Description of experiment
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Needed compounds:
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ethanol : CH3CH2OH
sodium azide : NaN3
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=N,Cu
coordination
Summary:
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Copper(II) ions are capable of forming a mixed azide/ammonia complex which has
energetic properties.
Description:
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Take 200 mg of copper sulfate 5-hydrate and dissolve in an as small as possible
amount of warm water. To this, add drops of ammonia (12%): A blue precipitate
appears. When more drops are added, then the precipitate redissolves again. A
deel royal blue solution is obtained.
Keep adding drops until the precipitate just has redissolved. Do not add more
ammonia: The final result is a totally clear liquid, which is very dark blue.
This is a solution of the tetrammine copper(II) complex, which has a deep royal
blue color.
Dissolve over 100 mg of sodium azide in a few drops of water and add this
solution to the still warm deep blue solution: Each time when a drop of the
concentrated solution of sodium azide is added, the color seems to go to green
and at some spots even green/brown. On shaking the liquid becomes deep blue
again. When all of the sodium azide is added, then the liquid still is deep
blue, but the color is not as bright, it is somewhat more dull blue, not the
beautiful royal blue.
Carefully heat this dark blue liquid, but do not boil it: The color shifts from
dark blue to dark blue/green. Apparently some of the ammonia ligands are
replaced by azide ligands and this results in a shift of color.
Add an equal volume of 96% ethanol to the liquid: A beautiful deep blue/purple
precipitate is formed, which fairly quickly settles at the bottom. Let it stand
for an hour or so, then all of it has settled at the bottom. The liquid above
the precipitate is fairly dark green/cyan and clear. The dark blue/purple
precipitate can be rinsed with ethanol and dried and gives a nice dark
blue/purple powder when it is totally dry. This precipitate looses its deep
blue color when it is heated in a flame. It does not
Decant the liquid from the precipitate and allow the liquid to stand for a few
days: Nice dark green crystals are formed, which have a glittering appearance.
The seem to be needle-shaped, but the crystal mass was somewhat messy, so their
shape could not be determined 100% sure. They were nice and glittering though.
Put the dark green crystals on a filter paper and make them mostly dry. Then
allow them to dry completely in a petri dish in a warm place: A dry
non-hygroscopic solid is obtained, which consists of many small (salt-grain
sized) glittering granules.
Put some of the dark green solid on the tip of an old screw driver and put this
in the flame of an alcohol burner: A lot of crackling noise is produced, but no
hard bangs are obtained. Some of the larger crystals give a high-pitched PENG,
but the sound is not really impressive. When the crystals puff away, they
produce a fine cloud of black particles, which give a beautiful green/cyan
color to the flame of the alcohol burner.
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The dark blue/purple solid most likely is (impure) [Cu(NH3)4]SO4.H2O. It has no
energetic properties and becomes light blue when heated in a flame. When heated
it looses its ammonia.
The dark green solid most likely is a mixed copper(II) ammine/azide complex,
possible of basic character. This complex has some energetic properties, but it
certainly is not massively explosive. Under other conditions, this experiment
is said to result in formation of Cu(NH3)2(N3)2, which is said to have highly
energetic properties. In the experiment, described here, the dark blue liquid
was heated after adding the azide. This may result in formation of a basic
complex. Adding much more azide, and not heat