Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonia : NH3
cupric chloride dihydrate : CuCl2 . 2H2O
hydrazine dihydrochloride : N2H4.2HCl
Class:
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elem=N,Cu
coordination
redox
Summary:
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Hydrazine seems to be able to reduce copper(II) to copper(I) in mildly acidic
environments. In alkaline environments the reaction is fast and most likely
elemental copper is produced in the reaction.
Description:
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Sequence 1:
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Dissolve some copper(II) chloride in water: A blue/green solution is obtained.
Prepare a fairly concentrated solution of hydrazine dihydrochloride in water
and add the solution of copper(II) chloride to this solution: The solotion
becomes green, due to complex formation where copper(II) is coordinated to
chloride.
Heat the solution: A colorless gas is produced in the form of very fine
bubbles. Well before the solution really boils, a lot of fine bubbles are
produced. The color of the solution also gradually changes from green to pale
yellow/green. After a few minutes of boiling/bubbling, the liquid only has a
pale yellow color with a greenish hue, but it does not become colorless.
Stopper the test tube, assuring that no fresh air can go into the test tube,
and shake well: A lot of fine bubbles are produced, but even after some time,
the liquid does not become colorless.
Add the pale yellow/green solution to a small amount of a 5% ammonia solution:
A grey precipitate is formed, which has a somewhat reddish/brown hue.
Add much more ammonia: The precipitate mostly dissolves, but a dark brown/red
solid remains, which is flocculent and does not quickly settle at the bottom.
Sequence 2:
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Dissolve some copper sulfate in water and add this to a fairly dilute solution
of hydrazine dihydrochloride: The solution becomes light blue. No reaction can
be observed.
Add this solution to a large excess amount of 5% ammonia: The solution becomes
dark blue, but immediately a precipitate is formed. The dark blue color quickly
disappears again, when the liquid is shaken, and a colorless gas is produced. A
flocculent brown precipitate is formed again.
Remark: Hydrazine reduces copper(II) in mildly alkaline solutions and destroys
the dark blue copper(II) complex of ammonia. Apparently, hydrazine is capable
of reducing the copper all the way to the metal. This is different from the
behavior with hydroxyl amine, which reduces the blue copper(II) ammine complex
to a colorless copper(I) complex.
In acidic environments, copper(II) seems to be reduced to copper(I), but only
slowly and at elevated temperature. It is not clear why the solution does not
become completely colorless.