Description of experiment
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Needed compounds:
-----------------
ferric chloride : FeCl3 . 6H2O
hydrogen peroxide : H2O2
ammonia : NH3
sulphuric acid : H2SO4
hydrochloric acid : HCl
sodium sulfite : Na2SO3
ammonium thiocyanate : NH4SCN
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
------
elem=Cu
coordination
precipitation
redox
Summary:
--------
Copper (II) reacts with thiocyanate in a complex way.
Description:
------------
Copper sulfate and ammonium thiocyanate are allowed to react with each other
and to stand for a while. The liquid is acidified and a large quantity of
sodium sulfite is added. The liquid is shaken until the sodium sulfite
is dissolved and then the precipitate is allowed to settle.
With an excess amount of NH4SCN and acidified with H2SO4,
the final result is the following (I):
---------------------- < surface of liquid
pale yellow, slightly
opaque liquid (color
of sulphur suspension)
---------------------- < top of white layer
very fine white solid
---------------------- < top of black layer
thick black layer
---------------------- < top of grey layer
white/grey solid
====================== < bottom of glass tube
With an excess amount of copper sulfate and acidified with
HCl, the final result is the following (II):
---------------------- < surface of liquid
yellow/green, slightly
opaque liquid
---------------------- < top of white layer
very thin white layer
---------------------- < top of dark grey layer
thick dark grey layer
====================== < bottom of glass tube
If (I) is shaken and allowed to settle again, then the situation changes.
The black precipitate becomes lighter, until it is pale grey. The change
from black to pale grey takes a few minutes. If the precipitate has settled,
then the liquid above it is almost colorless and clear.
If (II) is shaken and allowed to settle again, then a similar phenomenon
is observed. The precipitate has turned pale grey and the liquid is
dirty green (mossy color).
Add all of (I) and (II) to each other and shake well and let settle: The
result of this action is a pale grey precipitate and a pale yellow/green
clear liquid above the precipitate.
The liquid is decanted from the precipitate and the precipitate is washed
once with water. After this, the precipitate is divided in three equal
portions (III), (IV) and (V).
Add H2O2 (3%) to (III): No visible reaction, no formation of bubbles of
oxygen.
Add NH3 (15%) to (IV): No clear change, liquid turns slightly pale blue.
The precipitate does not change, it does not dissolve. After one minute
the liquid turns deep blue at the surface (probably due to oxidation of
a colorless Cu(I) complex with ammonia to the intensely colored Cu(II)
complex with ammonia by oxygen from the air).
Add H2O2 (3%) to the liquid with ammonia and the white precipitate in it:
A violent reaction occurs, a lot of gas is evolved. All of the white
precipitate can be dissolved easily by adding sufficient H2O2. The final
color of the liquid is deep blue.
Add a slightly acidified solution of FeCl3.6H2O to (V): The liquid becomes
intensely colored and opaque. The color of the liquid is red/brown and the
white precipitate can still be recognized in the red/brown liquid.
Remark:
-------
The black precipitate of copper(II) and thiocyanate apparently reacts
with acidic sulfite solutions and yields a white solid. Probably a redox
reaction between the sulfite and the black compound occurs, which changes
the black compound to a white compound.