Alcohols: Manufacture of Ethanol

Specification Point 4.32C (Paper 2C Only):

Know that ethanol can be manufactured by: reacting ethene with steam in the presence of a phosphoric acid catalyst at a temperature of about 300°c and a pressure of about 60-70 atm, the fermentation of glucose, in the absence of air, at an optimum temperature of about 30°c and using the enzymes in yeast

Ethanol:

Ethanol (C₂H₅OH) is the type of alcohol found in alcoholic drinks such as wine and beer. It is also used as fuel for cars.

1. Reacting ethene with steam (hydration):

A mixture of ethene and steam is passed over a hot catalyst of phosphoric acid at a temperature of 300°c and 60-70 atmospheres of pressure and is then condensed into a liquid for use:

Ethene   +   Water   →   Ethanol

  C₂H₄           H₂O            C₂H₅OH         

Equation showing the reaction that takes place between ethene and steam                 

2. Fermentation of glucose:

Sugar or starch is dissolved in water and yeast is added to be fermented at 30°C with the absence of oxygen for a few days. Yeast contains zymase enzymes (biological catalysts) break down starch or sugar to glucose. The yeast respires anaerobically using the glucose to form to ethanol and carbon dioxide:

 Glucose   →   Ethanol   +   Carbon Dioxide

 C₆H₁₂O₆       2C₂H₅OH      2CO₂                         

Equation showing the fermentation of glucose by anaerobic respiration

Comparing methods

Specification Point 4.33C (Paper 2C Only):

Understand the reasons for fermentation, in the absence of air, and at an optimum temperature

Fermentation: a chemical process in which microorganisms such as yeast act on carbohydrates to produce ethanol and carbon dioxide. Sugars and starch are examples of carbohydrates.