CIE IGCSE Chemistry

Revision Notes

11.2.5 Alcohols

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Alcohols

  • All alcohols contain the hydroxyl (-OH) functional group which is the part of alcohol molecules that is responsible for their characteristic reactions
  • Alcohols are a homologous series of compounds that have the general formula CnH2n+1OH
  • They differ by one -CH2 in the molecular formulae from one member to the next

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

  • Ethanol (C2H5OH) is one of the most important alcohols
    • Ethanol can also be represented by its structural formula CH3CH2OH
  • 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 complete combustion:

C2H5OH (l) + 3O(g) → 2CO2 (g) + 3H20 (l)

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 approximately 300 °C
    • The pressure used is 60 atmospheres (6000kPa)
    • 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 25 and 35 °C with the absence of oxygen for a few days
    • Yeast contains enzymes that catalyse the break down of 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 → 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

Make sure you learn the conditions for both hydration and fermentation.

Comparing Methods of Ethanol Production

Extended

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

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