Mandatory Experiment 5.2
Preparation and properties of ethyne
Student Material
Theory
Calcium dicarbide reacts with water producing ethyne and calcium hydroxide:
CaC2 + 2H2O → C2H2 + Ca(OH)2
The gas usually contains hydrogen sulfide and phosphine as impurities, caused by the
hydrolysis of traces of calcium sulfide and calcium phosphide present in the calcium
dicarbide. The impurities, which cause an unpleasant odour, can be removed by bubbling
the gas through acidified copper sulfate solution.
Chemicals and Apparatus
Calcium dicarbide
Water
Acidified copper(II) sulfate solution
Limewater
i
n
Acidified potassium manganate(VII) solution
i
Bromine water
Teat-pipette
Test tubes, solid stoppers and stoppers with two holes
Delivery tubes
Trough
Retort stands and clamps
Procedure
NB: Wear your safety glasses.
(1) Preparation
1. Place 2-3 pieces of calcium dicarbide in a test tube and set up the apparatus as in
the diagram.
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2. Add water from the teat-pipette, a few drops at a time, until all the calcium
dicarbide has reacted. The gas produced is collected in test tubes by displacement
of water, the test tubes being stoppered under water when filled with gas. The first
test tube filled can be discarded, as it contains a mixture of air and ethyne.
(2) Investigation of Properties
1. Ignite the gas in one of the test tubes. Describe the flame. Add a few drops of
limewater to the test-tube and shake well. Describe what happens.
2. Add a few drops of a solution of bromine water to a test tube of gas, stopper quickly
and shake well. Describe what you see.
3. Add a few drops of acidified potassium manganate solution to a test tube of gas,
stopper quickly and shake well. Describe what you see.
Questions relating to the experiment
1. Write an equation for the combustion of ethyne, and select a product
that may have caused a colour change in the limewater.
2. Draw the structural formula of a possible product of the reaction in
Investigation 2. What does the observed result indicate?
3. What does the observed result in Investigation 3 indicate?
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4. Why should the first test tube be discarded?
5. In what way do the products of combustion, as described in the
equation, differ from what you observed in Investigation 1? Explain.
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Teacher Material

Ensure that the teat-pipette used is large enough to hold sufficient water for all the
calcium dicarbide to react.

Ethyne prepared by the reaction of water with calcium dicarbide contains
impurities such as hydrogen sulfide and phosphine. These are removed by passing
the mixture of gases through acidified copper(II) sulfate solution. The hydrogen
sulfide is completely removed as precipitated copper(II) sulfide (this happens as
the solubility product of copper(II) sulfide is extremely small (Ksp = 4 x 10 –36).

The phosphine is a stronger ligand than water, and uses its lone pair to bond with
the copper ion. Partial (or total) substitution of water by PH3 in Cu(H2O)62+
occurs. In this way the phosphine is removed from the gas mixture.

Acidic conditions are used to counteract any basic products formed by the
reaction of water with calcium dicarbide (e.g. phosphine). Under basic conditions,
copper ions precipitate out as copper(II) hydroxide.

During the purification process, the solution appears to turn green as a dark
precipitate is formed. However, this colour change is only apparent. Filtration
results in a blue filtrate that is a little lighter in colour than the original solution
(due to the removal of some hydrated copper(II) ions). The solid obtained in the
filter paper is a dark brown colour.

To ensure maximum removal of impurities, the ethyne should be bubbled slowly
through a solution of acidified copper(II) sulfate that has considerable depth.

Bromine water should be diluted to give a pale yellow solution. This is done to
ensure that decolourisation occurs readily.

Acidified potassium manganate(VII) solution should be diluted further to give a
pale pink solution. This is done to ensure that decolourisation occurs readily.

When testing for unsaturation, an alternative to adding bromine water or acidified
potassium manganate(VII) solution to a test tube of gas is to bubble the gas
directly into a small volume of solution as the gas is being produced.

The KMnO4 may undergo a colour change rather than a complete decolourisation,
and some students should be able to explain this in terms of the variable oxidation
states of transition elements and the associated colours.
Preparation of reagents
Bromine water can be purchased directly from laboratory suppliers. Alternatively, it can
be made up by shaking 0.5 cm3 of bromine in a fume cupboard with 100 cm3 of water. If
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bromine water is being prepared from bromine, it is essential to do this in the fume
cupboard, wearing rubber gloves.
The solution deteriorates with time. Store in a tightly stoppered brown bottle.
Acidified potassium manganate(VII) solution can be prepared by mixing 10 cm3 of
approximately 0.02 M KMnO4 with 20 cm3 of dilute H2SO4.
Acidified copper(II) sulfate solution: Prepare 500 cm3 of a saturated solution of
copper(II) sulfate pentahydrate (CuSO4.5H2O). Carefully add 10 cm3 of concentrated
sulfuric acid.
Limewater is prepared by adding about 200 g of calcium hydroxide to 2.5 l of water.
Calcium hydroxide has a low solubility in water. Shake vigorously at intervals over a
period of time and finally allow to settle. Decant off the clear limewater solution from
above the undissolved solids. Top up the vessel with deionised water and repeat the
process.
Calcium dicarbide can deteriorate in storage, and should be tested in advance. It may
last better if it is stored in a desiccator after the seal has been broken on the container.
Quantities per working group
2-3 pieces of calcium dicarbide are required.
A few drops of bromine water, of acidified KMnO4 solution, and of limewater will
suffice. If excess is used, the required colour changes may not materialise.
Safety considerations

Safety glasses must be worn.

The bromine solution should be used in a fume cupboard only, and skin contact
should be avoided.
Chemical hazard notes
Calcium dicarbide
: Severe eye hazard. Tins must not be opened with steel
implements, as sparks may cause an explosion. Contact with water results in the release
of an extremely flammable gas.
Bromine
: The vapour is highly toxic by inhalation. The liquid causes severe
burns to eyes and skin. The aqueous solution attacks lungs, eyes and nose.
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Acidified potassium manganate(VII) solution
i: Oxidising agent - the solution used
here is irritating to the skin and eyes. Wash any splashes off the skin.
i:
Calcium hydroxide
Limewater
i:
Irritant to eyes, skin and lungs.
Splashed droplets in the eye cause severe irritation.
n:
Copper(II) sulfate
Harmful if ingested. Eye and skin irritant.
Concentrated sulfuric acid
: Corrosive and oxidising. Use with extreme caution.
Add acid to water when diluting.
Disposal of wastes
Add the residue in the reaction vessel to the used acidified copper(II) sulfate solution.
Neutralise with 10% sodium carbonate solution, and dilute with water. Flush to foul
water drain.
Suggested solutions to student questions
1. Write an equation for the combustion of ethyne, and select a product,
which may have caused a colour change in the limewater.
C2H2 + 21/2 O2 → 2CO2 + H2O
The carbon dioxide turns limewater milky.
2. Draw the structural formula of a possible product of the reaction in
Investigation 2. What does the observed result indicate?
H
H
\
/
C =C
/
\
Br
Br
or
1, 2 dibromoethene
H H
| |
Br – C – C - Br
| |
Br Br
1, 1, 2, 2, tetrabromoethane
(Allow products with two hydrogen atoms in the molecule and in
which there is one OH group to every Br atom.)
The decolourisation indicates that the ethyne is unsaturated.
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3. What does the observed result in Investigation 3 indicate?
This also indicates unsaturation.
4. Why should the first test tube be discarded?
It contains mostly air.
5. In what way do the products of combustion, as described in the equation,
differ from what you observed in Investigation 1? Explain.
The smoky, sooty flame indicates that combustion is incomplete due to
insufficient oxygen, and particles of carbon are evident.
Industrial Uses
Industrially ethyne is burned while mixing with pure oxygen. This produces a clear flame
with a temperature of about 3000 0C, which is suitable for cutting and welding, and is
used in an oxyacetylene torch for those purposes.
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