Description of experiment
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Needed compounds:
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bleach : NaClO
sulphuric acid : H2SO4
potassium bromide : KBr
Class:
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elem=Br,Cl
precipitation
redox
Summary:
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Bromide is oxidized by sulphuric acid, even when diluted with water.
Bromine precipitates from water, when it is created in high concentrations.
Description:
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Add appr. 45 ml of concentrated H2SO4 (appr. 96%) to a solution of appr. 80 g
of KBr in 100 ml of water. The H2SO4 is added slowly in order to allow the
liquid to cool down after each addition of H2SO4: After addition of all H2SO4,
the liquid is orange. The liquid fumes, when in contact with air. At the
bottom of the flask, large crystals (KHSO4???) are formed. The liquid also
contains smaller crystals, which circulate around in the flask (the liquid
moves upwards in the centre of the flask and moves downward at the glass
walls). The crystals move with the liquid. This can be explained physically
by assuming that heat is lost at the glass walls, resulting in a slightly
denser liquid, which sinks to the bottom, while at the center the warmer
liquid goes up. After a day of standing all crystals are at the bottom of
the flask and the liquid is yellow and clear.
Add some bleach (appr. 10% active chlorine) to the yellow liquid (appr.
2 parts of bleach to 3 parts of the yellow liquid): As soon as the bleach
mixes with the yellow liquid a finely dispersed red precipitate is formed.
The liquid becomes completely turbid and opaque. The red precipitate
sinks to the bottom in the course of half a day. Finally, a thick dark
drop of liquid bromine is at the bottom of the flask and the liquid is
clear and orange/red. Above the liquid, there is a thick orange/red vapour
of elementary bromine.