Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sulphuric acid : H2SO4
sodium nitrite : NaNO2
chromium (III) chloride hexahydrate : CrCl3.6H2O
Class:
------
elem=N,Cr
coordination
Summary:
--------
Chromium (III) chloride initially gives a green solution in water, which on
standing turns violet/grey. The chromium/chloride complex slowly is changed to
plain aqueous chromium and free chloride ion. This takes a few days before
completion.
With nitrite ions, a purple complex is formed.
Description:
------------
Sequence 1:
------------
Add some dark green chromium chloride (hexahydrate) to water: The solid quickly
dissolves and the liquid becomes deep green. The green color is a really nice
and saturated color.
Add a spatula full of sodium nitrite to the green solution: The solid quickly
dissolves and the liquid remains green. In the course of a few minutes,
however, the color changes to a bright and deep red/purple.
Heat the liquid, until it almost starts boiling: A colorless gas is evolved,
the gas mixture above the liquid turns light brown. The liquid itself turns
green, but not as bright and saturated as the initial green color. The
colorless gas most likely is NO.
Sequence 2:
-----------
Add some dark green chromium chloride (hexahydrate) to water: The solid quickly
dissolves and the liquid becomes deep green. The green color is a really nice
and saturated color.
Let the liquid stand for at least a few days: The color has changed to a
purplish/grey/blue shade, just as plain aqueous chromium (III) ions. So, the
chloro-complex is converted to a plain aqua-complex, the chloride ligands are
replaced by water ligands.
Add a spatula full of sodium nitrite to the grey/purple/blue solution: The
solid quickly dissolves and the liquid turns dull green. In the course of a few
minutes, however, the color changes to bright and deep red/purple, just as in
sequence one. So, the end-result is the same.
Add some dilute sulphuric acid: A colorless gas is produced and the air above
the mix becomes brown. The liquid becomes dull blue. The color resembles that
of plain aqueous chromium ion, but it is somewhat more blue. So, either there
still is some complex formation with nitrite, NO or NO2 or the blue color is
due to some N2O3 in the liquid.