Alcohol (WJEC GCSE Chemistry)

How is ethanol made?

Fermentation

• Ethanol (C₂H₅OH) is one of the most important alcohols
• It is the type of alcohol found in alcoholic drinks such as wine and beer
• It is also used as fuel for cars and as a solvent
• 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 form carbon dioxide and ethanol
• 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 respire anaerobically using the glucose to form ethanol and carbon dioxide:

C₆H₁₂O₆ + Enzymes → 2CO₂ + 2C₂H₅OH

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

Hydration of ethene

• Alkenes also undergo addition reactions with steam in which an alcohol is formed. Since water is being added to the molecule it is also called a hydration reaction
• The 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
• This is a more efficient method of producing ethanol for industrial uses
• The structure and formulae of simple alcohols can be found in Naming Alcohols

How to test for an alcohol

• We can test for alcohols by using acidified potassium(VI) dichromate solution
• Acidified means that the potassium dichromate(VI) is in a solution of dilute acid (such as dilute sulfuric acid) 

• The evidence of a chemical reaction is the change in colour from orange to green

Testing for an alcohol

 

Acidified potassium(VI) dichromate will turn from orange to green when added to an alcohol