Alcohols | Chemistry for Grade 11 (IGCSE) PDF Download

Alcohols

• Alcohols are characterized by the presence of the hydroxyl (-OH) functional group, which is responsible for their distinctive chemical reactions.
• They constitute a homologous series of compounds with the general formula C_nH_2n+1OH.
• Each member of this series differs from the next by one -CH₂ unit in its molecular formula.
  
• Ethanol (C₂H₅OH) is a significant alcohol with various applications.
• Its structural formula, CH₃CH₂OH, is an alternative representation.
• Found in alcoholic beverages like wine and beer, ethanol is a prevalent type of alcohol.
• It serves as a fuel for automobiles and functions as a solvent in various applications.
• When alcohols undergo combustion in excess oxygen, they generate carbon dioxide (CO₂) and water (H₂O).
• Ethanol, specifically, undergoes complete combustion according to the equation: C₂H₅OH (l) + 3O₂ (g) → 2CO₂ (g) + 3H₂O (l).

The Manufacture of Ethanol

• Ethanol production involves two primary methods:
  • One method is the hydration of ethene with steam.
  • The other method involves the fermentation of glucose.
• Each approach carries its own set of pros and cons that are carefully evaluated.

Hydration of ethene

• A blend of ethene and steam undergoes a process where it's directed over a heated catalyst of phosphoric acid.
• This reaction typically occurs at temperatures around 300°C.
• The pressure applied is approximately 60 atmospheres, equivalent to 6000 kilopascals (kPa).
• The resulting gaseous ethanol is then cooled and condensed into a liquid state, ready for utilization.
  

Fermentation of Glucose

• Sugar or starch is dissolved in water, and yeast is introduced into the solution.
• The mixture is then subjected to fermentation, typically maintained at temperatures ranging between 25 and 35 °C, without the presence of oxygen, for several days.
• Yeast contains enzymes that facilitate the breakdown of starch or sugar into glucose.
• If the temperature is too low, the reaction rate will be sluggish; conversely, if it's too high, the enzymes may become denatured.
• The yeast undergo anaerobic respiration, utilizing glucose to produce ethanol and carbon dioxide.
  C₆H₁₂O₆ → 2CO₂ + 2C₂H₅OH
• Once the alcohol concentration reaches approximately 15%, the yeast are deactivated, prompting the emptying of the reaction vessel and the initiation of the process anew.
• This characteristic defines ethanol production through fermentation as a batch process.

Comparing Methods of Ethanol Production