Mandatory Experiment 7.5
Preparation of ethanoic acid and an investigation of some of its
properties
Student Material
Theory
A sodium dichromate solution acidified with sulfuric acid can be used to oxidise a
primary alcohol to a carboxylic acid. Primary alcohols are first oxidised to the
corresponding aldehyde by a mixture of these reagents. To ensure complete oxidation to
the acid two conditions in particular are important:
1. Using more oxidising agent than alcohol (i.e. the ethanol is the limiting reagent).
2. Refluxing the reaction mixture for 20 to 30 minutes.
Na2Cr2O7/H2SO4
C2H5OH
————→ CH3COOH
(The balanced equation for the reaction is:
3C2H5OH + 2Cr2O72- + 16H+ → 3CH3COOH + 4Cr3+ + 11 H2O)
Chemicals and Apparatus
Dilute sulfuric acid
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Sodium dichromate
Concentrated sulfuric acid
Anti-bumping granules
Ethanol
Deionised water
Universal indicator paper
Magnesium ribbon
Anhydrous sodium carbonate
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Safety glasses
PVC gloves
Quickfit apparatus for distillation
Dropping funnel
Retort stands and clamps
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Water bath
Ice bath
Graduated cylinder
Graduated disposable pipettes
Thermometer
Bunsen burner
Tripod
Wire gauze.
Procedure
NB: Wear your safety glasses.
(a) Preparation
1. Put a few anti-bumping granules and 10 cm3 of dilute sulfuric acid in the round
bottomed or pear shaped flask.
2. In a fume cupboard, add in 9 g of sodium dichromate and dissolve by careful
swirling. Use a small dry funnel to avoid crystals of dichromate being caught on
the neck of the flask. Mop up any spilt drops as sodium dichromate solution is
highly irritating to the skin and eyes, and may also cause damage to clothes and
the work surface.
3. Slowly with swirling and cooling in an ice bath, add 6 cm3 of concentrated
sulfuric acid. Set up the apparatus as shown in Fig. 1 for reflux distillation with
addition. If necessary, use a clamp to hold the water outlet in the sink.
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Fig. 1
4. Mix 2 cm3 of ethanol and 10 cm3 of deionised water in the dropping funnel.
5. Add the solution from the dropping funnel dropwise down the condenser, while
swirling the contents of the flask and cooling it if necessary to prevent too
vigorous a reaction.
6. Remove the dropping funnel and still head from the top of the assembled
apparatus.
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7. Boil the mixture gently using a water-bath for about half an hour.
8. Cool the apparatus, dismantle and rearrange for distillation as in Fig. 2. Direct
heating without a water bath must be used, as the boiling point of the mixture will
eventually exceed 100 0C.
Fig. 2
9. Distil off about 15 cm3. This is aqueous ethanoic acid.
(b) Properties
Divide the distillate into three portions in test tubes.
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1. Odour: Smell the distillate by carefully wafting some of the vapour towards your
nose. Compare its smell to that of ethanol, and record your observations in a table
of data copied into your practical report book from the table below.
2. Test with universal indicator paper: Dip some universal indicator paper in the
distillate and record your observations.
3. Test with magnesium: Coil the 5 cm clean strip of magnesium loosely, drop it
into one of the test tubes, and swirl. Record your observations.
4. Test with sodium carbonate Add 1 g of anhydrous sodium carbonate powder
into the second test tube, and swirl. Record your observations.
5. Esterification (i) With care add 2 drops of concentrated sulfuric acid to the third
test tube.
(ii) Add 1 cm3 of ethanol and warm gently. Carefully smell the reaction product.
Record your observations.
Test
Observation
Deduction
Odour
Universal indicator paper
Magnesium strip
Anhydrous sodium
carbonate
Ethanol with concentrated
sulfuric acid
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Questions relating to the experiment
(1) Explain the term reflux distillation.
(2) Explain why it was thought necessary in stage 5 of the preparation to add the alcoholwater mixture through the condenser.
(3) Name three possible impurities present in the final distillate.
(4) What colour change happened in the reaction vessel during the reaction? Name the
species responsible for each colour.
(5) Assuming the density of ethanol is 0.80 g cm-3, calculate the number of moles of
ethanol used in the experiment.
(6) Calculate the number of moles of sodium dichromate used in this experiment, given
that its formula is Na2Cr2O7.2H2O.
(7) Given that from the balanced equation 3 C2H5OH ≡ 2Na2Cr2O7, show clearly that the
ethanol is the limiting reactant.
(8) Given that from the balanced equation 1 C2H5OH ≡1 CH3COOH, calculate the
theoretical yield in grams of ethanoic acid.
(9) If the actual yield of ethanoic acid were required, further purification of the distillate
would be necessary. Explain how this might be carried out.
(10) Find the percentage yield of ethanoic acid in this experiment, given that the actual
yield was 1.35 g.
(11) What is the function of the anti-bumping granules?
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Teacher Material
Preparation
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The use of Bibby clips to hold the parts of the assembled Quickfit apparatus
together is recommended.
The mixing of sulfuric acid with water is a strongly exothermic reaction. The acid
must be added to the water (and not the water to the acid) to prevent a violent
reaction, with acid being splashed out of the container. This stage of the
experiment should be monitored closely by the teacher.
Ethanol is flammable so a hot plate or mantle would be preferable to a bunsen
burner as a heat source.
If a magnetic stirrer/hotplate is used, then a magnetic follower can be used instead
of anti-bumping granules.
There is quite a danger that the student will add the alcohol solution to the
acidified dichromate too quickly, causing a violent, exothermic reaction. This part
of the preparation must be supervised very carefully.
The use of a round-bottomed flask is preferable to that of a pear-shaped flask in
this experiment. There is more danger of a violent reaction if the pear-shaped
flask is used.
The use of a 250 cm3 round-bottomed flask, if available, is preferable to that of a
100 cm3 round-bottomed flask in this experiment, as it facilitates better mixing of
the reactants.
During the reaction the orange dichromate ion (Cr2O72-) changes to the green
chromium(III) ion (Cr3+), which when concentrated can look quite dark.
During refluxing, the flow of cooling water should be adjusted so that the vapour
front is visible not more than half way up the Liebig condenser. If this is not done,
the yield is likely to be reduced, as an appreciable amount of the very volatile
ethanal that is initially formed may escape to the atmosphere.
It is essential that an open arrangement, as shown in Fig. 2, is used for the
distillation stage.
As the first 5 cm3 of distillate will contain relatively little ethanoic acid, it could
be collected separately, and the next 10 cm3 collected used for the tests.
A small scale or semi-micro method is available for those schools in possession of
BASF mini-labs or similar equipment. Increased safety, reduced waste and a
shorter time required for the practical are three of the advantages of this
alternative method.
If the ethanoic acid is to be further purified by redistillation, direct heating
without a water bath must be used, as the boiling point of ethanoic acid is 116 –
118 0C.
Properties
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Tests for hydrogen and carbon dioxide gases could be attempted where
appropriate.
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The ester ethyl ethanoate has a very distinct sweet "glue-like" odour.
Preparation of reagents
Dilute sulfuric acid:
(Always dilute sulfuric acid by adding the acid to water and not the other way round.)
100 cm3 of the concentrated acid is added slowly to about 800 cm3 of deionised water
containing about 20 ice cubes. The mixture is stirred and made up to 1 litre in a
volumetric flask with deionised water. The flask is stoppered and inverted a number of
times.
This solution when made up to a litre is approximately 2 mol l-1. Great care is needed
during dilution as the mixture can get very hot and spray acid out of the container.
Quantities per working group
For the preparation:
Dilute sulfuric acid (10 cm3)
Sodium dichromate (9 g)
Concentrated sulfuric acid (6 cm3)
Anti-bumping granules (a small amount)
Ethanol (3 cm3)
Deionised water (10 cm3)
For investigating properties
Universal indicator paper (5 cm)
Magnesium ribbon (5 cm)
Anhydrous sodium carbonate (1 g)
Ethanol (1 cm3)
Concentrated sulfuric acid (2 drops)
Safety considerations
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Safety glasses must be worn.
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The use of gloves is essential.
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Hair should be tied back if necessary.
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Chemical hazard notes
Sodium dichromate(VI)
is a strong oxidising agent. The solution used here is
concentrated and will be highly irritating to the skin and eyes, and may be a sensitizer by
skin contact. Wash any splashes off the skin- otherwise damaged skin may ulcerate. The
solid salt may cause cancer by inhalation of dust. Avoid raising dust during preparation
of the solution – prepare the solution in a fume cupboard.
Concentrated sulfuric acid
causes severe burns to eyes and skin. Due to its very
considerable heat of reaction with water, it is essential that the acid be added to water
when it is being diluted.
Ethanol
is highly flammable; keep away from sources of ignition.
Magnesium ribbon
retina.
burns with an intense white light that can be damaging to the
Concentrated glacial ethanoic acid
has a strong smell of vinegar and is
corrosive to skin and tissue. The vapour is very irritating to the lungs.
Sodium carbonate
Wear eye protection.
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is irritating to the eyes and skin, and its dust irritates the lungs.
Disposal of wastes
In a fume cupboard, add sodium metabisulfite to convert any residual
dichromate into chromium(III) ions (green solution). Dilute with excess water and flush
to foul water drain.
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Specimen results
Test
Observation
Deduction
Odour
Vinegar odour
Ethanoic acid present
Universal indicator paper
Changes colour from green
to red
Solution is acidic
Magnesium strip
Effervescence
Hydrogen gas generated
Anhydrous sodium
carbonate
Effervescence
Carbon dioxide gas
generated
Ethanol with concentrated
sulfuric acid
Sweet odour and oily
droplets
Ethyl ethanoate produced
Suggested solutions to student questions
(1) Explain the term reflux distillation.
Reflux distillation involves condensing the vapour from a boiling liquid in such a way as
to return the condensed material to the reaction vessel. In this way a reaction may be
carried out at quite a high temperature while preventing the loss of any of the reactants or
products.
(2) Explain why it was thought necessary in stage 5 of the preparation to add the alcoholwater mixture through the condenser.
When the alcohol water mixture meets the acidified dichromate solution a strongly
exothermic reaction occurs. Without the presence of the condenser the reagents could
spray out.
(3) Name three possible impurities present in the final distillate.
The distillate may contain, along with the ethanoic acid, some water, ethanal and ethyl
ethanoate.
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(4) What colour change happened in the reaction vessel during the reaction? Name the
species responsible for each colour.
During the reaction, the orange dichromate ion (Cr2O72-) changes to the green
chromium(III) ion (Cr3+).
(5) Assuming the density of ethanol is 0.80 g cm-3, calculate the number of moles of
ethanol used in the experiment.
Mass of ethanol = volume x density
= 2 cm3 x 0.80 g cm-3
= 1.6 g
Moles of ethanol = Mass / Molar mass
= 1.6 g / 46 g mol-1
= 0.035 mol
(6) Calculate the number of moles of sodium dichromate used in this experiment, given
that its formula is Na2Cr2O7.2H2O.
Moles of sodium dichromate = Mass / molar mass
= 9 g / 298 g mol-1
= 0.03 mol
(7) Given that from the balanced equation 3 C2H5OH ≡ 2Na2Cr2O7, show clearly that the
ethanol is the limiting reactant.
2 mol Na2Cr2O7 ≡ 3mol C2H5OH
So 0.03 mol of dichromate would oxidise 1.5 x 0.03 mol of ethanol.
= 0.045 mol of ethanol
0.035 mol (of ethanol) < 0.045 mol
Ethanol is the limiting reactant.
(8) Given that from the balanced equation 1 C2H5OH ≡1 CH3COOH, calculate the
theoretical yield in grams of ethanoic acid.
1 mol C2H5OH ≡1 mol CH3COOH
0.035 mol C2H5OH ≡ 0.035 mol CH3COOH
Mass of ethanoic acid = moles x molar mass
0.035 mol x 60 g mol-1 = 2.1 g
(9) If the actual yield of ethanoic acid were required, further purification of the distillate
would be necessary. Explain how this might be carried out.
Redistil the aqueous ethanoic acid collecting the fraction that boils between 116 0C and
118 0C.
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(10) Find the percentage yield of ethanoic acid in this experiment, given that the actual
yield was 1.35 g.
The theoretical yield of ethanoic acid was 2.1 g.
% yield = Actual yield x 100 / Theoretical yield
= 1.35 g x 100 / 2.1 g
= 64%
(11) What is the function of the anti-bumping granules?
The function of the anti-bumping granules is to maintain a smooth gentle boiling by
preventing super-heating.
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