Description of experiment
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Needed compounds:
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iodine : I2
thionyl chloride : OSCl2
Class:
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elem=S,I
redox
Summary:
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Iodine dissolves in thionyl chloride. It does not react. It gives a purple
solution, looking very similar to a purple solution of iodine in an alkane or
chlorinated hydrocarbons.
When water is added, then a remarkable reaction occurs. Sulphur is formed.
Description:
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Add a small amount of crushed iodine to 1 ml or so of thionyl chloride: The
iodine slowly dissolves. The liquid becomes deep purple. Only a very small
amount of iodine already gives a dark purple color.
Add appr. 10 ml of water: Immediately a yellow solid is formed and the thionyl
chloride quickly reacts exothermically with the water. When the reaction is
finished, the liquid is turbid and yellow with a brown hue.
Dilute with a lot more water: The liquid becomes turbid and pale yellow/white.
The liquid has a smell of sulphur dioxide, but it is not clean sulphur dioxide.
The smell is dirty, maybe there is some smell of sulfide as well.
The yellow solid almost certainly is elemental sulphur.
This reaction is remarkable. One would not expect elemental sulphur in this
reaction. Thionyl chloride contains sulphur in oxidation state +4 and with
water it gives the reducing sulphur dioxide. One would expect the iodine to
oxidize the sulphur dioxide with formation of sulphuric acid and hydroiodic
acid.