Preparation of Ethyne

Describe the colour of calcium dicarbide?

grey-brown or sandy, due to the presence of impurities

What impurities can be found in calcium dicarbide?

calcium sulfide CaS

calcium phosphide Ca₃P₂

calcium nitride Ca₃N₂

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

1. using a spatula, some calcium carbide is placed in a Buchner flask, it must not be touched as moisture from the hand could start the reaction

2. water is added slowly using a dropping funnel

3. fizzing takes place in the Buchner flask, a white solid calcium hydroxide is formed and the Buchner flask becomes warm, as an exothermic reaction is occurring

4. the ethyne gas is bubbled through acidified copper sulfate solution, to remove impurities of hydrogen sulfide, phosphine and ammonia as ethyne does not dissolve

5. bubbles of gas start to come through the delivery tube, a few test tubes of the gas are collected, the first few are discarded as they only contain displaced air

What safety precautions are taken when preparing ethene?

• since ethynes an explosive misture with air, no naked flam should be brought near the gas preparation apparatus, and all tests should be performed away from the apparatus

• after the ethyne is collected the apparatus should be dismantled to ensure no more ethyne is produced

Describe the physcial properties of ethyne:

• ethyne is a colourless gas with a sweetish smell, but if impurities are still present, it will not smell nice

• insoluble in water; it is only soluble in organic solvents such as cyclohexane and chloroform

Describe the combustion of ethyne:

• lid is removed from the gas jar and a lit taper is held at the mouth of the gas jar

  • a more yellow-luminous, smokey flame and great deal of soot is observed

  • the soot is a result of unburnt carbon, due to lack of sufficient oxygen to burn it completely

Describe the tests for unsaturation on ethyne

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

  • colour change from yellow-orange to colourless is observed

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

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

  • colour change from purple to colourless is observed

  • the very reactive triple bond reacts with the potassium permanganate to produce a colourless product

Write an equation for the complete combustion of ethyne in excess oxygen:

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O