Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium sulfite : Na2SO3
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
------
elem=S,Cu
coordination
redox
Summary:
--------
Copper(II) ion and sulfite ion react in aqueous solution. A dirty green/yellow
precipitate is formed when solutions with these ions are mixed. When there is
excess sulfite, then on slight heating the precipitate redissolves and the
liquid becomes colorless.
Apparently a coordination complex is formed with copper(I). This coordination
complex is very air-sensitive. It reacts with air, giving a dirty
brown/green/yellow precipitate.
Description:
------------
Prepare a solution of copper sulfate and add an excess amount of a solution of
sodium sulfite to this: A dirty green/yellow precipitate is formed. On shaking,
this precipitate is divided through all of the liquid.
Heat the liquid somewhat: The precipitate dissolves again. Immediately after
all of the precipitate has dissolved, the liquid is yellow with a green/brown
tinge. After a while, the liquid becomes colorless. At the surface, the liquid
remains fairly intense yellow/green and liquid, sticking at the glass, becomes
turbid and dirty yellow/brown with a green tinge.
Shake the test tube for a while assuring that there is a lot of contact with
fresha air: The liquid becomes yellow/green and slightly turbid.
Stop shaking: The liquid slowly turns clear and colorless again.
--------------------------------------------------------
It seems that at first, copper(II) gives a precipitate with sulfite, but on
heating there is internal oxidation/reduction of this copper(II) sulfite
precipitate. The copper(I) goes into solution as a colorless complex with more
sulfite or with oxidation products of sulfite. This complex is very easily
oxidized by oxygen from the air and on oxidation it appears that newly formed
copper(II)sulfite appears, giving rise to the dirty yellow/green color.