Preparation of ethene

            In this preparation the reaction takes place in a round-bottomed flask; the gas produced is bubbled through 10% aqueous sodium hydroxide solution in a wash-bottle and then collected via a delivery tube over water.

Hazards:

            Sulphuric acid is corrosive; ethanol and ethene are flammable. There is substantial production of sulphur dioxide in this reaction which should therefore be done in a fume cupboard.

 

  • Place 20 cm3 of ethanol in a 200 cm3 round-bottomed flask, and add slowly with cooling and shaking 40 cm3 of concentrated sulphuric acid.
  • Why is the sulphuric acid added slowly with cooling and shaking?
  • The ethanol contains some water, and the dilution of sulphuric acid is strongly exothermic. Ethanol and sulphuric acid, although miscible, have very different densities, so need shaking in order to mix them properly.
  • Add to the flask about 2 - 3 g of clean dry sand, and assemble the apparatus arranging for the flask to be heated over a sand bath.
  • What is the reason for adding clean dry sand?
  • The sand acts as fine anti-bumping granules and provides small nuclei on which gas bubbles can grow.
  • Why is the flask heated over a sand-bath?
  • The sand bath spreads the heat out so that the flask is heated evenly. This reduces the chance of the flask breaking and ensures that there are no hot spots in the reaction mixture which could lead to excessive charring, for example.
  • Heat the sand-bath until the liquid effervesces gently; the liquid becomes progressively darker in colour.
  • Why does the liquid have to effervesce gently?
  • Gentle effervescence avoids large amounts of sulphuric acid coming off as droplets if the reaction gets too violent.
  • Why does the liquid become darker?
  • Sulphuric acid is an oxidising agent, and removes not only water from ethanol but also hydrogen, leaving the carbon behind. This charring is unavoidable. The sulphuric acid is reduced to sulphur dioxide.
  • Allow the gas to escape from the delivery tube for a few minutes, then collect a small sample of the gas in an inverted water-filled test tube. Close the tube with a bung, then light the tube of gas at a safe distance from the apparatus. If the tube contains ethene, it burns with a clear blue flame; if the ethene is still mixed with air the mixture in the test tube ignites with a sharp report. Allow the gas to escape from the tube until a test-tube of the gas proves to be pure ethene. Then collect several gas-jars of ethene.
  • Why is the gas allowed to escape for a few minutes?
  • The first portion of the gas evolved is simply displaced air from the apparatus.
  • Why is the 'safe distance' mentioned?
  • Ethene is flammable and it is important not to ignite the gas issuing from the delivery tube.
  • Why does the gas ignite 'with a sharp report' if it is not pure ethene?
  • An air-ethene mixture is explosive over a certain range of compositions.
  • Disconnect the delivery tube connecting the round-bottomed flask to the wash bottle before turning off the heat.
  • Why must the tube be disconnected before the heat is turned off?
  • Two reasons: as the gas cools it will contract, and this would cause the solution in the wash bottle to suck back; and quite a lot of sulphur dioxide is produced, and this would react with the sodium hydroxide washing solution. This would also cause suck-back. The reaction of sucked-back sodium hydroxide solution with hot concentrated sulphuric acid would be dangerously violent.
  • The residue in the flask should be disposed of by allowing it to cool, and then pouring into a large volume of cold water.
  • Why must the residue be poured into a large volume of cold water?
  • The remaining sulphuric acid must be diluted so that the mixture does not get too hot and boil the water, causing spitting of hot sulphuric acid. This is less likely if the acid is not itself hot.
  "May her rest be long and placid,
She added water to the acid.
The other girl did what we taught her,
And added acid to the water."