Alcohol (WJEC GCSE Chemistry)

Making Alcohol

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

Testing for 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