Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sodium thiocyanate : NaSCN
nitric acid : HNO3
hydrochloric acid : HCl
sodium nitrite : NaNO2
Class:
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elem=N
redox
Summary:
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Nitrous vapors react with thiocyanate, building a red/brown compound, which
disappears again, when left in contact with air.
Description:
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Sequence 1:
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Add some solid sodium nitrite to HCl (10% by weight): The solid dissolves in
the acid and starts fizzling. Brown vapours are formed and the liquid becomes
pale green/blue. When shaken for a while, locking out any air, the gas
mixture becomes colorless again and the liquid becomes pale green.
Add some solid NaSCN: As soon as the test tube is opened, brown vapors appear
where there is contact with air. If the solid NaSCN is kept in the brown vapors,
then it reacts with these vapors. It becomes red/brown and starts bubbling.
The solid, which is thrown in the liquid, quickly dissolves and the liquid
becomes fairly dark red/brown. A fairly large amount of gas is evolved from
the liquid (overpressure in the test tube). The gas is colorless.
It looks as if the NaSCN reacts with brown NO2 gas and not with NO. It is
observed, that the red/brown discoloration is fastest, where the brown color
of NO2 is strongest.
A remarkable observation is that the NaSCN reacts with the gas mixture and
bubbles during this reaction, without having contact with the liquid.
Sequence 2:
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Add some solid sodium nitrite to some nitric acid (2 mol/l): The solid
dissolves, while fizzling, and brown vapors are evolved.
Keep some solid NaSCN in the brown vapors: The solid becomes red/brown. This
experiment shows that the choride of the hydrochloric acid is not essential
for the reaction with NaSCN.
Take away the NaSCN and keep in air: The red/brown color disappears again.
The solid does not become white anymore, it becomes light yellow.
Sequence 3:
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Mix a little hydrogen peroxide (3%) with nitric acid (2 mol/l) and add some
sodium nitrite: The solid dissolves. While fizzling, brown vapors are
evolved.
Add a small quantity of solid NaSCN to the liquid: The solid dissolves, the
liquid becomes red/brown. The red/brown compound is not destroyed by hydrogen
peroxide, it may even help with the formation of this compound.