Description of experiment
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Needed compounds:
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sodium sulfite hepta hydrate : Na2SO3 . 7H2O
nitric acid : HNO3
potassium nitrite : KNO2
sodium bromide : NaBr
Class:
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elem=Br,N
redox
Summary:
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Sodium bromide is not oxidized by concentrated nitric acid. However, when
a minute quantity of nitrite is added, suddenly the reaction starts and
quickly all bromide is oxidized.
Description:
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Add some solid sodium bromide to concentrated nitric acid (appr. 60% by
weight): The solid does not appear to dissolve, or the solid dissolves and
is replaced by sodium nitrate. The liquid remains almost colorless.
After vigorous shaking, there still is a large quantity of a white solid,
which does not dissolve.
Sequence 1:
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Add a very small amount of potassium nitrite (just a few little crystals of
size a few tenths of mm): As soon as the crystals touch the liquid they
dissolve and a brown gas mixture, probably NO/NO2, is evolved (bubbling).
If the liquid is shaken, then it turns darker quickly, from almost colorless,
through yellow, orange, dark brown. At a certain moment the liquid becomes
turbid and droplets of bromine separate from the liquid. A large drop of
bromine is collected at the bottom of the bottle and a thick red/brown
vapor of bromine is above the liquid.
Sequence 2:
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Add a crystal of solid Na2SO3.7H2O to the solution, prepared from nitric acid
and sodium bromide: No visible changes are observed. The crystal of sodium
sulfite does not dissolve, however, its structure seems to change slowly.
On vigorous shaking the crystal still does not dissolve, but after a while
a faint brownish color can be observed above the liquid (is this NO2 or Br2?).
The liquid itself remains almost colorless or pale yellow.