Description of experiment
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Needed compounds:
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formic acid : HCOOH
bromine : Br2
Class:
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elem=C,Br
redox
Summary:
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Bromine dissolves fairly well in pure formic acid, but no redox reaction
occurs, not even on heating. When water is added to the mix, then a redox
reaction sets in, in which bubbles of gas are produced (probably CO2).
Description:
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Add a drop of pure bromine to a few 100's of ml of pure (>98%) formic acid: The
drop settles at the bottom of the test tube under a layer of formic acid. The
formic acid layer becomes red/brown.
Shake the test tube for some time: More of the bromine dissolves, the layer of
formic acid becomes dark red. Not all of the bromine dissolves, a small blob of
bromine remains at the bottom. Also quite some bromine escapes as gas/vapor and
the air above the formic acid is brown/orange.
Heat the liquid in the test tube: The bromine starts refluxing in the test
tube, drops being collected at the glass wall and running back into the formic
acid. The bromine does not react though.
Dump the acid and bromine in a small amount of water: The liquid settles at the
bottom and a drop of bromine also settles at the bottom. At places with a high
concentration of formic acid, quite some colorless gas is produced. Most
likely, the bromine oxidizes the formic acid to carbon dioxide, but it seems
that this reaction only runs at an appreciable rate in the presence of water.
Remark: This experiment was repeated with 85% formic acid, and with that concentration
the result is the same. Even when the acid is diluted to 50% there still hardly
is any formation of carbon dioxide. When the acid is diluted to 20% or so, then
quite some CO2 is formed and with some moderate heating the liquid can be made
colorless, all bromine being reduced to colorless bromide.