Preparation of Ethene

Write a balanced equation for the formation of ethene?

C₂H₅OH —Al₂O₃→C₂H₄ + H₂O

Key steps + observations in the procedure for the preparation of ethene:

1. some ethanol is poured into a boiling tube, and glass wool is added to soak up the ethanol and holds it in place at the end of the boiling tube

2. a small heap of aluminium oxide (alumina) is place midway along the boiling tube

3. the aluminium oxide is gently heated by the Bunsen burner, the ethanol is not heated directly as it would vaporise and pass over the aluminium oxide too quickly without being dehydrated to form ethene

4. bubbles of displaced air from the apparatus are allowed to escape for a short time before 5 test tube and a glass jar of ethene are collected

5. once finished, the apparatus is raised so that the delivery tube is no longer in the water, and only then is the Bunsen burner turned off; this prevents suck back

What safety precautions are taken when preparing ethene?

• stopper on the boiling tube must be air tight, and the Bunsen burner must not be allowed near the mouth of the boiling tube as any gas that escapes is flammable

• suck back occurs when the Bunsen burner is turned off before the delivery tube is removed from the water

  • the alcohol vapour in the boiling tube cools, creating a vacuum, and the water is sucked back to fill the space

  • the cold water can cause the hot boiling tube to crack or shatter

  • to avoid suck back, raise the apparatus in order to remove the delivery tube from the water before turning off the Bunsen

Describe the physical properties of ethene:

• ethene is a colourless gas with a sweetish smell

• collected over water, therefore it must be insoluble in water otherwise it would have dissolved; it is only soluble in organic solvents such as cyclohexane and chloroform

Describe the combustion of ethene:

• stopper is removed from a test tube and a lit wax taper is held at the mouth of the test tube

  • a yellow-luminous, slightly smoky flame is observed

• when gas has stopped burning some limewater is added to the test tube, which is then stoppered and shaken

  • limewater turns milky due to the production of CO₂

Describe the tests for unsaturation on ethene:

• bromine water is added to a test tube of ethene, which is stoppered and shaken

  • colour change from yellow-orange to colourless is observed

  • bromine bonds with the very reactive alkene to form a new colourless compound

• acidified potassium permanganate is added to a test tube of ethene, which is stoppered and shaken

  • colour change from purple to colourless is observed

Write and equation for the combustion of ethene:

C₂H₄ + 3O₂ → 2CO₂ + 2H₂O

Write an equation for the reaction between limewater and carbon dioxide:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O