7.3 - Preparation and Properties of Ethene

The reaction is:

$$\text{C}_2\text{H}_5\text{OH} \xrightarrow{\text{Al}_2\text{O}_3, \, \text{heat}} \text{C}_2\text{H}_4 + \text{H}_2\text{O}$$

Materials and Apparatus Required

Chemicals

• Ethanol
• Aluminium oxide ($Al₂O₃$) powder
• Limewater
• Bromine water
• Acidified potassium manganate(VII) solution

Apparatus

• Large heat-resistant glass boiling tube with a one-holed rubber stopper
• Glass wool
• Test tubes and stoppers
• Delivery tube
• Trough for water displacement
• Retort stand and clamp
• Bunsen burner
• Teat pipette

Safety Precautions

• Wear safety glasses at all times.
• Ethanol is highly flammable; keep away from open flames.
• Bromine water is toxic and corrosive. Handle with care in a fume cupboard.
• Acidified potassium manganate (VII) is a strong oxidising agent and can irritate the skin and eyes.
• Calcium hydroxide (limewater) is an irritant to the eyes and skin.

Method

1. Add ethanol to the large boiling tube to a depth of about 2 cm.
2. Insert glass wool into the boiling tube to soak up all the ethanol.
3. Place about 2 g of aluminium oxide halfway along the boiling tube.
4. Set up the apparatus with a delivery tube leading into a trough filled with water to collect the gas by displacement.
5. Heat the aluminium oxide catalyst strongly, and occasionally heat the ethanol gently to produce ethene gas.
6. Collect ethene in test tubes by displacement of water, discarding the first test tube as it contains air.
7. Once gas bubbles stop emerging, remove the delivery tube from the water before turning off the Bunsen burner to prevent the suck-back of water into the hot glass.

Investigating the Properties of Ethene:

1. Combustion: Ignite a test tube of ethene and observe the characteristics of the flame.
2. Limewater Test: Add a few drops of limewater to a test tube of ethene, stopper the tube, shake, and observe if the limewater turns milky, indicating the presence of carbon dioxide.
3. Bromine Water Test: Add a few drops of bromine water to a test tube of ethene, stopper, shake, and note if the bromine water decolourises, indicating unsaturation.
4. Acidified Potassium Manganate (VII) Test: Add a few drops of acidified potassium manganate (VII) to a test tube of ethene, stopper, shake, and observe if the purple solution turns colourless, indicating unsaturation.

Results

• Combustion: Ethene burns with a yellow, smoky flame due to incomplete combustion, producing carbon and carbon dioxide.
• Limewater Test: The limewater turns milky, confirming the presence of carbon dioxide ($CO₂$) from the combustion of ethene.
• Bromine Water Test: The bromine water is decolourised, indicating that ethene is unsaturated.
• Acidified Potassium Manganate(VII) Test: The purple solution becomes colourless, indicating that ethene is an unsaturated hydrocarbon.

Example Questions with Answers