These are detailed Revision Notes for Topic 3.5 of AQA A-Level Chemistry (Alcohols). They were written by me using a combination of the textbook and class notes. I will also be uploading the other topics and creating bundles.
Topics Included:
- Alcohols - introduction
- Ethanol production
- Th...
Alcohols
15.1 Alcohols – introduction
15.1 Alcohols - Introduction
- Functional group -OH. 15.2 ethanol production
- General formula CnH2n+1OH 15.3 the reactions of alcohols
- Polyhydric alcohols contain more than one hydroxyl group.
- The C-O-H angle is 105o because the lone pairs repel each other producing a smaller bond angle than
the usual 109.5o bond angle.
- Alcohols are primary, secondary or tertiary based off of how many other groups are bonded to the
carbon that has the -OH group.
- The -OH group allows for hydrogen bonding between molecules, giving alcohols higher melting and boiling
points than alkanes of a similar size.
- Small alcohols are soluble in water because it can hydrogen bond to the water, however longer chains
cannot because the polar chain cannot hydrogen bond, so they are immiscible.
15.2 Ethanol Production
- Ethanol is used as an intermediate in the manufacture of other organic chemicals. It can be made in
two ways:
Hydration of alkenes:
- Ethene (made from the fractional distillation and cracking of crude oil) reacts with steam using a
phosphoric acid catalyst.
CH2=CH2 + H2O à C2H5OH (phosphoric acid catalyst)
- It is a fast continuous reaction, and it gives essentially pure ethanol.
Fermentation:
- Carbohydrates from plants are broken down into sugars and converted to ethanol by the enzymes in
yeast.
- It is an anaerobic respiration process.
C6H12O6(aq) à 2C2H5OH(aq) + 2CO2(g)
- This reaction is slow at low temperatures, but the enzymes do not work at high temperatures, so it is
done at 35oC.
- No air is present in the reaction to prevent the oxidation of ethanol to ethanoic acid.
- Fermentation stops after about 15% alcohol and it is removed by fractional distillation (its boiling point
is less than water, 78%).
- Biofuel ~ a fuel derived or produced from renewable biological sources.
- Ethene can be produced by dehydrating ethanol made from fermentation of sugar giving it a renewable
source.
- At the moment we get it from crude oil.
- Ethanol fuel is used in petrol; however, it releases CO2 into the atmosphere, so it is not ideal.
- Ethanol made by fermentation is carbon neutral because the CO2 released when it’s burnt is
unbalanced by the CO2 absorbed by CO2 absorbed: CO2 released:
the plant for photosynthesis. Photosynthesis: Fermentation:
- However, there are extra carbon 6H2O + 6CO2 à C6H12O6 + 6O2 C6H12O6 + 2C2H5OH + 2CO2
costs with the energy needed to Combustion:
both transport, harvest and grow 2C2H5OH + 6O2 à 4CO2 + 6H2O
the crops and the fuel and fertilisers 6 mol CO2 6 mol CO2
required for this.
- Also, burning food for fuel is unethical, as it is taking food away from others.
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