Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
nitric acid : HNO3
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
coordination
redox
Summary:
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When dichromate is reduced by means of adding hydrogen peroxide, then after
the initial transient a green compound is formed, even when the acid in
which the dichromate is dissolved is nitric acid (nitrate is supposed to
not form a coordination complex with chromium (III)). Is this green compound
a coordination complex with nitrate? or with hydrogen peroxide?
This coordination complex can easily be destroyed by heating.
Description:
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Sequence 1:
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Dissolve some K2Cr2O7 in dilute HNO3 (2 mol/l) and add an excess amount of
H2O2 (6% by weight): Liquid becomes intense blue/indigo and starts foaming.
However, fairly quickly, the intense blue color fades and the bubbling
ceases. The final color is green. The liquid is clear.
Heat the liquid and boil gently for a while: The liquid becomes violet under
tungsten light and bluish/grey under SL light. This is the color of plain
aqueous chromium (III).
Sequence 2:
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Dissole some K2Cr2O7 in dilute HNO3 (2 mol/l) and heat the liquid, until it
almost starts boiling: The liquid becomes dark orange/red. The dichromate
tends to darken a little when heated.
Add an excess amount of H2O2 (appr. 10% by weight) carefully in small
quantities at each step: The dichromate reacts violently with the hydrogen
peroxide, resulting in strong foaming. After all dichromate has reacted,
the resulting liquid is bluish/grey under SL light. This is the color of
plain aqueous chromium (III). The initial transient of blue/indigo can be
observed in this experiment, but only for a fraction of a second. Due to
the heat, the compound decomposes very quickly.