CIE IGCSE Chemistry

Revision Notes

14.2.3 Alcohols

Alcohols

  • Family of organic compounds that all contain the -OH functional group
  • This is the group of atoms responsible for their chemical properties and reactions

 

the_first_three_alcohols table, IGCSE & GCSE Chemistry revision notesDiagram showing the first three alcohols

 

Ethanol

  • 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
  • Alcohols burn in excess oxygen and produce CO2 and H2O
  • Ethanol undergoes combustion:
CH3CH2OH + 3O2 → 2CO2 + 3H2O

The manufacture of ethanol

  • There are two methods used to manufacture ethanol:
    • The hydration of ethene with steam
    • The fermentation of glucose
  • Both methods have advantages and disadvantages which are considered

Hydration of ethene

  • A mixture of ethene and steam is passed over a hot catalyst of phosphoric acid at a temperature of 330°C
  • The pressure is used 60-70 atmosphere
  • The gaseous ethanol is then condensed into a liquid for use

 

Addition- Water to Ethene, IGCSE & GCSE Chemistry revision notesA water molecule adds across the C=C in the hydration of ethene to produce ethanol

 

Fermentation of glucose

  • 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 starch or sugar to glucose
  • 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%, 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

Exam Tip

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

Extended Only

Comparing Methods of Ethanol Production

Comparing methods of ethanol production table, IGCSE & GCSE Chemistry revision notes

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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