Description of experiment
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Needed compounds:
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nitric acid : HNO3
sodium hydroxide : NaOH
hydrogen peroxide : H2O2
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
redox
Summary:
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Dichromate reacts with hydrogen peroxide, yielding chromium (III) as the
final product, in acidic environments. In alkaline environments, chromium
(III) yields chromate with hydrogen peroxide.
Description:
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Dissolve some K2Cr2O7 is dilute HNO3 (appr. 2 mol/l) and add a large excess
amount of H2O2 (appr. 6% by weight): The liquid becomes intense blue, a gas
is evolved and then it quickly turns green.
Divide the green liquid in three parts.
Boil one part for a while: The green liquid turns blue/grey/purple, like
compounds of the plain aqueous chromium (III) ion.
Add an excess amount of a solution of NaOH to the second part of the green
liquid: A greenish precipitate is formed, which quickly dissolves again. The
liquid becomes yellow/green and clear.
Add an excess amount of HNO3 (appr. 2 mol/l): The liquid remains yellow.
Add the third part of the green liquid to the acidic yellow liquid: The
resulting liquid is dark blue. This shows that chromium (III) was converted
to chromium (VI) in the alkaline environment.