Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
ammonia : NH3
hydrogen peroxide : H2O2
bleach : NaClO
sodium chlorate : NaClO3
sodium hydroxide : NaOH
nickel sulfate : NiSO4 . 6H2O
Class:
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elem=Ni,Cl,O
redox
precipitation
Summary:
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Nickel in nickel (II) hydroxide is oxidized to a higher oxidation state by
hypochlorites, but not by chlorates. The oxidation product is reduced by
hydrogen peroxide and ammonia. Heating of the oxidation product makes it
more resistant to reduction and to breakdown by acids.
Description:
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Add a solution of sodium hydroxide and sodium chlorate to a solution of nickel
sulfate: Formation of a pale green precipitate of nickel (II) hydroxide.
Add some bleach (appr. 4 % active chlorine) to a solution of nickel sulfate:
Formation of a 'dirty'-green precipitate, which slowly turns darker. Finally
the precipitate is black or very dark brown. A gas is evolved (probably this
is oxygen). Evolution of the gas continues slowly for a long time.
The liquid with the black precipitate is called BLACK.
Add some hydrogen peroxide (appr. 10% by weight) to BLACK: The black
precipitate turns light green (plain color of nickel (II) hydroxide). A small
amount of a gas is evolved (probably oxygen).
Add some ammonia (appr. 5% by weight) to BLACK: The black precipitate
dissolves, the liquid becomes light purple/blue and clear. Above the liquid
a white smoke/fume can be observed. The change from turbid and black to clear
and purple/blue is not momentaneous, it takes approximately 10 seconds.
Apparently the addition of ammonia causes reduction of the black compound to
nickel (II), which then forms the well-known blue/purple nickel-ammine complex.
Add some HCl (appr. 10% by weight) to BLACK: Liquid becomes clear almost
instantaneously and light green. No visible evolution of a gas.
Heat BLACK for a while (gently boiling): Precipitate remains black, but it
appears as if the particles are getting larger.
Add an excess amount of ammonia (5% by weight) to the heated BLACK: The
precipitate does not dissolve anymore, not at room temperature, not on
heating.
Add an excess amount of HCl (10% by weight) to the heated BLACK with ammonia:
Now the precipitate does dissolve, but it does not dissolve quickly. Besides
this, some white smoke is formed, due to formation of NH4Cl from HCl-fumes
and NH3.