Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
hydrochloric acid : HCl
copper sulfate penta hydrate : CuSO4 . 5H2O
ferric chloride : FeCl3 . 6H2O
bleach : NaClO
potassium ferricyanide : K3 [Fe(CN)6]
sodium sulfide : Na2S . 3H2O
Class:
------
elem=S,Fe,Cu
redox
precipitation
Summary:
--------
Sulfide gives precipitates with some metals, which do not dissolve in
strong acids. These sulfides, however, can easily be dissolved, when an
oxidizing agent is used.
Sulfide is easily oxidized by moderately strong oxidizers.
Description:
------------
Sequence 1
-----------
Add a solution of potassium ferricyanide to a solution of sodium sulfide:
The liquid becomes pale yellow and turbid. Solid sulphur precipitates from
the liquid. The reaction is not instantaneous, it takes several seconds.
Sequence 2
-----------
Add some bleach (mixture of NaClO, NaCl and NaOH in an aqueous solution)
to a solution of sodium sulfide: the liquid instantaneously turns pale
yellow and turbid. The sulfide is oxidized immediately.
Add much more bleach to the pale yellow solution: the liquid remains pale
yellow and turbid.
Heat the liquid: The liquid remains pale yellow and turbid. The solid
sulphur is not further oxidized by the bleach.
Add an excess amount of HCl (10% by weight) and some additional bleach
to the milky liquid: The liquid remains turbid and becomes pale yellow/
green. Above the liquid the yellow/green color of Cl2-gas can clearly
be observed. Slowly the liquid becomes less and less turbid, but this takes
several minutes. The solid sulphur is oxidized further, but this process
proceeds slowly. Even after 15 minutes, the liquid still was not completely
clear, although at that time it was possible to see something behind 2 cm
of the liquid.
Sequence 3
-----------
Add a solution of ferric chloride to a solution of sodium sulfide: Formation
of a black precipitate, smell of H2S.
Add some bleach: The black precipitate turns orange/brown, like rust. The
sulfide is oxidized by the bleach and the new precipitate probably is
ferric hydroxide, mixed with sulphur.
Sequence 4
-----------
Add a solution of sodium sulfide to an excess amount of a solution of copper
sulfate: formation of a dark brown (almost black) precipitate. After the
precipitate has settled, the liquid above the precipitate is pale blue/green.
Add some bleach: The dark brown precipitate turns blue/green. This change is
not instanteneous. It takes a few tens of seconds, before everything is
changed and it was necessary to shake the test tube, in order to bring the
dark brown precipitate in intimate contact with the bleach.
Add some HCl (10% by weight): The liquid quickly becomes clear. A strong
suffocating odour of chlorine can be observed.
Sequence 5
-----------
Add a solution of sodium sulfide to an excess amount of a solution of copper
sulfate: formation of a dark brown (almost black) precipitate. After the
precipitate has settled, the liquid above the precipitate is pale blue/green.
Add a large excess amount of HCl (10% by weight): The precipitate does not
dissolve, no odour of H2S.
Add a small amount of bleach: Part of the precipitate dissolves, liquid
becomes clear, no milkyness of finely divided sulphur.
Add more bleach: the rmeaining part of the precipitate dissolves, the liquid
becomes completely clear and its color is green/blue.
Remarks:
Sulfide can be oxidized in alkaline environments, instantaneously when a
strong oxidizer is used, more slowly, when a less potent oxidizer is used.
The oxidation product is finely divided sulphur.
Further oxidation can be accomplished, but if this must go through the
finely divided sulphur, then it goes very slowly. If a direct path exists
for oxidation of sulfide in strongly acidic environments by a strong
oxidizer, then further oxidation proceeds much more quickly. This is the
case when the insoluble copper sulfide is oxidized in a fairly strong
solution of HCl.
A theoretical explanation for the observations may be the following:
Oxidation of sulfide is rapid.
Oxidation of sulfur is slow(er). Once present as a solid, floating around in
the liquid, the oxidation becomes much slower, because of less area of contact.
When copper sulfide is oxidized in acidic environment, the sulfide is rapidly
oxidized. Before the resulting sulfur can separate it is already further
oxidized.