Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
hydrogen peroxide : H2O2
potassium bromate : KBrO3
chromium(III) oxide : Cr2O3
Class:
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elem=Cr,Br
redox
Summary:
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Chromium(III) oxide is extremely inert when it is calcined. It does not
dissolve in the common acids, nor in solutions of common bases. The solid,
however, can be oxidized by a solution of a bromate.
Description:
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Put a small amount of solid calcined chromium(III) oxide in a test tube and add
a spatula full of potassium bromate. Add a few ml of water and swirl: Some of
the potassium bromate dissolves, but only a minor part.
Heat the liquid: All of the potassium bromate dissolves, but the chromium oxide
is not affected. It remains suspended in the liquid as fine green particles.
Let the liquid stand for an hour or so: The green solid quickly settles, and
slowly white crystals of potassium bromate settle at the bottom as well.
Add a single drop of 20% sulphuric acid and heat again: Initially, nothing
specials seems to happen besides the dissolving of the crystals of potassium
bromate. But after a minute or so, slowly some bromine is released, the air
above the liquid becomes orange/brown. At the same time, the liquid becomes
yellow/brown and the chromium oxide slowly dissolves. It takes many minutes to
dissolve an appreciable part of the chromium oxide, but it definitely
dissolves.
Allow the liquid to stand for a few minutes: Some of the remaining chromium
oxide settles at the bottom, and the liquid above it is yellow/brown. This of
course could be due to the presence of bromine. For this reason a test for
dichromate was performed.
Put 100 ml of water, 1 ml of 3% hydrogen peroxide and several drops of dilute
sulphuric acid in an erlenmeyer and swirl. To this very dilute H2O2/H2SO4
solution add a few drops of the yellow/brown liquid: Immediately, the liquid
turns deep blue. This is due to formation of the peroxo compound CrO5 and this
shows that the yellow/brown liquid contains hexavalent chromium.
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This is a suitable method of dissolving potteries chrome green (calcined Cr2O3)
and it opens up a method of obtaining soluble chromium compounds from a simple
and easy to obtain chemical. KBrO3 can be made fairly easily from KBr by
electrolysis. The Cr2O3 only dissolves if a small amount of acid is added as
well. The acid acts as a catalyst. In the reaction itself, more acid is
produced and so, once it is started, it continues at a higher rate.