Ethanol

Ethanol (C2H5OH) is the most examined alcohol in 0620. Its two production methods give a classic comparison question, and its reactions cover the whole of alcohol chemistry.

Key facts

Property	Value
Formula	C2H5OH (molecular C2H6O)
Mr	2(12) + 6(1) + 16 = 46
Functional group	-OH (hydroxyl)
Homologous series	Alcohols
Key property	Colourless, volatile, flammable liquid; miscible with water
Main uses	Biofuel; alcoholic drinks; solvent

Where ethanol appears in 0620

• Alcohols: the reference member and its reactions.
• Fuels: ethanol as a renewable biofuel.
• Alkenes: made by hydration of ethene, and dehydrated back to ethene.
• Carboxylic acids and esters: oxidation to ethanoic acid and esterification.

Production

Fermentation

C6H12O6(aq) -> 2C2H5OH(aq) + 2CO2(g)

Yeast supplies the enzyme; conditions are 25-37 C, no air (anaerobic), in aqueous sugar solution. Fermentation stops near 15% ethanol (higher concentrations kill the yeast), and the ethanol is separated by fractional distillation.

Hydration of ethene

C2H4(g) + H2O(g) -> C2H5OH(g)

Conditions: steam, a phosphoric acid catalyst, about 300 C and 60 atm.

Comparison

Feature	Fermentation	Hydration
Raw material	Sugar (plants)	Ethene (crude oil)
Renewable?	Yes	No
Rate	Slow (batch)	Fast (continuous)
Purity	Impure (needs distillation)	Pure
Temperature	Low (25-37 C)	High (300 C)

Key reactions

Combustion

C2H5OH(l) + 3O2(g) -> 2CO2(g) + 3H2O(l)

Burns with a clean blue flame; used as a biofuel.

Oxidation to ethanoic acid

C2H5OH + 2[O] -> CH3COOH + H2O

Using warm acidified potassium dichromate(VI), which turns from orange to green, or bacteria in air (making vinegar).

Esterification and dehydration

C2H5OH + CH3COOH ⇌ CH3COOC2H5 + H2O (concentrated H2SO4 catalyst) makes the ester ethyl ethanoate.

C2H5OH(g) -> C2H4(g) + H2O(g) (aluminium oxide catalyst, heat) is dehydration back to ethene.

Ethanol as a biofuel

Ethanol made by fermentation is often called carbon-neutral: the CO2 given off when it burns is roughly balanced by the CO2 taken in by the crop as it grows by photosynthesis. It burns more cleanly than petrol, and it is renewable because more sugar crops can be grown. The trade-offs are that farmland is used for fuel rather than food, and the fermentation and distillation themselves use energy.

Compare the production of ethanol by fermentation and by the hydration of ethene. Give one advantage of each method. (4 marks)

Mark scheme

• Fermentation uses sugar from plants, a renewable raw material; hydration uses ethene from crude oil, which is finite/non-renewable [1]
• Advantage of fermentation: it uses a renewable raw material / a lower temperature, so less energy is needed [1]
• Advantage of hydration: it is faster / a continuous process / gives a purer product [1]
• A clear point comparing rate or purity for both methods (e.g. fermentation is slow and impure, hydration is fast and pure) [1]

Examiner note: give a genuine advantage of each method, not the same idea twice. A common slip is to praise fermentation for being renewable and then say nothing distinctive about hydration.

Common exam mistakes

• Swapping the two sets of conditions. Fermentation is yeast at 25-37 C; hydration is steam with a phosphoric acid catalyst at about 300 C and 60 atm.
• Writing the oxidation product as ethanal or ethene. Ethanol is oxidised to ethanoic acid (CH3COOH).
• Calling all ethanol renewable. Only fermentation ethanol is renewable; hydration ethanol comes from crude oil.
• Leaving combustion unbalanced. It is C2H5OH + 3O2 -> 2CO2 + 3H2O.