Fermentation (Edexcel GCSE Chemistry): Revision Note

Exam code: 1CH0

Stewart Hird

Last updated

Fermentation

  • 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

Examiner Tips and Tricks

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

Purifying the Product

  • Fermentation produces a dilute solution of ethanol which needs to be separated from the reaction mixture.

  • This is done using fractional distillation.

  • The mixture is heated to 78 ºC which is the boiling point of ethanol but below that of water (100 ºC).

  • The ethanol evaporates and its vapours pass through a condenser, where they cool and condense, forming liquid ethanol.

  • The water and any other impurities remain behind in the reaction flask.

  • When the temperature starts to increase to 100 ºC heating should be stopped. The water and ethanol have now been separated.

Fractional distillation of a mixture of ethanol and water, IGCSE & GCSE Chemistry revision notes

Diagram showing how fractional distillation is used to separate ethanol from water in the laboratory

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Stewart Hird

Author: Stewart Hird

Expertise: Chemistry Content Creator

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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