Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium iodide : KI
potassium bromide : KBr
hydrochloric acid : HCl
sodium nitrite : NaNO2
Class:
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elem=N,Br,I
redox
Summary:
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Nitrite is decomposed by hydrochloric acid (as it is done by all acids),
but when the decomposition products are not allowed to escape, then
further reactions occur.
When bromide is added, then a compound is formed, which definitely is not
bromine. What is this compound?
When iodide is added, then iodine is formed.
Description:
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Sequence 1 (bromide experiment):
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Add a spatula of sodium nitrite to some hydrochloric acid (10% by weight):
The solid dissolves and a gas is evolved. The gas mixture in the test tube
becomes brown.
Shake the liquid and gas mixture in the test tube and do not allow the gas
mixture to escape into the air: The brown gas dissolves in the acid and slowly
the brown color disappears. The liquid becomes pale green and the gas mixture
becomes almost colorless (or pale green, which is hard to observe, due to the
thin layer of gas). The smell of the gas mixture is not the smell of chlorine,
although it is a choking odour.
Add some solid potassium bromide to the test tube (not allowing much of the
gas mixture to escape): The solid dissolves (not as easy as in plain water),
but finally all potassium bromide has dissolved. The liquid becomes greenish
brown and the gas mixture becomes brown. Some overpressure is formed in the
test tube. When some of the gas mixture is allowed to escape, then a strong
smell of chlorine can be observed, although the color of the gas mixture is
brown. The brown color of the liquid and the gas mixture are not like the
color of bromine. Bromine vapor and concentrated aqueous bromine solutions
are much more red/orange. Here the liquid becomes fairly dark sepia-like and
the gas mixture becomes pure brown.
Open the test tube under water, with the dark sepia/brown liquid downwards:
As soon as the test tube is open, the brown liquid sinks down and when it is
diluted, its color changes to bright yellow. The gas mixture remains brown.
Bubble the gas mixture into an erlenmeyer, completely filled with water: The
brown bubbles loose their color completely. The gas, collected in the erlen-
meyer is colorless and the liquid in the erlenmeyer is colorless as well.
This supports the assumption that the brown vapor is not bromine. The brown
gas probably is NO2, although the smell would suggest it is chlorine. (Is
it a mixture of both? or is it another compound, consisting of nitrogen,
oxygen and chlorine?).
Let a large bubble of air go into the erlenmeyer, while still under water:
The gas mixture, trapped in the erlenmeyer, becomes brown again. On stoppering
and shaking of the erlenmeyer with the brown gas mixture and water, the
gas mixture becomes colorless again.
Sequence 2 (iodide experiment):
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Add a spatula of sodium nitrite to some hydrochloric acid (10% by weight):
The solid dissolves and a gas is evolved. The gas mixture in the test tube
becomes brown.
Shake the liquid and gas mixture in the test tube and do not allow the gas
mixture to escape into the air: The brown gas dissolves in the acid and slowly
the brown color disappears. The liquid becomes pale green and the gas mixture
becomes almost colorless (or pale green, which is hard to observe, due to the
thin layer of gas). The smell of the gas mixture is not the smell of chlorine,
although it is a choking odour.
Add some solid potassium iodide to the test tube (not allowing much of the
gas mixture to escape): When the iodide is added, the top part of the
gas mixture in the test tube becomes brown (this shows that an important
constituent of the gas mixture above the pale green liquid is NO). The
crystals of potassium iodide become brown, even before they fall into the
liquid. The crystals quickly dissolve and they cause the liquid to foam
strongly (a thick grey/black slurry covers the liquid and this prevents the
gas evolved from bubbling, instead of foaming). When the liquid is shaken,
it becomes dark brown, with some black particles in it. The gas mixture
becomes colorless again. Inside the test tube there is noticeable pressure
buildup.
When the test tube is opened, then the gas mixture becomes brown again. When
the brown gas mixture touches the liquid, the liquid is covered by a crust
of iodine, with many blisters of gas bubbles in it. The brown gas mixture
reacts with the iodide in the liquid, such that it is covered by an insoluble
crust of iodine. A gas is evolved as well in this reaction, this causes
blistering of the crust.