Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
potassium bromide : KBr
potassium bromate : KBrO3
formic acid : HCOOH
Class:
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elem=C,Br
redox
Summary:
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Potassium bromate does not react with concentrated formic acid, but when
some bromide or hydrochloric acid is added, then the reaction proceeds
quickly, carbon dioxide being produced.
Description:
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Sequence 1:
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Add some potassium bromate to formic acid (appr. 85% by weight): The solid
goes to the bottom of the liquid and does not react. It does not dissolve
(or just a small part dissolves).
Add a small amount of hydrochloric acid (appr. 20% of the volume of formic
acid and the concentration of the hydrochloric acid was appr. 30% by weight):
The hydrochloric acid mixes with the formic acid. The liquid starts bubbling
and becomes yellow. The gas mixture above the liquid becomes pale green
(chlorine or a mixture of chlorine with some bromine). At a certain moment,
the formation of gas goes fairly quickly. The gas mixture appears to become
lighter, apparently the major part of the gas is CO2.
At the end of the reaction, when the evolution of the gas becomes slower,
the color of the liquid intensifies. At the end when no more gas is evolved,
the liquid is bright orange and the gas mixture above the liquid is brown/
orange and contains some bromine (it is not a thick bromine vapor, but one
can easily observe the brown/orange color of the bromine vapor).
Sequence 2:
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Put some potassium bromate with a large excess amount of potassium bromide
in a test tube and add some formic acid (appr. 85% by weight): As soon as
the acid is added, the liquid becomes red/brown and after a few seconds
evolution of a gas starts. The evolution of gas goes faster and faster and
the temperature of the reaction mixture rises, making it very hot. At
a certain moment the reaction became too violent and for safety is was
slowed down under a cold running tap assuring that no water entered the
test tube. The liquid was very hot and by keeping it under the running water
it cooled down again, resulting in a more controllable rate of reaction.
The gas, produced in the reaction, is almost colorless. After the reaction,
the liquid is just yellow and above the liquid only a very pale color of
bromine vapor can be observed. The potassium bromide only partly dissolved,
most of the bromide still is solid.
Add some water (approximately twice the original volume of the liquid): The
solid potassium bromide now dissolves and the color of the liquid becomes
more pale due to dilution.
Remark:
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Apparently, the bromate oxidizes the formic acid to carbon dioxide, but
an intermediate in the form of bromine or chlorine is needed in order to
do so. Bromate-only does not result in oxidation.