Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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potassium metabisulfite : K2S2O5
sodium metabisulfite : Na2S2O5
Class:
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elem=S
redox
Summary:
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Sulfur is an extremely fascinating element in that it is capable of existing in
a wide range of oxidation states under normal conditions, from -2 (as in
Hydrogen sulfide) up through +6 (sulfuric acid). The following experiment
demonstrates simple redox chemistry involving Sulfur dioxide gas created in
situ.
Description:
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In this experiment, the only necessary reagent is a metabisulfite salt, the
common varieties of which are the potassium and sodium salts. Both of these
compounds are readily available from home-brewing suppliers, since they are
widely used for sterilizing equipment and preserving wine. When heated, both
compounds rapidly decompose, releasing Sulfur dioxide.
Na2S2O5 -> Na2SO3 + SO2
Further strong heating (most likley does not occur under these reaction
conditions) may cause the sulfite to decompose further, releasing more SO2.
Na2SO3 -> Na2O + SO2
Into a test tube, place a small scoop or spatula of sodium or potassium
metabisulfite. Hold the test tube at the very top with a heatproof clamp and
apply a source of heat, such as a torch or even an alcohol burner to the very
end of the tube. Very quickly, especially with a propane torch, droplets of
golden yellow elemental sulfur condense on the unheated walls of the test tube.
Further heating directed upon the droplets leads to a thermochromic color
change to red and the vaporization of the droplets. So
This reaction is believed to proceed by a simple redox mechanism involving
"disproportionation" of the sulfur dioxide into elemental sulfur and sulfur
trioxide. SO2 has sulfur in a +4 oxidation state. In the reaction, one in three
sulfur atoms is reduced all the way to the 0 state, while the remaining two are
oxidized to the +6 state.
3SO2 -> 2SO3 + S
Additional side reaction contributing to the effect may include the reduction
of sulfur dioxide by the sodium bisulfite. The rapidity of the sulfur
deposition upon heating seems to indicate that the above reaction, occuring in
the gas phase, is the most prevalent.