Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium hydroxide : NaOH
hydrochloric acid : HCl
nitric acid : HNO3
cadmium selenide : CdSe
cadmium sulfide : CdS
Class:
------
elem=Cd;S;Se
redox
Summary:
--------
Cadmium selenide reacts vigorously with nitric acid, producing nitrous
fumes and a yellow/orange solid, which is filled with many small gas bubbles
and hence remains floating on the liquid.
Cadmium sulfide gives a similar reaction, but now a pale yellow solid is
formed. This yellow solid is sulphur.
Description:
------------
Remark: The cadmium selenide may contain some cadmium sulfide, the color of
the chemical used is dark red, but this does not assure that it is totally
free of sulfide.
The cadmium sulfide used is bright yellow and is known not to contain any
selenide.
Sequence 1:
-----------
Add some nitric acid (appr. 50% by weight) to some solid cadmium sulfide: A
vigorous reaction occurs, some nitrous fumes are produced and a pale yellow
chunk of some solid is formed. It floats on the liquid, it is filled with
gas bubbles. There is no smell of H2S.
Take out the chunk and put it in a concentrated solution of NaOH: No visible
reaction. After a minute or so, however, the liquid is very pale yellow.
Heat the NaOH solution, together with the yellow mass: Now the liquid becomes
intense yellow brown and the solid dissolves. A typical smell of hot sulfur
can be observed. The liquid becomes almost clear.
Add some hydrochloric acid to the intense yellow liquid: A strong smell of
rotten eggs can be observed, the liquid becomes light yellow and turbid.
Sequence 2:
-----------
Add some nitric acid (appr. 50% by weight) to some solid cadmium selenide: A
vigorous reaction occurs, some nitrous fumes are produced and an orange
chunk of some solid is formed. It floats on the liquid, it is filled with
gas bubbles.
Take out the chunk, rinse several times in tap water, and put it in a
concentrated solution of NaOH: The chunk dissolves in the NaOH-solution, part
of it becomes black / dark grey. Finally, the chunk has dissolved completely.
At that point the liquid is fairly dark orange/brown, some tiny black particles
still exist, the liquid is almost clear.
The dark orange/brown color may be due to formation of a seleno-sulfide
compound. This color can also be observed, when elementary selenium is
dissolved in a solution containing sulfide.
Add an excess amount of a solution of Na2SO3: The liquid becomes light yellow
and almost clear. The orange color disappears.
Add an excess amount of hydrochloric acid (appr. 30% by weight): The liquid
becomes orange red and after a while, a dark red precipitate is formed. This
dark red precipitate probably is selenium.
BEWARE: Sequence 2 MUST be done outside with wind coming from behind. The
reaction with cadmium selenide and nitric acid probably just yields nitrous
vapors (nasty on their own already), but there also is a chance of making
H2Se, which is really nasty. The latter is cumulative!