Describe laboratory method of preparation of ethene. Suggest a chemical test to distinguish ethene from ethane. [4+1]

Laboratory Preparation of Ethene

Ethene ($\mathrm{C}{}_2\mathrm{H}{}_4$) can be prepared in the laboratory by dehydration of ethanol using a concentrated acid catalyst. The reaction involves heating ethanol with concentrated sulphuric acid (H₂SO₄) at 170°C (or by passing ethanol vapor over heated alumina, $\mathrm{Al}{}_2\mathrm{O}{}_3$).

Concentrated Sulphuric Acid (H₂SO₄)Concentrated Sulphuric Acid (H₂SO₄)

Reaction:

$$\mathrm{C}{}_2\mathrm{H}{}_5\mathrm{OH}\stackrel{\mathrm{conc}\cdot \mathrm{H}{}_2\mathrm{SO}{}_4,170°\mathrm{C}}{\to }\mathrm{C}{}_2\mathrm{H}{}_4+\mathrm{H}{}_2\mathrm{O}$$

Procedure:

1. Setup:

   • Take ethanol in a round-bottom flask and add a few drops of concentrated sulphuric acid (acts as a catalyst and dehydrating agent).
   • Fit the flask with a thermometer and a condenser connected to a gas collection setup (e.g., an inverted gas jar filled with water).

2. Heating:

   • Heat the mixture gradually to 170°C (using a heating mantle or oil bath).
   • Ethanol vaporizes and undergoes dehydration, producing ethene gas and water vapor.

3. Collection:

   • The ethene gas is less soluble in water than ethanol, so it escapes and can be collected by downward displacement of water (or by bubbling through a solution like bromine water).

4. Purification (Optional):

   • Pass the gas through a drying agent (e.g., anhydrous calcium chloride, CaCl₂) to remove water vapor.
   • Collect the dry ethene gas in a gas jar or use it directly for testing.

Safety Precautions:

• Use fume hood or work in a well-ventilated area (ethene is flammable).
• Handle concentrated H₂SO₄ with care (corrosive, exothermic reaction).
• Avoid open flames near the setup (risk of explosion).

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Chemical Test to Distinguish Ethene from Ethane

Ethene is an alkene (contains a C=C double bond), while ethane is an alkane (only C-C single bonds). This difference allows us to use bromine water (Br₂ in water) as a chemical test.

etheneethene+Br₂(aq)C₂H₄Br₂

Test Procedure:

1. Take two separate test tubes:

   • One contains ethene gas (from the preparation above).
   • The other contains ethane gas (from a cylinder or prepared by heating sodium propionate with soda lime).

2. Add a few drops of bromine water (orange-brown color) to each test tube and shake gently.

Observation & Conclusion:

Alternative Test (Combustion):

• Burn both gases in a flame test:
  • Ethene burns with a sooty (yellow) flame (due to incomplete combustion, forming carbon particles).
  • Ethane burns with a clean blue flame (complete combustion).

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ethanolethanol+H₂SO₄170°Cetheneethene+H₂O