Alcohols: –OH group; ethanol combustion
Alcohols are a homologous series with the hydroxyl group –OH and general formula . The first four are methanol, ethanol, propan-1-ol and butan-1-ol, each differing by CH2 (Mr rises by 14). Ethanol (, Mr 46) is the key example. Its complete combustion in excess oxygen gives carbon dioxide and water: .
Ethanol manufacture: two routes
Two routes make ethanol. Fermentation (Core): glucose with yeast at ~37°C, anaerobic — — a batch process from renewable glucose giving impure ethanol (needs distillation). (Extended) Catalytic hydration: ethene + steam, phosphoric acid catalyst, ~300°C, 60–70 atm — a continuous process from non-renewable ethene giving pure ethanol.
(Extended) Oxidation to ethanoic acid
(Extended) Acidified potassium manganate(VII) or potassium dichromate(VI) oxidises ethanol to ethanoic acid, converting –OH into –COOH: (oxidation = hydrogen lost). Colour change: acidified purple → colourless; acidified orange → green. Ethanoic acid is a carboxylic acid (Mr 60); the carbon chain stays intact.
Drawn from real examiner reports.
Ethene, not ethane, for hydration
The catalytic-hydration route to ethanol starts from ethene (), an alkene with a C=C double bond, not ethane (). Ethane is a saturated alkane and cannot add steam. Remember "-ene" signals the double bond that allows addition — only ethene can be hydrated.
W22 P31 Q7(b)(i)(ii); S23 P31 Q7(e) — ethane given as the addition-route starting material.
Fermentation conditions are specific
Vague fermentation answers lose marks. State all four precisely: substrate glucose (not "sugar" or "starch"), catalyst yeast (not just "enzyme"), temperature ~37°C (not just "warm") and anaerobic conditions (oxygen excluded). Too hot denatures the yeast enzyme; too cold and it is very slow. Word equation: glucose → ethanol + carbon dioxide.
W22 P31 Q7(b)(i)(ii); S23 P31 Q7(e) — fermentation conditions vaguely stated.
Balance ethanol combustion (3 O2)
The balanced combustion of ethanol is . A common slip is writing or dropping the water. Balance in order C, then H, then O: the right side holds 7 O, and ethanol supplies 1, so 6 more come from .
Keep the two route conditions apart
(Extended) Keep the two ethanol routes apart. Fermentation is low temperature (~37°C), yeast, atmospheric pressure; catalytic hydration is ~300°C, 60–70 atm, phosphoric acid. "Use a high temperature to speed up fermentation" is wrong — it denatures the enzyme. "No catalyst needed for hydration" is also wrong. Yeast would not survive 300°C.
–OH ≠ –COOH functional group
The alcohol functional group is –OH (hydroxyl), with one oxygen; the carboxylic-acid group is –COOH (carboxyl), with two oxygens. Candidates often give –COOH for ethanol or –OH for ethanoic acid. Ethanol contains –OH; only after oxidation does the product (ethanoic acid) contain –COOH. Count the oxygens to tell them apart.
W22 P31 Q7(a)(i); W22 P41 Q6(c); W22 P42 Q6(d) — –OH confused with –COOH.
KMnO4 purple→colourless, not green
(Extended) Match the oxidising agent to its colour change. Acidified potassium manganate(VII) goes purple → colourless; acidified potassium dichromate(VI) goes orange → green. Writing "orange to green" for manganate(VII), or "purple to colourless" for dichromate(VI), swaps the two. Both signal that ethanol has been oxidised to ethanoic acid.
Compare must cover both routes
On a compare ethanol question, give both methods for every point: feedstock, catalyst, temperature, pressure, batch/continuous. "Fermentation uses yeast" alone earns nothing — contrast it with hydration's phosphoric acid catalyst. One mark per comparative point.
Balance combustion C, H, then O
Balance any alcohol combustion in order C → H → O. Set carbon dioxide from the carbons and water from the hydrogens, then count oxygen on the right, subtract the O in the alcohol, and halve for the coefficient. Water is liquid (l) at RTP.
State fermentation conditions in full
For fermentation of glucose to ethanol, name the four conditions explicitly: glucose, yeast, ~37°C, anaerobic. Avoid "enzyme", "warm" or "no air" alone — the mark scheme wants the specific substrate, catalyst, numerical temperature and the word anaerobic.
Pair reagent with its colour change
(Extended) State any oxidation colour change with its named reagent: acidified potassium manganate(VII) purple → colourless, or acidified potassium dichromate(VI) orange → green. Give the direction, not just two colours, and do not swap the reagents.
Alcohol — a member of the homologous series characterised by the hydroxyl functional group (–OH), with general formula (also written ).
Ethanol (, relative formula mass ) is the alcohol of primary importance at Cambridge IGCSE level: it is the product of fermentation and catalytic hydration, and it undergoes combustion and (Extended) oxidation.
Fermentation — the conversion of glucose to ethanol and carbon dioxide by the action of yeast enzymes, carried out under anaerobic conditions at approximately 37 °C.
Catalytic hydration (Extended) — the direct addition of steam across the C=C double bond of ethene using a phosphoric acid catalyst at approximately 300 °C and 60–70 atm.
Oxidation of ethanol (Extended) — the conversion of the –OH group to a –COOH group using an oxidising agent (e.g. acidified or acidified ), giving ethanoic acid.
Define alcohol and state its functional group.
Ethanol is used as a fuel in spirit burners. It burns in excess oxygen to produce carbon dioxide and water.
(a) Write a balanced symbol equation for the complete combustion of ethanol, .
(b) State the type of reaction occurring during combustion.
