Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium bromide : KBr
potassium iodide : KI
thionyl chloride : OSCl2
Class:
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elem=S,I,Br
redox
Summary:
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Iodide ion reacts with thionyl chloride, most likely giving iodine, and some
sulphur(II) species.
Potassium bromide does not dissolve in thionyl chloride (at least, not visibly).
No reaction occurs.
Description:
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Sequence 1:
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Take 1 ml of thionyl chloride and add a small amount of solid potassium
iodide: The solid at once is covered by a dark brown/red layer. This brown/red
material dissolves in the thionyl chloride, giving a clear and beautiful
red solution, resembling the color of a dark red wine. This color differs a
lot of the color of iodine, dissolved in e.g. ligroin, chloroform or carbon
disulphide. The color also is very different from the color of a solution
of iodine in ethanol or aqueous solution of KI.
Pour the deep red liquid in some water: The thionyl chloride reacts with
the water, giving SO2 and HCl. The deep red color disappears, and the
liquid becomes turbid. A foul smell (not like SO2, but more like H2S) is
present as well. If a small amount of water is used, then the liquid becomes
deep yellow (iodide + sulfite in acidic medium forms an adduct), and turbid.
If a large excess of water is used, then the liquid becomes more like milk
and only pale yellow.
Probably, iodide ion is oxidized by thionyl chloride to iodine, which dissolves
with a beautiful deep red color. The thionyl chloride itself then is reduced
to some sulphur(II) compound, or sulphur(I)? On addition of water, this
lower oxidation state compound of sulphur then disproportionates, giving
SO2 (sulphur(IV)) and sulphur.
Sequence 2:
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Take 1 ml of thionyl chloride and add a small amount of solid potassium
bromide: The solid sinks to the bottom and nothing seems to happen anymore.
The solid remains white, the liquid remains colorless (or maybe it becomes very
pale yellow, but that is all).
Add a few drops of water: The liquid now becomes orange, and a lot of gas
is produced (HCl + SO2, and most likely also some HBr).
Add more water: Vigorous reaction, a lot of gas is produced, finally, all is
dissolved. The resulting solution is colorless.