Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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methanol : CH3OH
sulphuric acid : H2SO4
potassium dichromate : K2Cr2O7
sodium dichromate anhydrous : Na2Cr2O7
Class:
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elem=Cr
redox
Summary:
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Sodium dichromate anhydrous dissolves in methanol quite well, but does not
react with it. When a little concentrated sulphuric acid is added, then
suddenly a very violent reaction occurs.
With potassium dichromate, this violent reaction does not occur. The latter is
nearly insoluble in methanol and hence it hardly can react with it.
Description:
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Take 2 test tubes and in each of them put appr. 3 ml of 99.9% methanol.
To one test tube, add a small amount of potassium dichromate and swirl: The
solid quickly settles at the bottom and does not dissolve. After a long time of
shaking the test tube, only a very faint yellow color can be observed in the
methanol.
To the other test tube, add a similar amount of solid anhydrous sodium
dichromate: The material dissolves quite well. A bright orange solution is
obtained and a little solid material remains at the bottom.
To each of the test tubes, add a big drop of concentrated sulphuric acid: In
the test tube with potassium dichromate hardly anything visible occurs.
However, after standing for several minutes, there is a dark green haze above
the orange crystals of potassium dichromate. On shaking, the liquid becomes
pale bluish green and orange crystals remain at the bottom. In the test tube
with sodium dichromate a very violent reaction occurs, one second or so, after
the drop of sulphuric acid reaches the orange soluti
Add appr. 10 ml of water to both test tubes and shake: In the test tube with
potassium dichromate, the solid slowly dissolves and the liquid becomes brown
and clear. A mix of orange dichromate and green trivalent chromium gives it the
brown appearance. In the test tube which originally contained sodium
dichromate, the fine suspension of the green solid dissolves. The liquid
becomes clear and bluish green. The green solid apparently dissolves well in
the water.
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This experiment shows that dichromate ion is a good oxidizer, capable of
oxidizing methanol in the presence of acid, but the reaction only is fast for
the sodium dichromate, because of its solubility in methanol.