14.5 Alcohols
Ethanol can be formed in to ways:
- By fermentation: enzymes in yeast break down glucose (a simple sugar) to ethanol and carbon dioxide, giving out heat (exothermic). This can be done with any substance that contains cellulose, starch or It is done by grinding the source (e.g. corn or grapes) and treating it with enzymes to break down cellulose and starch into glucose. Leave it to ferment. Fractional distillation is used to get the ethanol from the mixture of substances.
- Ethene is obtained by cracking long-chain alkenes from The ethene reacts with steam (reversibly) in the following conditions: 570°C, 60-70atm and a catalyst (phosphoric acid). Low temperature gives a better yield, but high temperature is used to give a better rate of reaction.
*They can be compared like this (in the syllabus it says “describe” so I don’t know how much they want you to know):
| Fermentation | From ethene |
| Advantages:
-renewable source -good use of waste organic material (e.g. the apples which don’t look nice enough to be sold in shops.) |
Advantages:
-fast -continuous process -pure ethanol -smaller containers |
| Disadvantages:
-Lots of material needed to produce just 1 litre of ethanol so lots of big fermentation tanks needed. -Fractional distillation is expensive -Slow process -Batch process |
Disadvantages:
-oil is a non-renewable resource -lots of energy to make steam and get the right conditions -a lot of ethene is un-reacted, (and then recycled) |
Ethanol burns well in oxygen, giving out plenty of heat, as well as carbon dioxide and water.
Ethanol is used as a:
-solvent: to dissolve the things than water cannot. It evaporates easily, so it is used a solvent in glues, printing inks, perfumes and aftershave.
-fuel: added to or instead of petrol, because it burns cleanly