Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
nitric acid : HNO3
hydrochloric acid : HCl
ferric chloride : FeCl3 . 6H2O
copper sulfate penta hydrate : CuSO4 . 5H2O
silver nitrate : AgNO3
potassium metabisulfite : K2S2O5
Class:
------
elem=S,Ag,Cu,Fe
coordination
redox
Summary:
--------
This sequence of experiments shows that sulfite is capable of reducing
silver (I), copper (II) and iron (III), but that these reactions all
show their own peculiarities. Sulphur in the +4 oxidation state is a
mild reductant.
Description:
------------
Sequence 1:
------------
Add a solution of silver nitrate to a solution of potassium metabisulfite:
Formation of a white precipitate.
Boil liquid, with the precipitate: The precipitate changes color. It goes
from white --> beige --> sepia --> brown --> brown/grey --> grey (with a
brown hue).
Decant the colorless liquid and rinse with distilled water for two times
and then add dilute HCl (appr. 10% by weight) to the precipitate: The
precipitate does not change. Hence, the precipitate is not silver oxide,
otherwise it would have become white again, due to formation of silver
chloride.
Again, rinse the precipitate two times with distilled water and add some
nitric acid (appr. 50% by weight): Formation of a gas (probably NO and/or
NO2, the gas mixture above the liquid becomes brown). The precipitate
partly dissolves. However, part of the precipitate does not dissolve. This
part is light yellow. It looks as if part of the precipitate was silver,
strongly contaminated with something else.
Sequence 2:
------------
Add a solution of copper sulfate to a solution of potassium metabisulfite:
The liquid becomes green, it remains clear.
Boil the green liquid: The liquid shifts color towards yellow. At a certain
moment, a dark red precipitate is formed. The precipitate quickly sinks to
the bottom of the test tube. The liquid above the precipitate becomes pale
blue (was there an excess amount of copper sulfate?).
Decant the light blue liquid and add a little amount of dilute HCl (appr.
10% by weight) to the red precipitate: The liquid becomes turbid white/
pale yellow for a while and then it becomes clear again. The color is
yellow/brown. The precipitate was not copper metal, but copper (I) oxide,
which reacts with the dilute hydrochloric acid, forming CuCl, which does
not dissolve well. This compound quickly dissolves in the excess hydrochloric
acid, forming [CuCl2]- complex ions.
Add a lot of water: The liquid remains clear and becomes almost colorless,
just a faint blue color can be observed.
Sequence 3:
------------
Add a solution of ferric chloride (acidified with a small amount of HCl in
order to prevent hydrolysis of the ferric chloride) to a solution of
potassium metabisulfite: The liquid becomes deep red/brown, it remains clear.
Boil liquid for a while: The liquid becomes lighter and remains clear. After
a while, the liquid is almost colorless. The dark red/brown compound is a
coordination complex, which is a transient compound. The ferric center atom
is reduced to ferrous ions, destroying the dark red/brown complex.