Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
sulphuric acid : H2SO4
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
vanadium pentoxide : V2O5
Class:
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elem=V,O
coordination
redox
Summary:
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Hydrogen peroxide builds complexes with vanadium (IV) and vanadium (V)
species. These compounds are not stable and result in dissociation of
the complex and formation of vanadium (IV) compounds. The net result
of adding hydrogen peroxide to a solution containing vanadium (V) can
be reduction to vanadium (IV) with the formation of oxygen.
Description:
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Sequence 1:
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Dissolve some vanadium pentoxide in a solution of NaOH (needs heating) and
add an excess amount of sulfuric acid, resulting in a solution, containing
pervanadyl ([VO2]+, yellow solution). Add some hydrogen peroxide (3% by
weight) to the yellow solution: The liquid becomes brown/red and remains
clear.
Boil the brown/red liquid for a while: Evolution of a lot of gas, the
liquid turns yellow/green. Apparently all H2O2 is decomposed and part of the
yellow [VO2]+ is reduced to blue [VO]2+ (mix of blue and yellow gives green
liquid).
Sequence 2:
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Dissolve some vanadium pentoxide in a solution of NaOH (needs heating) and
add an excess amount of sulfuric acid, resulting in a solution, containing
pervanadyl ([VO2]+, yellow solution). Add some hydrogen peroxide (3% by
weight) to the yellow solution: The liquid becomes brown/red and remains
clear.
Add some sodium sulfite to the brown/red liquid and shake: The liquid becomes
sky blue. This change of color is not immediate, it goes through a range of
colors: red/brown -> orange -> yellow -> green -> blue. Finally all vanadium
is in the +4 oxidation state.
Drive off all excess SO2 by boiling and then slowly add a solution of NaOH,
while shaking well: First the liquid remains blue (apparently there was an
excess amount of acid), but when more NaOH is added, the liquid becomes
darker and the color changes through green, through green brown to the final
red/brown color. This red/brown compound strongly resembles the color of the
compound when H2O2 is added to acidic pervanadyl. Here however, we have
a vanadium (IV) compound in strongly alkaline environment.
Add an excess amount of H2O2 (3% by weight) to the red/brown alkaline liquid:
The liquid becomes almost colorless, just a little dirty yellow/green. It
remains clear. Adding much more NaOH does not result in any visible change.