Preparation and properties of Ethyne

Draw a labelled diagram to show a suitable arrangement of apparatus and chemicals for the preparation of ethyne from the reaction of calcium carbide with water and for the collection of the gas in test-tubes over water

What was observed in the reaction flask as the water was dropped onto the calcium carbide (CaC₂)?

Reaction mixture turns milky white / bubbling / vapour / effervescence / solid reacts / flask gets hot

Why would you expect the first test-tube of gas collected to be less pure than the others?

Air (oxygen, nitrogen) present

Explain why the first test-tubes of gas collected were discarded

Contain air

What was observed when a few drops of dilute bromine solution were added to one of the test-tubes of ethyne?

Brown decolourises (to colourless)

What was observed when a few drops of acidified, dilute potassium manganate(VII) solution were added to another one of the test-tubes of ethyne?

Purple decolourises (to colourless)

What information do the results of the two above tests give about ethyne?

Unsaturated / multiple bond

Describe how you would use one of the test-tubes of ethyne to confirm ethyne gas is flammable

Insert lighting paper into test-tube

Describe the flame observed when ethyne is burned in air

Bright (luminous) / yellow / sooty

Write a balanced equation for the combustion of ethyne in excess oxygen

2C₂H₅ + 5H₂O → 4CO₂ + 2H₂O

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?

Hotter / cleaner (less sooty, less smoky) / less bright / blue

Compare the observations made in the two combustions tests of ethene and ethyne

Ethyne smokier (sootier, brighter, more luminous) than ethene’s /

ethene flame cleaner (not as bright, less luminous) than ethyne’s /

ethyne smoky (sooty) / ethene clean

Name a reagent used to test the gas for unsaturation

Bromine (Br₂)

Write an equation for the reaction taking place during a test to show that ethyne is unsaturated. Name the organic product

C₂H₂ + Br₂ → C₂H₂Br₂

Dibromoethene

Give a major use of ethyne

Oxyacetylene flame / cutting metals / welding metals