Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrochloric acid : HCl
potassium bromide : KBr
sodium chloride : NaCl
cobalt sulfate : CoSO4.7H2O
copper sulfate penta hydrate : CuSO4 . 5H2O
aluminum : Al
Class:
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elem=Al;Cu;Co
redox
Summary:
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Aluminum reacts vigorously with water, when its passivating layer of oxide
is effectively destroyed. This can be achieved by using tetrachloro or
tetrabromo complexes of copper (II).
Acid can also be used to destroy the passivating layer, but this takes
considerably more time.
Cobalt has a similar effect as copper (II), but it is less pronounced.
Description:
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Sequence 1:
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Add some aluminum foil (plain household foil) to a solution of sodium chloride:
No visible reaction.
Sequence 2:
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Add some aluminum foil to a concentrated solution of copper sulfate: No
visible reaction. On warming to appr. 50C there still is no visible reaction.
Let liquid cool down and add some sodium chloride and shake: Within a matter
of seconds, when part of the sodium chloride dissolves, the aluminum starts
reacting vigorously. Some brown metallic copper is produced and a lot of gas
is produced (H2). The liquid becomes warm.
Sequence 3:
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Dissolve a fairly large amount of copper sulfate in a concentrated solution
of sodium chloride: The solid becomes covered by a green jelly-like compound,
which slowly dissolves. The liquid becomes green. When some heat is applied,
then the solid quickly dissolves and the liquid becomes fairly intense grass-
green. The green color is due to formation of the [CuCl4]2- complex ion.
Cool down the liquid: The green color shifts a little towards blue. When the
liquid has reached room temperature, it is cyan/green.
Add some aluminum foil: The foil reacts vigorously, within a few seconds
after immersion in the liquid. The liquid becomes turbid and dark brown
(metallic copper?). A lot of gas is produced and the liquid becomes warm.
Sequence 4:
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Add some aluminum foil to a concentrated solution of copper sulfate: No
visible reaction.
Add some potassium bromide and shake: Within a matter of seconds, when part of
the potassium bromide dissolves, the aluminum starts reacting vigorously. Some
brown metallic copper is produced and a lot of gas is produced (H2). The
liquid becomes warm. Before the aluminum reacts, the liquid becomes brown.
This brown color is due to formation of the [CuBr4]2- complex ion.
Sequence 5:
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Dissolve a fairly large amount of copper sulfate in concentrated hydrochloric
acid (appr. 30% by weight): The liquid becomes dark brown/yellow. Some heat
is needed in order to dissolve the copper sulfate quickly.
Let liquid cool down and add aluminum foil (careful, this experiment is not
without risk): The aluminum reacts extremely violent, almost immediately
after it is immersed in the liquid. The liquid becomes intensely hot and
the gas produced probably is a mixture of hydrogen and water vapour (boiling).
The evolution of gas and heat is so violent, that the liquid tends to be
swirled out of the test tube! After the reaction, the liquid is turbid and
greyish/blue.
Sequence 6:
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Add some aluminum to some concentrated hydrochloric acid (appr. 30% by weight):
At first glance, there is no reaction. After a minute or so, a nice, constant
evolution of gas can be observed. The evolution of gas, however, becomes
faster and faster. The temperature rises. After a few minutes, the reaction
is violent, until all aluminum has reacted. After the reaction, the liquid
is turbid and grey.
Sequence 7:
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Take 1 part of copper sulfate solution (appr. 0.5 mol/l) and mix with 3 parts
of hydrochloric acid (appr. 10% by weight): The liquid becomes light green/
blue.
Add some aluminum foil to the liquid: The foil dissolves, while developing
a large amount of gas. The reaction produces a fairly large amount of heat,
but it is not so violent, that it goes out of control (the risk of this
exists for sequence 5).
Keep a flame in front of the test tube: A 'whoop' sound can be heard. Another
time a louder popping sound can be heard. These noises are due to combustion
of hydrogen. At a certain time, the amount of hydrogen evolved during the
reaction was enough to keep the fire going at the open end of the test tube.
Sequence 8:
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Take some cobalt sulfate and add this to some concentrated hydrochloric acid
(appr. 30%): The solid dissolves, the liquid becomes dark blue/cyan. This
color differs from the color of a pink cobalt (II) solution with a lot of
chloride, when it is heated (such a solution has a much deeper royal blue
color).
Add some aluminum foil: The foil dissolves and the reaction becomes fairly
violent. Compared to using plain hydrochloric acid (sequence 6), the reaction
starts more quickly (within seconds), but the final reaction is not more
violent, than with plain hydrochloric acid. After the reaction,the liquid
still is dark blue/cyan, but it is turbid.
Dilute with a lot of water: The liquid becomes pale pink and a fairly large
amount of a black precipitate is formed, which slowly produces a colorless
gas.
Decant the liquid and rinse the precipitate a few times with water and then
add some concentrated nitric acid: The precipitate dissolves and the liquid
becomes pale pink. Apparently the precipitate was metallic cobalt.