Mandatory Experiment 7.4
Preparation of ethanal 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 an aldehyde. Further unwanted oxidation of the ethanol to ethanoic
acid can be minimised by:
(a) using more alcohol than oxidising agent ( i.e. the sodium dichromate is the limiting
reagent)
(b) distilling off the ethanal as soon as it is made.
Na2Cr2O7/H2SO4
C2H5OH
————→ CH3CHO
(The balanced equation for the reaction is:
3C2H5OH + Cr2O72- + 8H+ → 3CH3CHO + 2Cr3+ + 7H2O)
Chemicals and Apparatus
Dilute sulfuric acid
i
Sodium dichromate
Concentrated sulfuric acid
Anti-bumping granules
Ethanol
Ethanal
i
Potassium manganate(VII) solution
Fehling's solution no. 1
n
Fehling's solution no. 2
Silver nitrate solution
i
Sodium hydroxide solution
1
Aqueous ammonia solution
i
Dilute nitric acid
Safety glasses
PVC gloves
Quickfit apparatus for distillation
Dropping funnel
Plastic filter funnel
Retort stands and clamps
Conical flask
Beakers
Heating mantle or bunsen/tripod/wire gauze
Rubber tubing
Test-tubes
Water bath
Graduated cylinder
Thermometer
Graduated disposable pipettes
Procedure
NB: Wear your safety glasses.
(a) Preparation
1. Set up the apparatus as shown in Fig.1 for distillation with addition.
2. Put a few anti-bumping granules and 12 cm3 of water in the round bottomed or
pear shaped flask.
3. Slowly, with swirling and cooling under a cold water tap, add 4 cm3 of
concentrated sulfuric acid.
4. Using a fume cupboard, dissolve 10 g of sodium dichromate in 10 cm3 of water in
a clean small beaker and then add 10 cm3 of ethanol. Place this solution in the
dropping funnel, using a small plastic funnel. Rinse the beaker out and mop up
any spilt drops, as sodium dichromate is highly irritating to the skin and eyes.
5. Surround the conical flask, in which the ethanal distillate is to be collected, with
ice/water in a large beaker.
6. Heat the dilute acid until it boils and turn off the Bunsen burner or heating mantle.
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7. Slowly (a steady drip approach is recommended) add the alcohol mixture from
the dropping funnel so as to maintain a gentle boiling. (Warning: If the mixture
is added too rapidly, the distillate will be contaminated with chromium ions
carried over by spurting.) The ethanal distils off as it is formed.
Fig.1
8. Redistil the impure ethanal and collect the fraction boiling between 20 0C and
23 0C.
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(b) Properties
Oxidation by acidified potassium manganate(VII)
1. Using graduated disposable pipettes, place in a test-tube 2 cm3 of ethanal, 1 cm3of
potassium manganate(VII) solution and 4 cm3 of dilute sulfuric acid.
2. Warm the test-tube in a water-bath and shake gently. Observe and record any
colour change in a table of data copied into your practical report book (see the
table below).
Oxidation by Fehling's solution
1. Using a graduated disposable pipette, place in a test-tube 1 cm3 of Fehling's
solution no. 1.
2. Using a separate pipette, add 1 cm3 of Fehling's solution no. 2 – swirl the contents
so that the blue precipitate initially formed will dissolve.
3. Add 1 cm3 of ethanal, heat gently and shake.
4. Observe any change and record your observations.
Oxidation by Tollen's reagent
1. Using a graduated disposable pipette, place in a clean (new if possible) test-tube 3
cm3 of silver nitrate solution and 1 cm3 of sodium hydroxide solution.
2. Add aqueous ammonia solution drop by drop, with shaking, until the precipitate
formed in stage 1 is just dissolved.
3. Add two or three drops of ethanal, mix by shaking and warm in a water bath.
4. Observe any change and record your observations,
5. Rinse out the test-tube with dilute nitric acid and then water.
Test
Acidified potassium
manganate(VII)
Fehling's solution
Tollen's reagent
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Observation
Deduction
Questions relating to the experiment
1. What colour change happened in the reaction vessel during the reaction? Name the
species responsible for each colour.
2. Why is it necessary to turn off the Bunsen burner or heating element before the
reaction is started?
3. What is likely to happen if excess sodium dichromate was used by mistake?
4. Why is ethanal collected in a vessel embedded in a beaker of ice water?
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 3 moles C2H5OH require 1 mole Na2Cr2O7.2H2O for complete reaction,
show clearly that sodium dichromate is the limiting reactant.
8. What use is made of the Fehling's solution test in the Leaving Certificate Biology
course?
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Teacher Material
1. 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.
Both ethanol and ethanal are flammable so a hot plate or mantle would be
preferable to a Bunsen burner.
Once the mixture in the reaction vessel has been brought to the boil, further
heating is not required as the oxidation of ethanol is an exothermic reaction, and
gentle boiling can be regulated by the flow from the dropping funnel.
During the reaction the orange dichromate ion (Cr2O72-) changes to the green
chromium (III) ion (Cr3+).
Ethanal has a distinctive pungent odour, and cautious smelling will identify it.
The distillate is colourless and contains ethanal, ethanol, ethanoic acid and water.
Ethanal boils at 21 0C and if time permits further purification will occur if the
mixture is redistilled and the fraction boiling between 20 0C and 23 0C is
collected.
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.
2. Properties
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Fehling’s solution should be made up immediately before use by mixing equal
volumes of Fehling’s solution no. 1 and Fehling’s solution no. 2.
The silver mirror must not be allowed to dry in the test-tube as it might then be
explosive. It should be washed out with dilute nitric acid and with water.
Preparation of reagents
Potassium manganate(VII) solution:
1.5 g potassium manganate(VII) dissolved in 500 cm3 of water will give an approx. 0.02
M solution. The crystals are difficult to dissolve and the solution needs to be filtered
through glass wool before use. The solution deteriorates within a few days and is best
prepared fresh in small quantities.
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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 M. Great care is needed during
dilution as the mixture can get very hot and spray acid out of the container.
Fehling's solution no. 1:
20 g of copper(II) sulfate crystals are dissolved in 200 cm3 of water with 3 cm3 of dilute
sulfuric acid added and made up to 250 cm3.
Fehling' s solution no. 2:
80 g of sodium potassium tartrate crystals and 40 g of sodium hydroxide pellets are
dissolved in 200 cm3 of water and made up to 250 cm3. Special care is required as this
is a very caustic solution.
Silver nitrate solution:
4 g silver nitrate crystals are dissolved in deionised water and made up to 250 cm3 – this
makes an approximately 0.1 M solution. The solution must be made with deionised water
because halide ions in tap water will form a precipitate with the silver ions. The solution
should be stored in a brown bottle, as light tends to reduce silver ions. This solution is
also best prepared fresh and in very small quantities. Otherwise the silver mirror test may
fail.
Sodium hydroxide solution:
20 g of pellets dissolved in 200 cm3 of cold water and made up to 250 cm3 will make an
approximately 2 M solution. Special care is required as this is a caustic solution.
Aqueous ammonia solution:
40 cm3 of 0.88 ammonia solution made up to 250 cm3 makes an approximately 3 M
solution.
Dilute nitric acid:
30 cm3 of concentrated acid made up to 250 cm3 will give an approximately 2 M
solution. The concentrated acid is very corrosive and has a poisonous irritating vapour.
Use the fume cupboard. Store in a brown bottle as light tends to cause reduction to
nitrogen(IV) oxide.
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Quantities per working group
For the preparation:
10 g sodium dichromate
4 cm3 concentrated sulfuric acid
10 cm3 ethanol
A few anti-bumping granules
5 ice-cubes
For investigating properties
1 cm3 potassium manganate(VII) solution
4 cm3 dilute sulfuric acid
1 cm3 each of Fehling's solutions no. 1 and no. 2
3 cm3 silver nitrate solution
1 cm3sodium hydroxide solution
A few drops of aqueous ammonia
10 cm3 dilute nitric acid
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.
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
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: Highly flammable; keep away from sources of ignition.
Ethanal
i: Extremely flammable; irritating to eyes and respiratory system; keep
away from sources of ignition. Work in a well ventilated room.
Potassium manganate(VII) solution - oxidising agent - the solution used here will be
irritating to the skin and eyes. Wash any splashes off the skin.
Solid potassium manganate(VII)
n is a powerful oxidising agent. Contact with
combustible materials may cause fire, and it forms an explosive product with
concentrated sulphuric acid. It is harmful and should not be allowed to be inhaled,
ingested or come into contact with the skin.
Fehling's solution no. 1
n contains copper sulfate(VI) solution which is harmful by
ingestion or contact with skin and is irritating to eyes.
Fehling's solution no. 2
contains a concentrated solution of sodium hydroxide.
It is caustic- causes severe burns to skin and eyes. Splashes in eyes can cause permanent
damage in a few moments. In case of contact with eyes wash immediately with plenty of
water and seek medical advice.
Silver nitrate solution
damage may result.
i:
Irritant – very dangerous to the eyes. If ingested, internal
Sodium hydroxide solution
: Caustic- causes severe burns to skin and eyes.
Splashes in eyes can cause permanent damage in a few moments. In case of contact with
eyes wash immediately with plenty of water and seek medical advice.
Aqueous ammonia
: Pungent vapour toxic by inhalation; irritating to eyes and
respiratory system; in case of contact with eyes wash immediately with plenty of water
and seek medical advice.
Dilute nitric acid
: Corrosive and toxic. In case of contact with eyes wash
immediately with plenty of water and seek medical advice.
Disposal of wastes
Dilute with excess hot water and flush to foul water drain.
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Specimen results
Test
Observation
Deduction
Acidified potassium
manganate(VII)
Solution decolourised
Manganate(VII) reduced to
manganese(II) - ethanal
oxidised
Fehling's solution
Red/brown precipitate
formed
Tollen's reagent
Silver mirror formed
Copper(II) reduced to
copper(I) oxide - ethanal
oxidised
Silver(I) reduced to silver
metal - ethanal oxidised
Solutions to student questions
1. 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+)
2. Why is it necessary to turn off the Bunsen burner or heating element before the
reaction is started?
The reaction would become too vigorous, and some unreacted ethanol and other
material is likely to be forced over into the conical flask. Once the mixture in the
reaction vessel has been brought to the boil, further heating is not required as the
oxidation of ethanol is an exothermic reaction, and gentle boiling can be
maintained by regulating the flow from the dropping funnel.
3. What is likely to happen if excess sodium dichromate was used by mistake?
More ethanoic acid will be produced reducing the yield of ethanal
4. Why is ethanal collected in a vessel embedded in a beaker of ice-water?
Ethanal boils at 21 0C and must be kept cold in order to prevent losses
through evaporation.
5. Assuming the density of ethanol is 0.80 g cm-3, calculate the number of moles of
ethanol used in the experiment.
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Mass of ethanol = volume x density
10 cm3 x 0.80 g cm-3 = 8 g
Moles of ethanol = Mass / Molar mass
= 8 g / 46
= 0.17 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
= 10/298 mol
= 0.034 mol
7. Given that 3 moles C2H5OH require 1 mole Na2Cr2O7.2H2O for complete
reaction, show clearly that sodium dichromate is the limiting reactant.
3C2H5OH Ξ 1 Na2Cr2O7.2H2O
So 0.034 mol of dichromate will only oxidise 3 x 0.034 mol of ethanol
= 0.102 mol
The amount of ethanol actually present (0.17mol) is greater than 0.102 mol.
Sodium dichromate is therefore the limiting reactant.
8. What use is made of the Fehling's solution test in the Leaving Certificate Biology
course?
Fehling's solution is used to test for reducing sugars, i.e. saccharide molecules
which contain an aldehyde group (-CHO).
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