Description of experiment
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Needed compounds:
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potassium dichromate : K2Cr2O7
sulphuric acid : H2SO4
thionyl chloride : OSCl2
Class:
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elem=S,Cr
inorganic
Summary:
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Thionyl chloride is not soluble in concentrated sulphuric acid, or only
sparingly soluble.
The mix of thionyl chloride and sulphuric acid is capable of forming chromyl
chloride from potassium dichromate.
Description:
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Take 0.5 ml of thionyl chloride and add 1 ml of 96% sulphuric acid: The thionyl
chloride forms a layer on the sulphuric acid. With vigorous shaking, small
droplets of thionyl chloride are dispersed in the acid. The droplets slowly
move upwards and form a layer on top of the acid again.
No reaction seems to occur. The few percent of water in the acid is not
decomposed by the thionyl chloride. No gas is produced.
Add a spatula of solid potassium dichromate: The mix (droplets in acid) starts
bubbling and foaming slowly. A colorless gas is produced, but also a clearly
visible orange/red gas is produced. This gas/vapor almost certainly is
chromyl chloride, which is formed from dichromate, concentrated sulphuric
acid and hydrogen chloride. Probably, the addition of potassium dichromate
causes formation of hydrogen chloride, it might also be possible that CrO3,
formed from the acid and the dichromate, reacts directly with thionyl chloride,
with formation of SO2 and CrO2Cl2.
The liquid becomes dark red, and remains clear.
Pour the liquid in a large amount of water: A vigorous reaction occurs,
gas is produced. The water becomes bright green with a blue hue. All hexavalent
chromium is reduced to chromium(III).