⭐⭐⭐⭐ Medium to high because of heating, gas collection and safety.
2. Aim
To prepare ethene gas by dehydrating ethanol and to test the ethene produced using bromine water.
3. Quick Theory
Ethene is an alkene with formula C₂H₄. It contains a carbon-carbon double bond, C=C. Ethanol can be converted into ethene by removing water. This is called dehydration.
ethanol → ethene + water
C₂H₅OH → C₂H₄ + H₂O
Ethene is unsaturated because it contains a double bond. It can be tested with bromine water.
Ethene + bromine water → colourless solution
Examiner Tip: The key proof that ethene is unsaturated is that it decolourises bromine water from orange/brown to colourless.
4. Apparatus and Chemicals
Apparatus
Chemicals
Boiling tube or hard-glass test tube, delivery tube, rubber bung, clamp stand, Bunsen burner, trough, gas jars/test tubes, heatproof mat, mineral wool/glass wool.
Ethanol, aluminium oxide catalyst or concentrated sulfuric acid depending on method, bromine water.
Safety: Ethanol and ethene are flammable. Bromine water is harmful/irritant. Concentrated sulfuric acid is highly corrosive if used. Wear goggles and perform carefully in a well-ventilated laboratory.
5. Labelled Experiment Diagram
Preparation and Collection of Ethene
6. Procedure
Place a small amount of ethanol on mineral wool/glass wool near the end of a hard-glass tube.
Place aluminium oxide catalyst in the middle of the tube.
Set up the delivery tube so gas can be collected over water in an inverted gas jar or test tube.
Heat the aluminium oxide strongly first.
Then gently heat the ethanol so ethanol vapour passes over the hot aluminium oxide.
Allow the first few bubbles of gas to escape because they may contain air.
Collect ethene gas over water.
Test a sample of ethene with bromine water.
Observe the bromine water changing from orange/brown to colourless.
Examiner Tip: Heat the catalyst strongly first, then heat ethanol gently. This ensures ethanol vapour passes over a hot catalyst.
7. Observations
Stage
Observation
Meaning
Heating ethanol over catalyst
Gas bubbles are produced.
Ethene gas is being formed.
Collection over water
Gas collects in inverted gas jar/test tube.
Ethene is only slightly soluble in water.
Bromine water test
Orange/brown bromine water turns colourless.
Ethene is unsaturated.
Combustion test, if performed
Ethene burns with luminous/smoky flame.
Hydrocarbon combustion.
8. Test for Ethene
The main test for ethene is bromine water. Ethene is an alkene and contains a double bond, so it undergoes an addition reaction with bromine.
C₂H₄ + Br₂ → C₂H₄Br₂
Test
Positive Result
Conclusion
Add ethene to bromine water
Orange/brown bromine water turns colourless
Ethene is unsaturated.
Examiner Trap: Decolourising bromine water is the test for a carbon-carbon double bond, not just “any hydrocarbon”.
9. Why Ethene is Collected Over Water
Ethene is collected over water because it is only slightly soluble in water and does not react significantly with water under these conditions.
Examiner Tip: If asked why gas is collected over water, say it is insoluble or only slightly soluble in water.
10. Sources of Error and Improvements
Source of Error
Effect
Improvement
Apparatus not airtight
Ethene escapes and less gas is collected.
Check bung and delivery tube connections.
First gas collected contains air
Impure ethene sample.
Allow first few bubbles to escape before collection.
Catalyst not hot enough
Little or no ethene produced.
Heat aluminium oxide strongly before heating ethanol.
Ethanol overheated
May cause rapid boiling or unsafe vapour production.
Heat ethanol gently.
Water sucked back into hot tube
Tube may crack.
Remove delivery tube from water before stopping heating.
Bromine water too concentrated
Colour change harder to judge.
Use dilute bromine water.
11. Student Common Mistakes
Forgetting that ethene is made by dehydration of ethanol.
Writing ethane instead of ethene.
Forgetting that ethene contains a C=C double bond.
Using limewater instead of bromine water as the main test.
Saying bromine water turns from colourless to orange.
Forgetting to discard the first gas bubbles.
Stopping heating before removing the delivery tube from water.
Not mentioning that bromine water is decolourised.
12. Examiner Tips
Tip 1: Ethanol loses water to form ethene.
Tip 2: The test for ethene is bromine water turning orange/brown to colourless.
Tip 3: Ethene is collected over water because it is only slightly soluble.
Tip 4: Always remove the delivery tube from water before stopping heating to prevent suck-back.
13. Examiner Traps
Trap: Ethene is C₂H₄, not C₂H₆. C₂H₆ is ethane.
Trap: Bromine water goes from orange/brown to colourless, not the other way around.
Trap: Aluminium oxide is a catalyst/dehydrating surface; it is not the product.
Trap: The first gas collected may be air, so it should be discarded.