Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
manganese (IV) oxide : MnO2
potassium permanganate : KMnO4
Class:
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elem=Mn,Cl
redox
Summary:
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When solid potassium permanganate is added to concentrated hydrochloric
acid, then a gas is evolved, being chlorine. The liquid becomes almost
black. When the reaction is driven to an end by heating (almost boiling)
the liquid, then the final liquid does not become colorless, but green.
The green color is not a bright green, but it is brownish/sepia-like.
What is the constitution of this green/sepia compound?
A similar observation is done when hydrochloric acid is heated with
manganese dioxide. In the cold there is no visible reaction, but when
heated, chlorine is produced and a green compound is formed.
Description:
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Sequence 1:
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Add some solid KMnO4 to some HCl (30% by weight): As soon as the solid
touches the liquid, a gas is evolved and a crackling sound is produced.
After a while, the gas mixture above the liquid is pale green and the
characteristic pungent odour of chlorine can be observed. When the liquid
is heated, then the reaction becomes more vigorous and the liquid becomes
black, but when viewed through a thin layer (e.g. liquid, sticking to the
glass, when shaking), it looks green/sepia. When heating is continued, the
evolution of gas quickly stops and all KMnO4 has reacted. The liquid then
becomes lighter. Its final color after appr. 2 minutes of heating at the
boiling point of the hydrochloric acid is green/sepia.
Let liquid stand for a few days, with test tube capped: After one day, the
liquid is light yellow/brown, after two days, the liquid is very pale yellow,
after three days it is colorless.
Add one crystal of solid KMnO4 to the colorless liquid again: A crackling sound
is produced again. The liquid becomes black/dark brown.
Heat a little bit again: The crackling becomes louder, a pungent smell
of chlorine can be observed.
Cap the test tube, with the warm (not really hot) liquid and let stand for
two days: The liquid still is black. Apparently the presence of the chlorine
prevents the liquid from becoming colorless or pale green/yellow (due to
presence of chlorine).
Heat again and let boil gently for a minute: The liquid quickly becomes
green/sepia again. After boiling all chlorine is driven off. Its characteristic
smell is not observed anymore.
Dilute the liquid with a few times its volume of water: The complete liquid
becomes colorless.
Sequence 2:
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Add some hydrochloric acid to a spatula full of MnO2: No visible reaction.
When the liquid is shaken, it becomes dark grey, due to finely divided MnO2.
Heat until boiling: The liquid starts bubbling, already before it starts
boiling. The gas mixture above the liquid becomes pale green. The liquid
itself becomes dark green. When the liquid is boiled for a longer time,
then it becomes green with a brown hue.