To prepare ethyne and examine its properties

Theory

Ethyne is prepared by reacting calcium carbide (CaC₂) with water

Equation

CaC₂    +    2H₂O    →    Ca(OH)₂    +    C₂H₂

Procedure

1. Place calcium carbide in a bunker flask and attach a delivery tube to it.

2. Ensure acidified copper sulfate container is in between the calcium carbide and water trough.

3. Acidified copper sulfate captures the impurities (hydrogen sulfide, ammonia and phosphine) as ethyne is bubbled through it.

4. Calcium carbide forms bubbles (ethyne) when water reacts with it. The solution goes white because of calcium hydroxide forming.

5. Fill several test tubes of ethyne. The first test tube will contain air and can be disposed of.

6. Test ethyne test tubes to examine properties.

Physical properties

• Ethyne is a colourless gas with a sweet smell

• It is insoluble in water but dissolves in non-polar solvents Example: Cyclohexane

Combustion

Procedure:

- Place a lighted splint into a test tube of ethyne gas

- Add limewater to the test tube, stopper and shake

Result:

- Ethyne burns with a very bright, very smoky yellow flame, with a lot of soot formed as a result of unburnt carbon

- Limewater turns milky white proving carbon dioxide was formed in the combustion

Compare the flame observed when combusting ethene and ethyne

The flame observed when burning ethyne is brighter and smokier than when burning ethene

How does the flame used in oxyacetylene welding, where ethyne is burned in pure oxygen, differ from the flame observed when ethyne was burned in air in a combustion test?

In pure oxygen: flame is hotter, cleaner (no smoke)

Combustion test: burns with a very bright yellow smoky flame

Write a balanced chemical equation for the complete combustion of ethyne?

2C₂H₂    +    5O₂    →    4CO₂    +    2H₂O

Testing for unsaturation – Method 1: Using bromine water

Procedure:

- Add red bromine water to a test tube of ethyne gas, stopper and shake

Result: Red bromine water turns colourless – ethyne is unsaturated

Testing for unsaturation – Method 2: Using acidified potassium manganate (VII) (KMnO₄)

Procedure:

- Add purple acidified potassium manganate (VII) to a test tube of ethene gas, stopper and shake

Result: Purple acidified potassium manganate (VII) turns colourless - ethyne is unsaturated

Outline two safety precautions in this experiment

1) Ethyne is explosive with air- keep any naked flame away from the apparatus

2) Use a spatula and gloves to place calcium carbide into the flask- can be an irritant to skin

Describe the appearance of calcium carbide

Calcium carbide is a grey-black solid

Give three observations in the reaction flask as the reaction proceeds

1. Effervescence/Fizzing is observed

2. A white substance forms

3. The reaction flask gets hot

Explain why fizzing/effervescence occurs in the reaction flask

Ethyne gas is produced

Explain the white substance that forms in the flask as the reaction proceeds

The white substance forms as a result of the production of calcium hydroxide Ca(OH)₂

Explain why the flask gets warm as the reaction proceeds

The reaction between calcium carbide and water is exothermic (gives out heat)

Why must the products of the reaction be passed through acidified copper sulfate?

The ethyne formed will not be pure, passing it through acidified copper sulfate removes impurities

Name three impurities that could be present in the ethyne gas produced that are removed by

the acidified copper sulfate?

1. Hydrogen sulfide (H₂S)

2. Phosphine (PH₃)

3. Ammonia (NH₃)

Explain why these impurities may be present in the ethyne gas produced

Calcium carbide is often contaminated with calcium sulfide, calcium phosphide and calcium nitride

Why are the first few test tubes of gas collected discarded?

The first few test tubes consist mostly of displaced air and not ethyne gas

Give an industrial application of ethyne

Ethyne is a gas that is used in oxyacetylene welding and cutting – when ethyne is burned in oxygen it burns at extremely high temperatures (> 3000°C)