Description of experiment
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Needed compounds:
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nitric acid : HNO3
potassium dichromate : K2Cr2O7
propionic acid : CH3CH2COOH
acrylic acid : CH2CHCOOH
Class:
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elem=C
organic,redox
Summary:
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Acrylic acid fairly easily is oxidized in dilute aqueous solution, while propanoic acid is not.
The double bond between two C-atoms makes the molecule more reactive.
Description:
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Dissolve a small amount of potassium dichromate in dilute nitric acid (appr. 1 mol/l HNO3):
An orange solution is obtained.
Split the solution of potassium dichromate in two parts, call these part A and part B.
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To part A, add a few big drops of acrylic acid and swirl: No visible change occurs.
Heat the solution with part A until it boils: After a few minutes, the liquid does not have a
bright orange color anymore, but looks brown.
Set the test tube with part A apart: After a few hours, the liquid has cooled down, and it has a
bluish/grey color, the typical color of aqueous chromium(III) ion.
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To part B, add a few big drops of propanoic acid and swirl: No visible change occurs.
Heat the solution with part B until it boils: After a few minutes, the liquid still is bright
orange, the color seems to be slightly more intense.
Set the test tube with part B apart: after one day, the cooled down liquid is bright orange and
looks the same as when it was prepared.
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It looks as if the C=C bond in acrylic acid can be oxidized by dichromate in dilute aqueous acidic
solution, albeit with some difficulty (heating is needed and then still one has to wait quite a
long time before it is oxidized completely). Propanoic acid, which has no double bond, cannot be
further oxidized by dichromate.