Description of experiment
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Needed compounds:
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hydrogen bromide : HBr
sulphuric acid : H2SO4
potassium bromide : KBr
sodium chlorite : NaClO2
Class:
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elem=Cl,Br
redox
Summary:
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Chlorite ion and chlorine dioxide are sluggish oxidizers when in aqueous
solution, even in the presence of quite some acid. Bromide ion only is oxiduzed
very slowly and only partially.
Description:
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Add some solid potassium bromide (or sodium bromide) to a 30% solution of
NaClO2 in water and shake: The solid dissolves, the resulting liquid is
colorless.
Add some 20% sulphuric acid: The sulphuric acid forms a layer on top of the
colorless KBr/NaClO2 layer. Between the two layers is a fairly dark
yellow/brown layer and also some bubbles of gas are produced in that layer.
Shake the liquid: More bubbles of gas appear and a deep yellow gas is produced
with a somewhat orange tint. The color of the gas is not like the color of
bromine vapor, it is much more yellow. Soon, the entire test tube is filled
with the intenely colored gas mix.
Keep a flame near the open end of the test tube: The gas mix "burns" very fast,
almost explosively with a high-pitched flute-like sound. This is due to a high
ClO2-content of the gas mix. After the reaction, the gas mix is pale brown.
Shake the test tube another time: This time again a yellow/orange gas mix is
produced, but now the color is more orange-like, but still not really like
bromine vapor (which is more red).
Keep a flame near the open end of the test tube again: Also this time a
flute-like sound is produced when the gas decomposes, but it is not as violent
as the first time.
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Dissolve some KBr in a solution of 30% NaClO2 and add a 40% solution of HBr:
The liquid becomes deep red and an orange/yellow gas mix is produced. Bubbles
of gas escape from the liquid. Soon, the test tube is filled completely with
the gas mix.
Keep a flame near the open end of the test tube: The gas almost explosively
decomposes and a red/brown gas remains in the test tube.
This set of experiments shows that chlorite ion does form bromine when brough
in contact with bromide ions and acid, but the reaction certainly is not
complete and much more ClO2 than Br2 is formed. This is quite remarkable, given
the reactivity of ClO2 and knowing that it is a strong oxidizer one would
expect a much more complete and faster conversion of bromide ion to bromine.