Ethanol — , , an alcohol
Ethanol has the molecular formula (equivalently ). Its relative formula mass sums the of every atom: . There are six hydrogens — five in the part plus one in the part. The group is the functional group that makes ethanol an alcohol, and because oxygen sits in the formula too, ethanol is not a hydrocarbon.
Complete combustion — excess oxygen, giving only and water
Complete combustion needs a plentiful (excess) supply of oxygen: every carbon is fully oxidised to carbon dioxide and every hydrogen ends up in water, so no carbon monoxide or soot forms. The equation is , with states , , , . The reaction is exothermic, transferring energy to the surroundings — which is why ethanol is a fuel.
Uses of ethanol — solvent and fuel
0654 names exactly two uses, and both must be known by name. As a solvent: the group lets ethanol mix with polar substances such as water, while the hydrocarbon part of its structure dissolves many substances that water cannot — hence its use in perfumes, printing inks and methylated spirits. As a fuel: complete combustion releases energy, ethanol burns more cleanly than some other fuels, and it can be blended with or used instead of petrol in engines.
Drawn from real examiner reports.
Oxygen gas is diatomic, not single O
When balancing , treating oxygen as a lone atom "O" rather than the molecule doubles or halves the coefficient. Seven oxygen atoms are needed on the right; ethanol itself supplies one, leaving six; each molecule supplies two — so the coefficient is 3.
Flagged Jun 2022 P16 Q16, P42 Q8aii; Nov 2023 P11 Q15
Wrong ratio in reacting-mass sums
Take the ratio from the balanced equation for the substance actually asked about: ethanol : is , whereas ethanol : is and ethanol : is also . Candidates mix the two product calculations, or apply the ratio straight to the given mass without first dividing it by the of ethanol.
Miscounting the hydrogens in ethanol
The formula contains six hydrogen atoms — five in the part plus one in the part. Counting only five gives and leaves the hydrogen impossible to balance in whole numbers, so a single miscount corrupts the mass calculation and the equation together.
Naming a use vaguely scores nothing
The two mark-scheme answers are solvent and fuel. Replies such as "used in industry" or "used in the home" identify neither of the specification uses and gain no credit. If two uses are asked for over two marks, give the two names; add the reasoning (mixes with water and with non-polar substances; burns to release energy) only when the question asks you to explain.
Ethanol is not a hydrocarbon
A hydrocarbon contains hydrogen and carbon only — the word "only" is required for the mark. Ethanol, , also contains oxygen in its group, so it fails that definition however much it burns like one. Ethanol is an alcohol. Examiners also expect a hydrocarbon to be called a compound, not an element, an atom or a mixture.
Flagged Nov 2022 P32 Q8c; Nov 2023 P41 Q2a
"Complete" means excess oxygen, not finished
The word "complete" describes the oxygen supply, not the reaction running to the end. Complete combustion requires a plentiful (excess) supply of oxygen so that the fuel is fully oxidised. An answer that says only "ethanol burns to give carbon dioxide and water" has given the products but omitted the condition that makes the combustion complete.
Carbon monoxide means incomplete burning
Carbon monoxide and unburned carbon (soot) are products of incomplete combustion, in a limited supply of oxygen — never list them among the products of complete combustion. When asked why carbon monoxide matters, say it is poisonous/toxic (it binds to haemoglobin, so the blood carries less oxygen). Vague phrasing such as "hard to breathe" is not credited.
Flagged Jun 2023 P31 Q8cii; Nov 2023 P41 Q2d
Balance carbon, then hydrogen, oxygen last
Balance the elements found in one species first: 2 carbons give , 6 hydrogens give . Oxygen is in three species, so leave it last — 7 atoms are needed on the right, ethanol supplies 1, so gives the other 6. Then check C, H and O.
Word equation means words, not formulae
If the command is "write the word equation", answer in words: ethanol + oxygen carbon dioxide + water. Formulae written where words were asked for score nothing — and the reverse also applies, since "write the symbol equation" needs balanced formulae.
Match the command word and the tariff
State = name it only. Describe = say what happens. Explain = give the causal link, so for incomplete combustion that is limited oxygen, therefore carbon only partly oxidised, therefore carbon monoxide forms — not just "it is dangerous". Give as many points as there are marks.
Show the ratio, then the mass, with units
Reacting-mass answers carry method marks. State the ratio read off the balanced equation, divide the given mass by the of that substance, then multiply by the ratio and by the other . Always write the unit, g, on the final line — a bare number loses the accuracy mark.
Cambridge 0654 spec reference: Section C11 "Organic chemistry", sub-topic C11.6 "Alcohols". Core (both statements listed under C11.6): (1) describe the complete combustion of ethanol; (2) state the uses of ethanol as a solvent and as a fuel. There are no Supplement/Extended statements under C11.6.
| Term | Mark-scheme-precise meaning |
|---|---|
| Ethanol | An alcohol with molecular formula (equivalently ); ; contains the functional group |
| Alcohol | An organic compound whose molecule contains the functional group attached to a hydrocarbon chain |
| Complete combustion | Burning in a plentiful (excess) supply of oxygen, so that a fuel is fully oxidised, producing only carbon dioxide and water (no carbon monoxide or soot) |
| Incomplete combustion | Burning in a limited (insufficient) supply of oxygen, producing carbon monoxide and/or unburned carbon (soot) in addition to, or instead of, some of the carbon dioxide |
Complete combustion of ethanol (balanced symbol equation):
Ethanol reacts with oxygen to give carbon dioxide and water . This reaction is exothermic (it transfers energy to the surroundings) -- the reason ethanol is useful as a fuel.
Uses of ethanol (Core, both must be known by name):
| Use | Why ethanol is suited to this use |
|---|---|
| Solvent | The group lets it mix with polar substances (like water), while its hydrocarbon part also dissolves many substances water cannot (e.g. in perfumes, printing inks, methylated spirits) |
| Fuel | Complete combustion releases energy; ethanol burns cleanly and can be blended with or used instead of petrol in engines |
Name the two uses of ethanol given in the specification.
Ethanol has the molecular formula .
Calculate the relative formula mass, , of ethanol. (2 marks)
