Alcohols & Fermentation T
Swap one hydrogen of an alkane for an –OH group and you leave the hydrocarbons behind: the molecule now contains oxygen, and it belongs to the alcohols. The –OH is the functional group, so every alcohol shares the same chemistry. The first four are methanol (CH3OH), ethanol (C2H5OH, often written CH3CH2OH to show the structure), propanol and butanol.
Ethanol. Write the formula as CH3CH2OH (or C2H5OH) so the –OH functional group stays visible — C2H6O counts the same atoms but hides what makes it an alcohol.
The four reactions of alcohols
There are four reactions to know for the first four alcohols, and describing “what happens” in each is enough — you only ever need a balanced equation for combustion:
| Reaction | What you observe / what forms |
|---|---|
| …react with sodium | Fizzing — hydrogen gas is given off (gentler than sodium + water) |
| …burn in air | Clean flame; complete combustion to carbon dioxide + water, releasing energy |
| …are added to water | They dissolve, giving a neutral solution (pH 7) — no fizzing, no reaction |
| …react with an oxidising agent | They are oxidised to the matching carboxylic acid (ethanol → ethanoic acid) |
That last row is why an opened bottle of wine slowly turns to vinegar: oxygen from the air (helped by microbes) oxidises the ethanol to ethanoic acid.
Balance the equation for the complete combustion of ethanol. Same routine as section 3 — but don’t forget ethanol brings one oxygen atom of its own:
- Carbons: 2 → 2CO2. Hydrogens: 6 → 3H2O.
- Oxygens needed on the right: 4 + 3 = 7. Ethanol supplies 1, so O2 supplies 6 — that’s 3O2.
C2H5OH + 3O2 → 2CO2 + 3H2O
The uses of the first four alcohols follow from their properties: fuels (ethanol in spirit burners and as a biofuel blended into petrol; methanol as a fuel and chemical feedstock), solvents (ethanol dissolves substances water can’t — it carries perfumes, mouthwashes and methylated spirits), and ethanol is the alcohol in alcoholic drinks.
Fermentation — making ethanol with yeast
Industry makes ethanol two ways. Hydration of ethene (section 6) is fast and continuous but uses a crude-oil feedstock; fermentation uses renewable plant sugars and runs at gentle conditions. Exam questions ask for the conditions, so learn all three:
sugar (glucose) yeast⟶ ethanol + carbon dioxide
- Yeast — its enzymes catalyse the reaction, which is exactly why the temperature matters (next point).
- Warm, around 30 °C — too cold and the enzymes work slowly; too hot and they denature. (Offered 0 °C / 25–35 °C / 450 °C in a multiple-choice, take the middle.)
- No oxygen (anaerobic conditions) — air is kept out.
Fermentation produces an aqueous (dilute) solution of ethanol — the carbon dioxide bubbles off (it turns limewater milky, a favourite link to C8 gas tests). To concentrate the ethanol, the mixture is fractionally distilled — the same separation idea as section 2, now exploiting the boiling point difference between ethanol (78 °C) and water (100 °C).
The two routes to ethanol, side by side
Ethanol is manufactured two ways: fermentation of plant sugars (above) and hydration of ethene — ethene + steam over a catalyst (section 6). The same molecule, from completely different starting materials — which is why “compare the two methods” is one of C7’s most predictable long-answer questions (there’s one to try below). The marks come from making paired points: say something about each method on the same idea — raw material, then conditions, then rate — rather than everything about one and then the other.
| Fermentation | Hydration of ethene | |
|---|---|---|
| Raw material | Sugar from plants — renewable | Ethene from crude oil — finite |
| Equation | glucose → ethanol + CO2 | ethene + steam → ethanol |
| Conditions | Yeast, warm (≈30 °C), no oxygen | Catalyst, high temperature & pressure |
| Process & rate | Batch — relatively slow | Continuous — fast |
| Product | Dilute ethanol — needs fractional distillation | Pure ethanol, high yield |
| Energy & resources | Low temperature → less energy; renewable, but uses farmland | Uses finite crude oil; more energy (heat + pressure) |
🧪 Exam-style questions
Which gas is produced when sodium is added to ethanol? Tick (✓) one box.
Describe how ethanol is produced from sugar solution, naming the process. Build the answer, then compare with the model below.
Show answer
- Add yeast to the sugar solution. 1 mark
- Keep it warm (≈30 °C) in the absence of oxygen (anaerobic). 1 mark
- The process is fermentation — producing an aqueous solution of ethanol and carbon dioxide. 1 mark
Balance the equation for the complete combustion of ethanol.Type a balancing number in each box (leave it as 1 if no number is needed), then press Check. Remember ethanol’s own oxygen atom.
Show answer
- 2 carbons → 2CO2; 6 hydrogens → 3H2O. 1 mark
- Right side needs 7 O; ethanol provides 1, so 3O2: C2H5OH + 3O2 → 2CO2 + 3H2O. 1 mark
Ethanol is used in alcoholic drinks. Which two of these are other main uses of ethanol? Tick (✓) two boxes, then press Check.
Propanol is warmed with an oxidising agent. What is the organic product? Tick (✓) one box.
Ethanol can be produced by fermentation or by the hydration of ethene. Compare these two methods of producing ethanol. This is a levels-of-response question — make paired points (raw material, conditions, rate, product) and reach a justified conclusion. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): relevant points (about both methods) identified, given in detail and logically linked to form a clear account — ending in a justified conclusion.
- Level 2 (3–4): relevant points identified, with attempts at logical linking, but the account is not fully clear.
- Level 1 (1–2): points identified and stated simply, relevance not clear, no logical linking.
Marks come from comparing the methods point-by-point, not from listing everything about one then the other. Credit-worthy comparisons:
- Raw material: fermentation uses sugar from plants, which is renewable; hydration uses ethene from crude oil, which is finite.
- Conditions / energy: fermentation is warm (≈30 °C) with yeast; hydration needs a catalyst at high temperature and pressure, so it uses more energy.
- Rate & process: fermentation is a slow, batch process; hydration is fast and continuous.
- Product: fermentation makes a dilute solution that must be fractionally distilled; hydration makes pure ethanol in high yield.
Conclusion (needed for Level 3): fermentation is the more sustainable route (renewable, low energy); hydration is better for fast, pure, large-scale manufacture. 6 marks
Carboxylic Acids T
Oxidise an alcohol (section 7) and you arrive at the last homologous series of the topic: the carboxylic acids, with the functional group –COOH. The first four are methanoic acid (HCOOH), ethanoic acid (CH3COOH), propanoic acid and butanoic acid. Ethanoic acid is the one you’ve met: it’s the acid in vinegar.
Ethanoic acid. Count the –COOH carbon in the name: ethanoic acid has two carbons in total, one of them inside the functional group.
The three reactions to describe
| Reaction | What you observe / what forms |
|---|---|
| …react with carbonates | Fizzing — carbon dioxide is given off (plus a salt and water). The salts are named -anoates: ethanoic acid makes ethanoates |
| …dissolve in water | An acidic solution forms (pH below 7) — they behave as typical acids, just weak ones |
| …react with alcohols | With an acid catalyst (e.g. concentrated sulfuric acid), they form esters + water |
You are not expected to write balanced symbol equations for any of these — describing the reaction and naming the products is what the marks are for.
The ester to know by name is ethyl ethanoate, made from ethanol + ethanoic acid. Esters smell pleasant and are volatile (they evaporate easily), which is why they’re used in perfumes and flavourings — volatility gets the molecules to your nose. A question may also hand you the reaction the other way round: given that compounds A and B react to make an ester plus one other product, that other product is water — a fact that becomes the key to condensation polymerisation in section 10.
The acid’s –OH and the alcohol’s –H (blue) leave together as the water; the carbonyl and the alcohol’s O (red) join into the –COO– ester link.
Both are colourless liquids, so “describe a test to distinguish them” is a natural 2-marker. Add a carbonate (e.g. sodium carbonate) to each: the carboxylic acid fizzes (CO2), the alcohol does nothing. Universal indicator works too — red-orange in the acid, green (neutral) in the alcohol. Quoting the result for both liquids is what completes the answer.
Now that you’ve met them both, never call ethanol or ethanoic acid a hydrocarbon. If the formula contains an O, the molecule is not a hydrocarbon — hydrocarbons contain hydrogen and carbon only (section 1). Ethanol (CH3CH2OH) and ethanoic acid (CH3COOH) each carry oxygen, which is exactly what makes them an alcohol and a carboxylic acid rather than members of the alkane or alkene families.
Why carboxylic acids are weak acids Higher
A strong acid (HCl, sulfuric, nitric) ionises completely in water — effectively every molecule releases its H+. A weak acid only partially ionises: most molecules stay whole, and the ionisation is a reversible reaction sitting at an equilibrium that lies well to the left:
CH3COOH(aq) ⇌ CH3COO−(aq) + H+(aq)
- Ethanoic acid only partially ionises in water / the equilibrium lies to the left. 1 mark
- So a solution of ethanoic acid has a lower concentration of H+ ions than hydrochloric acid of the same concentration… 1 mark
- …giving it a higher pH (closer to 7) and slower reactions — e.g. with the same carbonate, the weak acid fizzes less vigorously.
The reasoning chain is always partial ionisation → fewer H+ → higher pH / slower rate. Note the fair-test caveat: this comparison only makes sense at the same concentration — concentration (mol/dm³) and strength (degree of ionisation) are different ideas.
🧪 Strong & weak acids — ionisation, concentration & pH
Both acids start at the same concentration. Notice the strong acid is far more acidic. Then drag its slider to dilute it — and see how far you have to go before it only matches the weak acid’s pH.
● H⁺ hydrogen ion · ● A⁻ acid anion (Cl⁻ or CH3COO⁻) · H–A un-ionised acid molecule
≈100% ionised · picture schematic
· picture schematic
pH scale — compare the two acids
🧪 Exam-style questions
What is the name of the ester produced when ethanol reacts with ethanoic acid? Tick (✓) one box.
Dilute ethanoic acid is added to sodium carbonate in an open flask on a balance. What happens to the mass, and why? Tick (✓) one box.
Vinegar contains ethanoic acid. 250 cm³ of a vinegar contains 12 g of ethanoic acid. Calculate the mass of ethanoic acid in 1.0 dm³ of this vinegar.
Show answer
- 1.0 dm³ = 1000 cm³, which is 1000 ÷ 250 = 4 portions of 250 cm³. 1 mark
- Mass = 4 × 12 1 mark
- = 48 g (i.e. the concentration is 48 g/dm³ — the C3 concentration idea in organic clothing). 1 mark
Solutions of ethanoic acid and hydrochloric acid have the same concentration. Why does the ethanoic acid solution have a higher pH? Tick (✓) one box.
Esters are used in perfumes because they smell pleasant and are volatile. What does volatile mean? Tick (✓) one box.
Ethyl ethanoate (an ester) is made by warming ethanoic acid with which reagent? Tick (✓) one box.