Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonia : NH3
hydrochloric acid : HCl
sodium thiocyanate : NaSCN
ammonium thiocyanate : NH4SCN
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
coordination
precipitation
redox
Summary:
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Copper (II) reacts with thiocyanate in a complex way.
Description:
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Experiment 1 (fairly concentrated solutions)
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Add an excess amount of a solution of NH4SCN to a solution of copper
sulfate: The resulting liquid is grass-green and clear. After approximately
half a minute the solution becomes opaque and the color changes from
grass-green to grey-green. After a few minutes one can see clearly that
black particles precipitate from the liquid. The liquid itself does not
become dark. It is a clear liquid with fairly large black particles in it.
The color of the liquid itself becomes dirty-green.
After a longer time of standing (all larger black particles have settled)
the color of the liquid changes from dirty-green to dirty green/yellow. Still
very small black particles precipitate from the liquid.
After an even longer time a second type of precipitate appears. The liquid
produces very fine grey/white particles, which form a thin layer on the
thicker black layer, which precipitated first. After a few hours the situation
is as follows:
---------------------- < surface of liquid
clear dirty-green/
yellow liquid
---------------------- < top of grey/white layer
thin grey/white layer
---------------------- < top of black layer
thick black layer
====================== < bottom of glass tube
Experiment 2 (dilute solutions):
---------------------------------
The same experiment is done with highly dilute solutions of ammonium
thiocyanate and copper sulfate: The result is that a clear grass-green
solution appears, which becomes opaque after several minutes (instead
of half a minute). A very finely divided precipitate appears, which is
grey. It takes exceedingly long for the precipitate to go to the bottom
of the test tube.
Experiment 3 (very concentrated solutions):
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The same experiment is done with very concentrated solutions of sodium
thiocyanate and copper sulfate: As soon as the two liquids come in contact
with each other a thick black precipitate appears, which goes to the
bottom of the test tube quickly. Many minutes later, a thin layer of a
grey precipitate covers the thick black layer. The grey precipitate is
finely divided.
Experiment 4 (fairly concentrated solutions):
----------------------------------------------
An experiment is done with a fairly concentrated solution of ammonium
thiocyanate and copper sulfate: When the liquids are mixed, a dark green
(but clear) liquid is formed. After a few seconds it becomes opaque and
a black precipitate goes to the bottom of the test tube quickly. After
a few minutes a thick black layer is at the bottom of the test tube and
a dirty yellow/green liquid is above the black layer, with finely divided
grey/white particles in it.
Adding acid:
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A portion of the black precipitate and the white/grey precipitate are
collected by decanting the yellow/green liquid from them. The two kinds
of precipitates become mixed and the total appearance after decanting
is dark grey. A large quantity of hydrochloric acid (10% by weight) is
added and the liquid is shaken:
No real change is visible. It might be that the particles of the light
part of the precipitate become slightly larger, but this is hard to
see. After some time a dark grey precipitate is in the test tube and
a pale clear liquid above it.
Decant the liquid and keep the precipitate and add an excess amount of
ammonia (15%): The precipitate completely dissolves and the liquid becomes
deep blue and clear.
Adding ammonia:
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A small portion of ammonia (15%) is added to the result of adding concentrated
solutions of copper sulfate and ammonium thiocyanate: The black precipitate
disappears on shaking, the liquid becomes deep blue. After 10 minutes of
standing, the liquid is deep blue and clear and a thick layer of a white
to white/grey precipitate is at the bottom of the test tube.
The blue liquid, together with the white precipitate are divided in two
equal portions.
To the first portion an excess amount of HCl (10%) is added: A black
precipitate is formed immediately. This result is called (A).
The second portion is allowed to stand for a while. The clear deep blue
liquid is decanted and an excess amount of NH3 (15%) is added to the
white/grey precipitate: The precipitate dissolves on shaking and the
liquid becomes deep blue and clear. This result is called (B).
(A) and (B) are added to each other: The resulting liquid becomes dark
blue and clear.
Remark:
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The results of the experiments depend on concentration of the thiocyanate
and copper (II) ions. It does not matter whether sodium thiocyanate or
ammonium thiocyanate is used. At least three different compounds appear
on reaction of copper (II) with thiocyanate:
- a grass-green coordination complex of copper (II) with thiocyanate
- a black solid
- a white solid