5.2 - Preparation and Properties of Ethyne

The reaction is as follows:

$$\text{CaC}_2 + 2\text{H}_2\text{O} \rightarrow \text{C}_2\text{H}_2 + \text{Ca(OH)}_2$$

Materials and Apparatus Required

Chemicals

• Calcium dicarbide ($CaC₂$)
• Acidified copper(II) sulfate solution
• Water
• Limewater ($Ca(OH)₂$ solution)
• Bromine water
• Acidified potassium manganate(VII) solution

Apparatus

• Test tubes with solid stoppers and stoppers with two holes
• Delivery tubes
• Teat-pipette
• Trough
• Retort stands and clamps

Safety Precautions

• Wear safety glasses throughout the experiment.
• Calcium dicarbide is a severe eye hazard. Avoid contact with water unless in a controlled setup, as it releases highly flammable ethyne gas.
• Bromine water is highly toxic and causes burns. Use it in a fume cupboard and avoid skin contact.
• Acidified potassium manganate(VII) is an oxidising agent and irritant. Wash off any splashes immediately.
• Handle concentrated sulfuric acid and copper(II) sulfate with care, as they are corrosive and harmful.

Method

1. Place 2-3 pieces of calcium dicarbide into a test tube.
2. Set up the apparatus to collect gas over water, using a delivery tube and a water-filled trough.
3. Using a teat-pipette, add water dropwise to the calcium dicarbide.
4. Collect the ethyne gas produced in test tubes by water displacement, discarding the first test tube of gas (which contains mostly air).
5. Stopper the test tubes filled with ethyne and carry out the following tests:

• Combustion: Ignite the gas and observe the flame. Record your observations.
• Limewater Test: Add a few drops of limewater to a test tube of ethyne and shake. Observe any changes (e.g., the limewater turning milky, indicating the presence of $CO₂$ )
• Bromine Water Test: Add a few drops of bromine water to a test tube of ethyne, stopper the tube, and shake. Observe if the brown colour of bromine water decolourises, indicating unsaturation.
• Potassium Manganate(VII) Test: Add a few drops of acidified potassium manganate(VII) solution to a test tube of ethyne. Stopper and shake the test tube. Note any colour change.

Results

• Combustion: The ethyne burns with a smoky, sooty flame, indicating incomplete combustion due to limited oxygen.
• Limewater Test: Limewater turns milky, indicating the production of carbon dioxide ($CO₂$).
• Bromine Water Test: The brown bromine water is decolourised, indicating that ethyne is an unsaturated hydrocarbon.
• Potassium Manganate(VII) Test: The purple solution becomes colourless or changes to a lighter shade, indicating a reduction reaction and the unsaturation of ethyne.

Example Questions with Answers