Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
hydrochloric acid : HCl
ligroin : C7H16
hydrogen bromide : HBr
sodium selenite : Na2SeO3
Class:
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elem=Se,Br
coordination
Summary:
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Sodium selenite dissolves in concentrated hydrobromic acid with a red/brown
color. When this solution is shaken with ligroin, then the ligroin layer also
turns red/brown. The compound, dissolved in the ligroin layer is a selenium
compound, as is demonstrated by the experiment, described in more detail below.
Description:
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Pour some concentrated hydrobromic acid (40% HBr by weight) in a test tube and
add a small amount of solid sodium selenite: The sodium selenite dissolves in
the acid. The liquid remains clear and obtains a fairly intense red/brown
color.
Add a small amount of ligroin (boiling range 40 ... 60 C) and shake vigorously
for a while: When shaking stops, then the ligroin layer quickly separates from
the aqueous acidic layer and the ligroin layer now also is red/brown. The
ligroin layer also is clear.
Using a pasteur pipette take away most of the ligroin layer, assuring that
nothing is taken from the aqueous layer and put the ligroin in a handwarm and
clean petri dish: The ligroin quickly evaporates and a thin residue remains on
the glass of the petri dish. A foul smell can be observed, so it is best to do
this part of the experiment outside or in a fume hood.
Add a few ml of hydrochloric acid (20% HCl by weight or so) to the petri dish
and swirl this acid, such that all of the glass is rinsed by the acid. After
this rinsing pour the acid in a test tube.
In a separate test tube, prepare a solution of sodium sulfite in some
hydrochloric acid and add this solution to the acid which was used to rinse the
petri dish: Quickly the liquid turns orange/red and it becomes turbid. The
sulphur dioxide (from the sulfite in the acidic solution) reduces the selenium
in the rinsing acid to elemental form, which becoms finely suspended in the
liquid.
So, from this experiment it can be concluded that the ligroin extracts some of
the red selenium compound from the aqueous layer and after evaporation of the
ligroin this remains as a residue on the glass of the petri dish. It is not
entirely clear what this extracted selenium compound is. It may be SeBr4, but
some sources also speak of a dissociated compound like SeBr2 + Br2 or Se2Br2 +
Br2. Anyway, a covalent selenium compound is formed in solution and this is
sufficiently stable such that it does not com