Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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citric acid : COOH CH2 COH(COOH) CH2 COOH
hydrochloric acid : HCl
potassium tetraperoxochromate (V) : K3CrO8
Class:
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elem=Cr
redox
Summary:
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Tetraperoxochromate does not dissolve well in water and is not stable.
In acidic environments it cannot exist at all. It decomposes, yielding a
large amount of oxygen. It is not capable of oxidizing hydrochloric acid
to chlorine, it simply decomposes, giving off all its oxidizing power
in the form of oxygen.
Description:
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Sequence 1:
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Add some solid K3CrO8 to some hydrochloric acid (30% by weight): The solid
quickly dissolves, while developing a gas (oxygen). The liquid turns green
(color of chromium (III)). No smell of chlorine can be observed. The solid
immediately dissolves as a green compound, no transient deep blue peroxo
acicic compound of chromium is observed.
Sequence 2:
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Add some solid K3CrO8 to water: The solid hardly dissolves. A gas is evolved
very slowly at the crystals. The liquid becomes pale yellow and slightly
turbid (it is not clear if this is due to very small solid particles or due
to very small bubbles of oxygen).
Add some citric acid: The citric acid dissolves. The evolution of gas is a
little faster now. The liquid becomes pale green now (color of chromium (III)).
Add a little amount of HCl (30% by weight) and shake a little bit: The
crystals of K3CrO8 now dissolve fairly fast, with the evolution of a gas
(oxygen). While the crystals dissolve, a deep blue compound is formed, which,
however, quickly disappears. Finally, the liquid is green. No smell of
chlorine can be observed.
Remark: The deep blue compound, which appears transiently, is an acidic
peroxo compound, probably its composition is CrO5, better represented by
CrO(O2)2.