Alcohols (Cambridge International IGCSE Chemistry)

Spec point — 11.6.2 · Describe the manufacture of ethanol by: fermentation of aqueous glucose at 25–35°C in the presence of yeast and in the absence of oxygen catalytic addition of steam to ethene at 300°C and 6000kPa /60 atm in the presence of an acid catalyst Describe the combustion of ethanol State the uses of ethanol as: a solvent a fuel

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 C_nH_2n+1OH
• They differ by one -CH₂ in the molecular formulae from one member to the next

Table 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

• Ethanol will undergo complete combustion and burn in excess oxygen to produce carbon dioxide and water

C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (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

A 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%, so the reaction vessel is emptied and the process is started again
• Ethanol production by fermentation is therefore a batch process

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

Comparing methods of ethanol production

Spec point — Describe the advantages and disadvantages of the manufacture of ethanol by: fermentation catalytic addition of steam to ethene

Comparing methods of ethanol production

Extended tier only