Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
potassium permanganate : KMnO4
sodium sulfite : Na2SO3
sodium persulfate : Na2S2O8
sulphuric acid : H2SO4
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
------
elem=Cu
redox
Summary:
--------
Copper builds higher oxides than CuO when strong oxidizing agents are
present in alkaline environments. Probably these are not copper (III)
compounds, but the oxo-ion probably is replaced by peroxo or superoxo.
Description:
------------
Sequence 1:
-----------
Add a solution of NaOH to a solution of copper sulfate: Formation of a bright
blue precipitate, which looks like a little sponge.
Add some H2O2 (3% solution): The precipitate becomes yellow/brown. Bubbles
of oxygen are evolved. After some time, the precipitate becomes green/brown.
Add more H2O2 (now a 30% solution is used): The precipitate becomes yellow/
brown again. After a while the precipitate has become very dark brown.
While standing, oxygen is evolved smoothly, without ever becoming vigorous.
Add an excess amount of dilute sulfuric acid (appr. 15% by weight): All of
the precipitate dissolves, the liquid becomes light blue and clear.
Sequence 2:
-----------
Add a solution of NaOH to a solution of copper sulfate: Formation of a bright
blue precipitate, which looks like a little sponge.
Add some solid Na2S2O8: The solid dissolves, no further changes are observed
at first glance. However, after appr. 1 minute, the precipitate has become
slightly darker. After appr. 5 minutes the precipitate has turned almost
black (dark blue). Finally the precipitate turns black completely.
Add an excess amount of dilute sulfuric acid (appr. 15% by weight): All of
the precipitate dissolves, the liquid becomes light blue and clear.
Sequence 3:
-----------
Add some solid Na2S2O8 to a solution of copper sulfate: The solid dissolves,
the liquid remains light blue.
Add a solution of NaOH: Formation of a blue precipitate, which quickly turns
black.
Add one drop of a solution, containing Mn2+ (which is prepared from a
slight excess amount of sodium sulfite, added to an acidic solution of
potassium permanganate). Shake the liquid after adding this drop and then
add an excess amount of dilute sulfuric acid (appr. 15% by weight): The
black precipitate dissolves, a colorless and odourless gas is evolved
(probably oxygen). In the liquid a minute quantity of a solid brown/black
compound remains. The liquid is light blue, just as a plain solution of
copper sulfate. The brown/black compound probably is MnO2.
Remark on sequence 3:
---------------------
Copper (II) hydroxide reacts with persulfate forming a black compound. A
similar effect is observed with nickel (II) hydroxide. The copper-compound
is not capable of oxidizing very small quantities of manganese (II) in
acidic solution to permanganate, the nickel (II) compound can do this (see
experiments with nickel, persulfate and manganese (II)).