ferric chloride enhances the oxidation of hydroquinone by hydrogen
peroxide considerably. Besides this, a coordination complex appears
to be formed when ferric chloride is added to an hydroquinone/peroxide
solution.
Description:
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Add a solution of hydroquinone to a 3% solution of H2O2: After a few minutes
the solution has turned brownish-pink. This liquid is called BROWNPINK.
Add a small quantity of a solution of ferric chloride to an excess amount
of BROWNPINK: An intensely dark brown/red compound is created. The liquid
becomes very dark, one cannot see clearly whether the liquid is dark but
clear or contains a dark precipitate. A small amount of gas is evolved
as well.
Add a little bit more of a solution of ferric chloride: The liquid turns
lighter. Over a period of just a few seconds, one can easily observe that
the liquid turns more and more pale. After approximately 1 minute, the
liquid is red/brown and clear. The temperature of the liquid has risen
notably. It is warm after the reaction, although not really hot.
Add a few drops of a solution of ferric chloride to BROWNPINK: The liquid
turns very dark brown and the temperature rises until the liquid is luke-
warm. After a few seconds the liquid turns more pale and pale again.
Finally, the liquid becomes pale brown (it looks like a concentrated solution
of ferric chloride). A little amount of gas is evolved as well.
Add a few more drops of a solution of ferric chloride: The liquid does not
change, the drops of ferric chloride simply mix with the liquid.
Remark: It appears that ferric chloride strongly enhances the speed with
which hydroquinone is oxidized by hydrogen peroxide (rise of temperature,
quick fading of intense color), without being needed itself. Even a few
drops cause the hydroquinone to be oxidized quickly. After fading of the
intense color addition of more ferric chloride has no effect. From this
can be concluded that the ferric chloride or one of the reaction products,
due to the addition of ferric chloride, catalyzes the reaction between
hydrogen peroxide and hydroquinone.