20. Preparation of Ethene

Summary

This experiment focuses on the preparation and examination of ethene, a colorless gas, by dehydrating ethanol using aluminum oxide as a dehydrating agent and catalyst. The process involves heating ethanol in a boiling tube, collecting ethene gas over water, and observing its properties. Ethene's insolubility in water, combustion to form carbon dioxide and water, and reactivity with bromine water and potassium permanganate are demonstrated, confirming the presence of a carbon-carbon double bond.

Takeaways

• 🧪 The experiment involves preparing a sample of ethene gas by dehydrating ethanol using aluminum oxide (Al2O3) as a dehydrating agent and catalyst.
• 🔥 A Bunsen burner is used to gently heat the aluminum oxide, which initiates the dehydration reaction at the end of the boiling tube where ethanol is held by glass wool.
• 🌡️ The boiling tube is positioned horizontally to ensure the ethene gas produced can be collected over water.
• 💧 Ethene is insoluble in water, which is demonstrated by its ability to be collected over water without dissolving.
• 🔥 Ethene burns with a flame, indicating it is flammable, and when burned in air, it forms carbon dioxide and water.
• 🥛 The presence of carbon dioxide after burning ethene is confirmed by adding limewater, which turns milky due to the formation of calcium carbonate.
• 🌿 Bromine water is used to test for unsaturation in organic compounds; the disappearance of the yellow color indicates the presence of a carbon-carbon double bond in ethene.
• 🧪 The addition of bromine across the double bond in ethene results in the formation of a colorless compound, 1,2-dibromoethane.
• 🌿 Potassium permanganate (KMnO4) is also used as a test for unsaturation, and its decolorization upon reaction with ethene confirms the presence of a carbon-carbon double bond.
• 📚 The experiment concludes with a demonstration of the chemical properties of ethene, highlighting its reactivity due to the presence of a carbon-carbon double bond.

Q & A

What is the purpose of the experiment described in the transcript?

The purpose of the experiment is to prepare a sample of ethene gas by dehydrating ethanol using a chemical reaction and to examine its properties.

Which substance is used as a dehydrating agent and catalyst in the experiment?

Aluminium oxide (Al2O3) is used as a dehydrating agent and catalyst in the experiment.

How is the boiling tube positioned during the reaction?

The boiling tube is clamped near its mouth and positioned horizontally during the reaction.

What is the role of glass wool in the experiment?

Glass wool is used to hold the ethanol in place while the reaction takes place.

How is the gas collected in the experiment?

The gas is collected over water, indicating that ethene is insoluble in water.

What happens when ethene is burned?

When ethene is burned, it produces a flame and carbon dioxide, which turns limewater milky.

What is the significance of the color change in bromine water when added to ethene?

The disappearance of the yellow color in bromine water indicates the presence of unsaturation, such as a carbon-carbon double bond, in the ethene molecule.

How does potassium permanganate (KMnO4) test for the presence of a carbon-carbon double bond in ethene?

The disappearance of the purple color of potassium permanganate to give a colorless solution is evidence for the presence of a carbon-carbon double bond in ethene.

What is the expected product when bromine is added to ethene?

The expected product when bromine is added to ethene is a colorless oily compound called 1,2-dibromoethane.

Why is it important to remove the delivery tube from the water after collecting the gas?

It is important to remove the delivery tube from the water to prevent a siphoning effect, which could occur if the boiling tube cools and the water is sucked back into the tube, potentially causing it to crack.

What general rule is followed when organic compounds like ethene burn in air?

When organic compounds like ethene burn in air, they generally form carbon dioxide and water.