Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
copper sulfate penta hydrate : CuSO4 . 5H2O
hydroxyl amine hydrochloride : NH2OH.HCl
Class:
------
elem=N,Cu
precipitation
coordination
redox
Summary:
--------
When a solution of copper sulfate is added to a solution of hydroxyl ammonium
chloride in dilute solution of NaOH, then a dirty green precipitate is formed.
At some places the precipitate becomes yellow. When the liquid is shaken, then
all of the precipitate dissolves and a colorless liquid is obtained. On
standing, a thin layer of solid material is formed on the surface of the
liquid. This layer has a dirty green color.
Hydroxyl amine reduces copper(II) at high pH and a colorless complex of
copper(I) is formed, which at really high pH becomes unstable with formation of
hydrous copper(I) oxide.
Description:
------------
Dissolve some solid NaOH and solid hydroxyl ammonium chloride (NH2OH.HCl) in
water: A colorless solution is obtained.
Add a small amount of a solution of copper(II) sulfate: A dirty green
precipitate is formed. At some spots where remains of solid NaOH are present,
the precipitate is yellow.
Shake the liquid with the precipitate: All of the precipitate dissolves and a
colorless solution is obtained. Most likely this is a complex of hydroxyl amine
with copper(I) ions, possibly with chloride being incorporated in the complex
as well.
Pour some of this liquid along a glass wall: An olive-green thin layer remains
sticking to the glass. One can see the formation of the olive-green layer in
seconds. Oxygen from the air oxidizes the complex in the liquid.
Allow some of the liquid to stand in contact with air for several hours: A thin
solid layer/crust is formed on top of the liquid. This crust has an olive-green
color. The liquid itself remains colorless.
After one day, shake the liquid: The crust breaks into many small particles
which float around in the colorless liquid. The particles do not dissolve (or
they dissolve very slowly).
Add a few drops of 10% hydrogen peroxide: Quickly, the liquid becomes turbid
and cyan/green. All dark green particles quickly disappear and a fine pale
green/cyan precipitate remains.
---------------------------------------------------
Hydroxyl amine is a good reductor of copper(II) ions at high pH. This leads to
formation of copper(I) which apparently forms a colorless complex with hydroxyl
amine and/or chloride ion. Addition of hydrogen peroxide leads to formation of
a copper(II) compound, most likely some oxychloride of copper(II).