Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
sodium thiosulfate : Na2S2O3 . 5H2O
sodium hydroxide : NaOH
sodium borohydride : NaBH4
hydrochloric acid : HCl
zinc : Zn
nitric acid : HNO3
rhenium : Re
Class:
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elem=Re
redox
Summary:
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Rhenium can be oxidized to colorless perrhenate [ReO4]-, with nitric acid.
With zinc it can be reduced to a yellow/green species in the presence of
hydrochloric acid at sufficient concentration. This species apparently is
not reduced any further with borohydride in alkaline environments. In
acidic environments, a dark brown/black compound can be formed easily,
due to reduction of thiosulfate to sulfide by the borohydride. The sulfide
forms a dark compound with rhenium.
With sulfite, perrhenate nor the yellow/green compound can be reduced to
a lower oxidation state.
Description:
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Initial preparation of green/yellow liquid:
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Add a small amount of rhenium metal to a few drops of concentrated nitric
acid and heat a little: The metal quickly dissolves. The liquid becomes
green at first and later it becomes light yellow. The green color probably
is due to the presence of NO2/N2O3 in the concentrated nitric acid.
Add 1 ml of concentrated hydrochloric acid (30% HCl) to the liquid: The
resulting liquid is very light yellow, almost colorless.
Dilute the liquid with the same amount of distilled water: The liquid now
becomes really colorless.
Add a spatula of zinc: The zinc dissolves, the liquid becomes yellow/green
and a lot of hydrogen is evolved. The liquid becomes warm. Finally, when all
zinc is dissolved, the liquid is clear and yellow/green.
Sequence 1:
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Dilute again with an equal volume of water and then take approximately 1/3th
of the volume of the now light yellow/green liquid. To this, add some solid
sodium hydroxide: The sodium hydroxide dissolves and the liquid becomes very
light yellow and turbid. The solid in the liquid is white, it probably is due
to the presence of Zn2+ ions in the liquid.
Add more of a solution of sodium hydroxide, such that the liquid becomes
strongly alkaline: The liquid becomes clear and very light yellow. The color
is lighter than one would expect on the basis of dilution only.
Add some solid sodium borohydride: The solid dissolves. No further visible
changes besides a very slow evolution of a colorless gas (the borohydride
very slowly decomposes, giving some hydrogen).
Prepare a fairly large amount of 2% HCl and pour the alkaline liquid in
this excess amount of acid: The liquid starts fizzling strongly, due to
decomposition of the borohydride. Besides this, a very strong smell of
hydrogen sulfide can be observed. The liquid also becomes very dark brown
and turbid. A dark solid is precipitated.
The thiosulfate is reduced to sulfide by the borohydride in the lightly
acidic environment and this sulfide in turns gives the dark compound with
the dissolved rhenium compound. Is this dark compound ReS2 or Re2S7?
Sequence 2:
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Add quite some sodium sulfite to the yellow/green liquid: The solid dissolves
and the liquid remains clear and yellow/green. There is a clear smell of
sulphur dioxide.
Add a small amount of solid sodium borohydride: The solid quickly dissolves,
while fizzling. Besides this, a very strong smell of hydrogen sulfide (rotten
eggs) can be observed. The liquid almost at once becomes turbid and very
dark brown/black.