Edexcel IGCSE Chemistry 复习笔记 4.5.3 Manufacture of Ethanol

• Ethanol can be synthesised by the hydration of ethene
• Ethene is a by-product of the cracking of hydrocarbons and is a valuable feedstock for making many chemicals
• The hydration reaction is very important industrially for the production of alcohols and it occurs using the following conditions:
  • Temperature of around 330ºC
  • Pressure of 60 – 70 atm
  • Concentrated phosphoric acid catalyst

   

• When the reaction is complete, the reaction chamber holds unreacted ethene, ethanol and water
• The contents are transferred to a condenser where ethene is separated easily as it has a much lower boiling point than ethanol and water:
  • Ethanol: 78^oC
  • Ethene: -103^oC
  • Water: 100^oC

   

• The ethanol and water are separated afterwards by fractional distillation

A water molecule adds across the C=C in the hydration of ethene to produce ethanol

• Ethanol (C₂H₅OH) is one of the most important alcohols
• It is used as fuel (for vehicles in some countries) and as a solvent
• It is the type of alcohol found in alcoholic drinks such as wine and beer
• It can be produced by fermentation where sugar or starch is dissolved in water and yeast is added
• The mixture is then fermented between 15 and 35°C with the absence of oxygen for a few days
• Yeast contains enzymes that break down sugar to alcohol
• If the temperature is too low the reaction rate will be too slow and if it is too high the enzymes will become denatured
• The yeast respires anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆ + enzymes → 2CO₂ + 2C₂H₅OH

• The yeast is killed off once the concentration of alcohol reaches around 15%, hence the reaction vessel is emptied and the process is started again
• This is the reason that ethanol production by fermentation is a batch process