Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium persulfate : Na2S2O8
hydrogen peroxide : H2O2
sodium carbonate : Na2CO3 . 10H2O
sodium metabisulfite : Na2S2O5
sulphuric acid : H2SO4
vanadium pentoxide : V2O5
Class:
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elem=V,O
coordination
redox
Summary:
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Vanadium (IV) does not readily yield precipitates with alkaline compounds.
Carbonate is not capable of precipitating this.
Hydrogen peroxide builds a complex with vanadium (V) and possibly with
vanadium (IV). Diverse coloured compounds are formed in sequence. What
is their constitution?
Description:
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Add some sulphuric acid (2 mol/l) to a mixture of solid vanadium pentoxide
and sodium metabisulfite: As soon as the acid touches the mixture, it starts
bubbling (production of SO2).
Cap the test tube and keep the SO2 trapped: Some overpressure is developed.
The liquid becomes turbid yellow when shaken. After appr. 1 minute it has
turned a little bit green (partial reduction of vanadium pentoxide by the
sulphur dioxide).
Heat the liquid until it boils (and uncap the test tube during this!): The
liquid becomes clear and blue and some solid vanadium pentoxide is left.
When the liquid is shaken it becomes greenish blue. Apparently there was
insufficient sodium metabisulfite for reduction of all vanadium pentoxide
or a large part of the SO2 is driven off.
Add a little bit more of sodium metabisulfate to the still warm liquid: A
fairly vigorous evolution of gas is observed. When the liquid is shaken, it
becomes bright blue and no solid remains. A strong smell of SO2 can be
observed.
Drive off all SO2 by boiling gently. This is done, until no smell of SO2
can be observed anymore. Add a solution of sodium carbonate carefully:
Evolution of a gas (CO2), liquid remains blue.
Add more solution of sodium carbonate: The liquid turns grey and the
evolution of gas ceases. The liquid remains clear. When even more solution
of sodium carbonate is added, then the liquid turns brown/grey. After a
while of standing, the liquid has turned greenish grey. The liquid remains
clear.
The greenish grey liquid is used as the basis for two different sequences.
This liquid is called GREENGREY.
Sequence 1:
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Add an excess amount of sulphuric acid (appr. 2 mol/l) to GREENGREY:
Formation of a gas (CO2), the liquid becomes light blue.
Add an excess amount of H2O2 (appr. 6% by weight): The liquid turns red/brown
and remains clear. The intensity of this red/brown colour is much larger than
the intensity of the blue color. When this red/brown liquid is shaken, then
it bubbles. Probably these are bubbles of oxygen, but they may also be
bubbles of CO2 (or both).
Heat the liquid until it starts boiling: Before it starts boiling, the
liquid starts bubbling fairly vigorously and quickly becomes lighter. After
appr. 30 seconds it is pale yellow and clear. This is assumed to be the
colour of vanadium (V) in strongly acidic environments [VO2]+. The peroxide
has oxidized vanadium (IV) to vanadium (V).
Again add some H2O2 (6%): The liquid turns red/brown again.
Heat again: The liquid starts bubbling again. As soon as the bubbling ceases,
the liquid becomes lighter quickly. As long as it bubbles, it remains dark
red/brown.
Sequence 2:
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Let stand GREENGREY for 20 minutes: The liquid has darkened quite a lot and
appears to be turbid. There are no clearly visible crystals, flakes or other
large particles, it looks as if there is a solid compound, colloidally
divided in the liquid. When a part of the liquid is diluted with a lot of
water, then this does not result in a milky liquid, but in a completely
clear green/grey liquid. It is hard to determine, whether this is a true
solution or a colloid.
Sequence 2a:
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Add an excess amount of sulphuric acid (2 mol/l) to the darkeded GREENGREY
liquid: The liquid turns light blue and clear. A gas is evolved (CO2).
Sequence 2b:
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Add an excess amount of H2O2 (6%) to the darkened GREENGREY liquid: The
liquid becomes moss-green and much lighter. It remains clear.
Heat the liquid: It starts bubbling fairly vigorously and after that it becomes
yellow. After a while of gently boiling, the liquid is clear and yellow.
Again add an excess amount of H2O2: The liquid becomes fairly intense red/brown.
Sequence 2c:
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Add an excess amount of Na2S2O8 to the darkened GREENGREY liquid: Liquid
becomes slightly lighter. Colour remains greenish. Slight evolution of a gas.
Heat and boil gently: Liquid becomes colorless and clear.
Add an excess amount of sulphuric acid (2 mol/l): Evolution of a gas. The
liquid becomes intense yellow and remains clear.
Add a little amount of H2O2 (6%): Liquid becomes red/brown, where the H2O2
touches the liquid. When all is shaken, the liquid becomes yellow/brown and
remains clear.