Alcohols, Fermentation and Ethanol
Alcohols contain the –OH functional group. The examinable core is the two routes to ethanol and the comparison between them.
1. The homologous series
Alcohols have the general formula CₙH₂ₙ₊₁OH and the functional group –OH (hydroxyl).
| Alcohol | Formula |
|---|---|
| Methanol | CH₃OH |
| Ethanol | C₂H₅OH |
| Propanol | C₃H₇OH |
| Butanol | C₄H₉OH |
Don’t confuse the general formulae: alkanes are CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH. A recorded error gave the wrong general formula for alkanes while working on alcohols.
Working out a formula: for octanol (8 carbons), n = 8, so it is C₈H₁₇OH.
Count the hydrogens carefully: 2n + 1 = 17, plus the one in the –OH group. Both an incorrect molecular formula for octanol and confusion over its hydrogen count were recorded.
Naming: the number shows which carbon carries the –OH — propan-1-ol vs propan-2-ol.
Incorrect naming of alcohols was recorded — include the position number when there is a choice.
2. Reactions of alcohols
Combustion
alcohol + oxygen → carbon dioxide + water
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Balancing methanol: CH₃OH + 1½O₂ → CO₂ + 2H₂O, or doubled: 2CH₃OH + 3O₂ → 2CO₂ + 4H₂O
Remember the oxygen already in the alcohol when balancing. Struggling to balance the methanol combustion equation was recorded — count the O in the –OH as well as in the O₂.
Alcohols burn with a clean flame, which is why ethanol is used as a fuel.
Oxidation
Ethanol is oxidised to ETHANOIC ACID (a carboxylic acid).
Two routes:
- Microbial — bacteria in the air; this is why wine turns to vinegar
- Chemical — warming with acidified potassium manganate(VII), which turns purple → colourless
Dehydration
Ethanol → ethene + water, passing the vapour over hot aluminium oxide (~300 °C) or concentrated sulfuric acid.
This is the reverse of making ethanol from ethene. Confusion about the dehydration reaction was recorded — dehydration removes water to give the alkene.
3. Making ethanol — the two routes
This comparison is the most examined part of the topic.
Fermentation
glucose → ethanol + carbon dioxide C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂
Conditions:
| Condition | Value |
|---|---|
| Catalyst | YEAST (which contains enzymes) |
| Temperature | ~30–35 °C |
| Conditions | anaerobic (no oxygen) |
| Feedstock | sugars from plants — renewable |
The catalyst is YEAST — confusion about this was recorded. Yeast supplies the enzymes that do the work.
The temperature must be warm but not hot. Too cold and the enzymes work too slowly; too hot and the enzymes DENATURE. A water bath keeps it steady — a student was recorded as unsure why one is used.
Fermentation is anaerobic — it needs no oxygen. It is closely related to anaerobic respiration in yeast, though the two terms were recorded as confused; fermentation is the industrial application of that process.
Knowing when it’s finished: bubbling of CO₂ stops. A recorded difficulty — the gas evolution is the visible sign.
Afterwards: the mixture is only about 15% ethanol, so it is purified by fractional distillation.
Hydration of ethene
ethene + steam → ethanol C₂H₄ + H₂O → C₂H₅OH
Conditions:
| Condition | Value |
|---|---|
| Catalyst | phosphoric acid |
| Temperature | ~300 °C |
| Pressure | ~60 atm |
| Feedstock | ethene from crude oil — non-renewable |
Uncertainty about the conditions for industrial ethanol production was recorded — learn both sets and keep them apart, as with the Haber and Contact processes.
The comparison
| Fermentation | Hydration | |
|---|---|---|
| Raw material | renewable (plants) | non-renewable (crude oil) |
| Rate | slow | fast |
| Process | batch | continuous |
| Purity | impure — needs distillation | pure |
| Temperature | low (~30 °C) | high (~300 °C) |
| Energy cost | low | high |
Fermentation vs hydration is a standard “compare and evaluate” question. Confusion between the two was recorded — the memorable contrast is renewable but slow and impure against fast and pure but from oil.
4. Uses of ethanol
- Solvent — in perfumes, cosmetics and inks
- Fuel — burns cleanly; used as biofuel
- In alcoholic drinks
5. Mistakes that cost marks
Wrong general formula for alcohols or alkanes.
Miscounting hydrogens in a longer alcohol.
Omitting the position number when naming.
Forgetting the oxygen in –OH when balancing combustion.
Saying the catalyst for fermentation is something other than yeast.
Using too high a temperature — the enzymes denature.
Swapping the fermentation and hydration conditions.
Saying fermentation gives pure ethanol.
Confusing dehydration with hydration.
Frequently asked questions
What is the functional group in alcohols? –OH (hydroxyl).
What is the general formula? CₙH₂ₙ₊₁OH.
What is the formula for ethanol? C₂H₅OH.
What are the products of alcohol combustion? Carbon dioxide and water.
What is ethanol oxidised to? Ethanoic acid — which is why wine becomes vinegar.
What is the equation for fermentation? C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂.
What catalyst does fermentation use? Yeast, which contains enzymes.
What temperature is used, and why not higher? About 30–35 °C — higher would denature the enzymes.
How do I know fermentation has finished? Bubbling of carbon dioxide stops.
What are the conditions for hydration of ethene? Phosphoric acid catalyst, 300 °C, 60 atm.
Quick revision checklist
- I know the –OH group and the general formula
- I can name and write formulae for the first four alcohols
- I can work out a formula for a longer alcohol
- I can balance combustion, counting the O in –OH
- I know ethanol oxidises to ethanoic acid
- I know dehydration gives ethene
- I know the fermentation equation and all its conditions
- I know yeast is the catalyst and why temperature matters
- I know how to tell fermentation has finished
- I know the hydration equation and conditions
- I can compare the two routes across all six rows
- I can give uses of ethanol
These notes cover alcohols, fermentation and ethanol in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons, with particular attention to the errors students make most often and the wording examiners reward. Always check the current syllabus for reaction conditions in your own exam series.
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