Experiment: Preparation of ethene by dehydration of ethanol

Preparing ethene by the dehydration of ethanol is a standard Form 5 practical in the Carbon Compounds chapter. It shows how an alcohol can be converted into an alkene by removing a molecule of water, and it links directly to the reactions of unsaturated hydrocarbons. This guide sets out the aim, the apparatus, a numbered procedure, the observations you should expect, and the science process skills that the practical papers reward.

Aim

To prepare a sample of ethene gas by the catalytic dehydration of ethanol, and to confirm that the gas produced is an unsaturated hydrocarbon.

Apparatus and materials

• Boiling tube fitted with a delivery tube and bung
• Retort stand, boss and clamp
• Bunsen burner
• Trough of water and beehive shelf
• Gas jars with cover slips (or test tubes) for collecting the gas
• Mineral wool (cotton wool)
• Absolute ethanol
• Porous pot pieces, unglazed porcelain chips, or aluminium oxide (the catalyst)
• Bromine water and acidified potassium manganate(VII) solution for testing the gas

Procedure

1. Soak a small piece of mineral wool in absolute ethanol and push it to the bottom of a horizontal boiling tube.
2. Place a loose plug of porous pot pieces (or aluminium oxide) in the middle of the tube, between the soaked wool and the mouth, and clamp the tube almost horizontally with a slight downward slope towards the mouth.
3. Fit the bung and delivery tube, and lead the delivery tube into a trough of water over a beehive shelf so the gas can be collected over water.
4. Heat the porous pot in the middle of the tube strongly first, then warm the ethanol-soaked wool gently so that ethanol vapour is carried over the hot catalyst.
5. Move the flame between the catalyst and the wool so that a steady stream of vapour passes over the heated catalyst.
6. Allow the first gas jar of gas to escape, because it is mostly displaced air, then collect the following jars of gas over water.
7. Before you stop heating, remove the delivery tube from the water to prevent water being sucked back into the hot tube (suck-back).
8. Test the collected gas by adding a little bromine water to one jar and a little acidified potassium manganate(VII) to another, and shaking.

Expected observations

When the catalyst is hot and ethanol vapour passes over it, bubbles of gas come off the end of the delivery tube and collect over water. The gas is colourless. When bromine water is shaken with a jar of the gas, the reddish-brown colour of the bromine water fades to colourless. When acidified potassium manganate(VII) is shaken with the gas, its purple colour fades. Near the ethanol-soaked wool, droplets of a colourless liquid may condense on the cooler glass; this is water removed from the ethanol.

Inference and conclusion

Ethanol is dehydrated over the hot catalyst: a water molecule is removed from each ethanol molecule to form ethene, an alkene with a carbon-carbon double bond. The decolourising of bromine water and of acidified potassium manganate(VII) shows that the gas is unsaturated, because these reagents are decolourised by the double bond. The conclusion is that heating ethanol vapour over a suitable catalyst removes water and produces ethene, confirming that alcohols can be converted into alkenes by dehydration.

Science process skills (Paper 3 style)

Making an inference. From the observation that the gas decolourises bromine water, infer that the gas contains a carbon-carbon double bond, that is, it is unsaturated.

Controlling variables. If the investigation compared the volume of gas collected using different catalysts, the manipulated variable is the type of catalyst, the responding variable is the volume of gas collected in a fixed time, and the controlled variables are the volume of ethanol, the mass of catalyst and the heating.

Operational definition. Define an unsaturated gas operationally: a gas is unsaturated if it decolourises bromine water at room conditions without light.

Communicating. Draw and label the apparatus, showing the position of the ethanol-soaked wool, the catalyst, and the collection of gas over water, so another student could repeat the set-up from your diagram.

Safety precautions

• Wear safety goggles, because ethanol vapour is flammable and bromine water is corrosive.
• Keep the mouth of the boiling tube pointing away from people, so nobody is hit if liquid spits.
• Remove the delivery tube from the water before you stop heating, so that cold water is not sucked back into the hot glass and cracks it.
• Keep the flammable ethanol away from other open flames, so the vapour does not ignite.
• Test the gas with bromine water in a fume cupboard or well-ventilated area, so that bromine fumes are not inhaled.

Common errors

• Not heating the catalyst first. If the catalyst is cold, ethanol vapour passes over it unchanged and no ethene forms; heat the catalyst strongly before warming the ethanol.
• Suck-back. Stopping heating with the delivery tube still under water lets cold water be drawn back and crack the hot tube; lift the tube out of the water first.
• Collecting the first jar as product. The first gas is mostly air pushed out of the apparatus; discard it and collect the later jars.
• Confusing dehydration with combustion. Ethene is made by removing water, not by burning ethanol; the tube is heated to vaporise and dehydrate, not to set the ethanol alight.
• Too little catalyst surface. Using a solid lump rather than porous chips gives too little hot surface for the reaction; use porous pieces so vapour meets a large hot area.

How our teachers use this experiment

In one-to-one SPM Chemistry lessons, taught in English from RM50 per hour, we make sure students can explain every part of this set-up, why the catalyst is heated first, why the first jar is discarded, and why suck-back must be avoided, because the practical papers reward reasons, not just steps. Dehydration of ethanol is a recurring theme across SPM Chemistry, and the same unsaturation tests reappear when alkenes and their reactions are examined, so securing this experiment prepares you both for the written papers and for the Paper 3 practical assessment (Paper 3 is a practical test assessing science process skills).