Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
ethyl acetate : CH3C(O)OCH2CH3
sodium dichromate dihydrate : Na2Cr2O7.2H2O
sulphuric acid : H2SO4
acetic acid : CH3COOH
ethanol : CH3CH2OH
Class:
------
elem=C,Cr
organic
redox
Summary:
--------
Ethylacetate is in equilibrium with ethanol and acetic acid, even in dilute
aqueous solution with some free acid in it. Usually, making esters requires
rather forcing conditions, like mixing an alcohol and an organic acid and
adding this mix to concentrated sulphuric acid and then heating the mix. This
experiment nicely demonstrates that there is an equilibrium, also in water.
Description:
------------
Sequence 1:
-----------
Take half a mol of ethanol and half a ml of acetic acid and mix these two
liquids. Add 2 ml of a 20% solution of sulphuric acid: The liquid becomes clear
and only a single phase is formed. All liquids mix completely. The liquid has a
pungent smell of acetic acid.
Stopper the test tube and let the liquid stand for a few days at room
temperature. After these days, open the test tube and smell its contents: There
still is a pungent smell of acetic acid, but one clearly can smell another
smell, which is less pungent and somewhat sweet. The liquid still is clear and
only a single phase exists.
Take the liquid and add sodium bicarbonate to the liquid, until addition of the
solid does not result in fizzling anymore. Let the foam settle again, and then
smell the liquid: Now a nice smell can be observed, especially if smelled from
a somewhat further distance. This is the smell of ethylacetate, which is formed
from the ethanol and acetic acid in the acidic aqueous solution.
Sequence 2:
-----------
Take half a ml of pure ethylacetate and add 2 ml of a 20% solution of sulphuric
acid and shake: Most of the ethyl acetate dissolves in the liquid.
Vigorously shake the liquid and before the ethylacetate separates in a separate
layer, quickly pour half of the liquid in another test tube. To one of the test
tubes, add a few drops of concentrated solution of sodium dichromate: The
resulting liquid becomes orange and remains orange. After several minutes, the
liquid still is orange. No dichromate is reduced to chromium(III).
Keep the other half of the liquid and stopper the test tube with that liquid.
Keep it stoppered for several days: No dramatic changes can be observed. It
might be that all of the ethylacetate dissolves, but a small amount of the
liquid may remain separated.
To the liquid, now several days old, add a few drops of concentrated solution
of sodium dichromate: The orange color quickly disappears and the liquid
becomes dark green. The liquid also becomes warm. So, immediately after
preparation, the liquid does not reduce dichromate. After a few days of
standing the dichromate quickly is reduced.
Remark: These two sequences demonstrate that ethylacetate and ethanol/acetic
acid are in equilibrium in acidic aqueous solution. When starting from ethanol
and acetic acid, after a few days there is ethylacetate. When starting from
ethylacetate, the liquid becomes reducing, but only after some time. This is a
strong indication that ethanol is formed.