Manufacture of Ethanol (Edexcel IGCSE Chemistry (Modular))

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Hydration of ethene

  • 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 300ºC

    • 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: 78oC

    • Ethene: -103oC

    • Water: 100oC

  • The ethanol and water are separated afterwards by fractional distillation

Hydration of ethene

Equation showing the hydration of ethene with steam to form ethanol

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

Examiner Tip

Make sure you learn the conditions for the hydration of ethene. 

Fermentation

  • Ethanol can also be produced by fermentation where sugar or starch is dissolved in water and yeast is added

  • The mixture is then fermented between 25 and 35°C (the optimum temperature is 30 °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 → 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

  • At the end, there is a mixture of ethanol and water which is separated by fractional distillation

Examiner Tip

Fermentation is an anaerobic process. Oxygen is not required for ethanol to be produced by fermentation.

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