AQA A Level chemistry - AS Unit 2: Section 3.2.10: Alcohols

3.2.10 Alcohols - Ethanol production

Students should:

know how ethanol is produced industrially by fermentation
know the conditions for this reaction and understand the economic and environmental advantages and disadvantages of this process compared with the industrial production from ethene
understand the meaning of the term biofuel
know that the term carbon neutral refers to an activity that has no net annual carbon (greenhouse gas) emissions to the atmosphere'
appreciate the extent to which ethanol, produced by fermentation, can be considered to be a carbon-neutral biofuel

Fermentation

Ethanol has been produced for thousands of years by the fermentation of sugars and starch using yeast (the basis for brewing and baking). Ethanol is the essential component of all alcoholic drinks. "Spirits" (like whisky or brandy) are prepared by distilling beer or wine.

Fermentation is the traditional method for making ethanol, both for drinking and for industrial purposes. Industrially, ethanol is very important as a solvent, a fuel or as a raw material for making other chemicals. The fermentation process uses yeast. Yeast is a living organism - a unicellular fungus. It obtains the energy it needs from simple sugars like glucose by anaerobic respiration

equation for the reaction is:-

glucose solution ethanol + carbon dioxide

The condition s required are: yeast / warmth/ no air (anaerobic)

C₆H₁₂O₆(aq) 2C₂H₅OH(aq) + 2CO₂(g)

The concentration of ethanol produced by the fermentation process reaches a maximum of about 12% by volume, at which stage the yeast is poisoned and dies.

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Advantages and disadvantages of fermentation

Fermentation is now almost totally superceded in the manufacture of ethanol by industrial processes involving the addition of steam to ethene.

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Biofuels

Biofuels are fuels made from recently living organisms. They can be divided into three categories:

1 First-generation biofuels are made largely from edible sugars and starches.
2 Second-generation biofuels are made from non-edible plant materials.
3 Third-generation biofuels are made from algae and other microbes.

Biofuels may be alcohols, or products of esterification reactions with naturally occurring fatty acids.

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Carbon footprint

The carbon footprint is the damage left behind after any process or activity in terms of the overall transformation of organic carbon into carbon dioxide. This has become an issue of major importance with the general consensus that the earth is warming up due to the increase in carbon dioxide levels caused by the industrialisation of society.

Industries are being developed that attempt to be as carbon neutral as possible, meaning that their activity tries to not produce any increase in the global carbon dioxide levels.

Even though it may be imagined that the production of biofuels such as ethanol, and their use, is carbon neutral, closer inspection reveals that overall it is not.

All aspects of industrial endeavour need to be taken into account.

1 The sugar cane is probably grown on land that otherwise would probably have forests sequestering (capturing and holding) carbon dioxide.
2 The care, irrigation and harvesting requires machinery and the installations themselves need a supply of electricity and other facilities.
3 The ethanol needs to be transported to the point of sale, which also uses fuel.

However, that said, biofuels reduce the carbon footprint of countries that would otherwise rely on fossil fuels for their energy supply.

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