Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium sulfite : Na2SO3
hydrogen peroxide : H2O2
ammonia : NH3
chrome alum : KCr(SO4)2 . 12H2O
Class:
------
elem=Cr
coordination
redox
Summary:
--------
Chromium (III) does not quickly build a coordination complex with ammonia,
such as is the case with copper (II) and nickel (II). Even in a slightly
alkaline environment of dilute ammonia, chromium (III) can be oxidized to
chromium (VI) by the action of hydrogen peroxide. Reduction, back to
chromium (III) is not easily accomplished in alkaline environment.
Description:
------------
Add a little quantity of dilute ammonia (5% by weight) to a solution of
chrome alum: A greyish/pale green precipitate is formed.
Add much more ammonia (excess amount of ammonia): The precipitate does not
dissolve, the precipitate floats through the liquid in the form of little
flakes.
Add an excess amount of H2O2 (3% solution) and heat the liquid to appr.
60 C: The liquid turns yellow like a lemon within a few minutes and it becomes
clear. The precipitate is converted to the yellow chromate [CrO4]2-.
Add a crystal of sodium sulfite: The liquid starts foaming (creation of
oxygen). The liquid remains lemon/yellow and clear. No chromate is reduced
to chromium (III).