Description of experiment
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Needed compounds:
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sodium persulfate : Na2S2O8
hydrochloric acid : HCl
copper sulfate penta hydrate : CuSO4 . 5H2O
ascorbic acid : C6H8O6
Class:
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elem=C,Cu
coordination
redox
Summary:
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Copper(II) ions form a pale green complex with a yellowish hue. When the
solution is heated to boiling, then the copper(II) is reduced to metallic
copper.
Description:
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Dissolve some copper sulfate in a few ml of water in test tube A.
Dissolve a similar amount of ascorbic acid in a few ml of water in test tube B.
Mix the two solutions: The blue solution of test tube A mixes with the
colorless solution of test tube B and at once, the color changes to pale green
with a yellow/brown hue. The liquid remains clear.
Heat the clear green liquid, until it just starts boiling: The liquid becomes
nearly colorless (very pale blue) and a compact red precipitate is formed. The
precipitate is dense and very quickly settles at the bottom.
Decant the liquid from the precipitate and add some concentrated hydrochloric
acid: The precipitate does not dissolve, nor does it seem to react. It remains
red. The acid becomes yellow. That can be explained, because a small amount of
the decanted liquid still was present and this liquid still contained a little
copper(II). In concentrated hydrochloric acid, copper(II) forms a strongly
colored yellow complex.
Add some solid sodium peroxodisulfate and heat the liquid: The precipitate
quickly dissolves and the liquid becomes intense yellow/brown.
Dilute with a lot of water: The liquid becomes very pale blue.
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Explanation: Copper(II) ions form a green complex with ascorbic acid. This
complex is fairly stable at room temperature, but on heating it decomposes. The
ascorbic acid then reduces the copper(II) to metal, which forms a red
precipitate.
The red precipitate really is the metal and not copper(I) oxide. On addition of
concentrated hydrochloric acid, the latter would dissolve at once. The metal
does not react with hydrochloric acid, but on addition of the strongly
oxidizing peroxodisulfate, the metal quickly dissolves and forms a strongly
colored complex with the concentrated hydrochloric acid.