Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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copper sulfate penta hydrate : CuSO4 . 5H2O
sodium hypophosphite : NaH2PO2
sulphuric acid : H2SO4
hydrochloric acid : HCl
Class:
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elem=P,Cl,Cu
redox
Summary:
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Copper(II) can be reduced to the very peculiar compound CuH by hypophosphorous
acid, but only under very specific conditions. The compound CuH is quite
unstable and easily looses hydrogen.
In the presence of chloride ions, no CuH is formed, but CuCl is formed instead.
Description:
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Dissolve some sodium hypophosphite in water and set this aside. This solution
is colorless and odorless.
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Prepare a solution of copper sulfate and add some of the solution of sodium
hypophosphite: No reaction occurs, the liquid remains clear and sky blue.
Heat the liquid until boiling: No reaction occurs, the solution remains sky
blue and clear. Hypophosphite ion is not capable of reducing copper(II) to a
lower oxidation state. The experiments below show that the free acid is needed
for reduction of copper(II).
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Prepare a solution of copper sulfate and add a small quantity of dilute
sulphuric acid and then add some of the solution of sodium hypophosphite: No
reaction occurs, the liquid remains clear and sky blue.
Heat the liquid: Well before the liquid starts boiling, a reaction occurs. A
brown solid is formed in the liquid, which clumps together and forms loosely
packed lumps. Also some copper metal is plated onto the glass.
Loosely stopper the test tube (e.g. with a ball of wadding or tissue paper) and
keep on heating: The brown material produces a colorless gas. At a certain
point in time, the formation of gas is quite vigorous. The color of the
material shifts slightly, it becomes a little more reddish brown.
Take away the ball of wadding or tissue paper and quickly hold the open end of
the test tube near a flame: A small whoop is produced with an orange flame. The
gas is hydrogen gas.
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Prepare a solution of copper sulfate and add a small quantity of dilute
hydrochloric acid and then add some of the solution of sodium hypophosphite: No
reaction occurs, the liquid remains clear and sky blue. The amount of chloride
is so low that there is no visible green color, due to formation of the
chloro-complex of copper(II).
Heat the liquid: Well before the liquid starts boiling, the blue color first
fades and at a certain point in time, the liquid becomes turbid and white. A
thick and dense white precipitate is formed, which quickly settles at the
bottom.
Add more hydrochloric acid: All of the white precipitate dissolves. A
completely colorless solution is obtained.
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The brown compound in the second experiment is CuH. This decomposes on heating
to Cu and H2. The Cu also is brown in this finely divided state and the change
from CuH to Cu is not easily observed.
The white compound in the third experiment is CuCl. This dissolves in excess
hydrochloric acid, giving the colorless CuCl2(-) ion. Normally, such solutions
with CuCl2(-) are very prone to aerial oxidation, but when excess
hypophosphorous acid is present, then the solution remains colorless.