The most frequent Carbon Compounds errors and how to fix them: wrong general formulae, confusing alkanes with alkenes, missing reaction conditions and catalysts, mis-numbered or repeated isomers, reversed ester names and unbalanced equations, each shown with the version that scores.
Carbon Compounds is generous with marks if you are precise, and unforgiving if you are not. Below are the mistakes we see most often, grouped by content standard. For each one we show what students write, why it loses marks, and the correct version. Read them beside your own answers.
Homologous series and formulae (10.2)
1. Using the wrong general formula. Students write CₙH₂ₙ for an alkane or CₙH₂ₙ₊₂ for an alkene. This makes every molecular formula wrong. Correct: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH, carboxylic acids CₙH₂ₙ₊₁COOH. Learn all four and check n each time.
2. Describing a homologous series loosely. Writing only “compounds that are similar” scores nothing. Correct: state that members share the same general formula and functional group, differ by CH₂, show a gradual change in physical properties, and have similar chemical properties.
3. Mixing up “saturated” and “unsaturated”. Students call an alkene saturated. Correct: saturated means only single carbon–carbon bonds (alkanes); unsaturated means at least one carbon–carbon double bond (alkenes).
Naming and nomenclature (10.2)
4. Forgetting the locant. Writing “butene” or “propanol” without a number loses the mark when the position matters. Correct: give the locant, but-1-ene, propan-1-ol, and number the chain so the functional group gets the lowest possible position.
5. Numbering from the wrong end. Students number a chain so the double bond or branch lands on a higher number. Correct: always number from the end that gives the functional group and substituents the lowest locants.
Reactions of alkanes and alkenes (10.3–10.4)
6. Calling an alkene reaction “substitution”. When bromine adds to ethene, students label it substitution. Correct: alkenes undergo addition (the double bond opens); alkanes undergo substitution, and only in ultraviolet light.
7. Omitting the condition for substitution. Writing CH₄ + Cl₂ → CH₃Cl + HCl with no condition loses a mark. Correct: state that ultraviolet light (or sunlight) is required.
8. Giving only one observation in a distinguishing test. Students say “the alkene decolourises bromine water” but say nothing about the alkane. Correct: give both results, the alkene decolourises brown bromine water at room temperature; the alkane shows no change.
9. Forgetting the products of incomplete combustion. Answers say “less carbon dioxide”. Correct: incomplete combustion produces carbon monoxide and soot (carbon) as well as water, because oxygen is limited.
Isomerism (10.5)
10. Drawing the same molecule twice. Students bend a straight chain and call it a new isomer. Correct: isomers must differ in the order the atoms are joined, not in how the drawing is rotated or bent. Name each one to check they are genuinely different.
11. Changing the molecular formula. While drawing isomers, students add or lose a hydrogen. Correct: keep the molecular formula fixed and make sure every carbon still has exactly four bonds.
Alcohols and carboxylic acids (10.6–10.7)
12. Confusing oxidation with dehydration. Students say heating ethanol with acidified potassium dichromate(VI) gives ethene. Correct: oxidation gives ethanoic acid (orange to green); dehydration (hot aluminium oxide, or concentrated sulfuric acid) gives ethene.
13. Wrong colour change or wrong reagent. Answers pair dichromate(VI) with “purple to colourless”. Correct: dichromate(VI) goes orange to green; manganate(VII) goes purple to colourless. Match the colour change to the named reagent.
14. Forgetting a carboxylic acid is still an acid. Students think ethanoic acid does not react with carbonates. Correct: it shows all acid reactions, with reactive metals (→ salt + hydrogen), carbonates (→ salt + water + carbon dioxide) and alkalis (→ salt + water).
Esters, fats and oils (10.8)
15. Naming the ester the wrong way round. Students write “ethanoic ethyl” or swap the parts. Correct: the alkyl part comes from the alcohol and the -anoate part from the acid, so ethanol + ethanoic acid gives ethyl ethanoate.
16. Omitting the catalyst in esterification. The equation is written with no catalyst. Correct: state concentrated sulfuric acid as the catalyst, and remember water is the other product.
17. Confusing fats and oils. Students say oils are saturated. Correct: fats are saturated and solid at room temperature; oils are unsaturated and liquid. Hydrogenating an oil raises its melting point.
The habit that fixes most of these
Almost every mistake above comes from skipping a detail: the exact formula, the condition, the second observation, or the balance of an equation. Before you move on from any answer, check four things, is the general formula right, is the condition or catalyst stated, is the equation balanced, and (for a test) have you given both the positive and the negative result. A one-to-one teacher can mark your answers against this same checklist so the slips are caught early, well before the SPM Chemistry written papers.
More slips worth knowing
18. Writing an unbalanced combustion equation. Students write CH₄ + O₂ → CO₂ + H₂O and stop. Correct: balance it, CH₄ + 2O₂ → CO₂ + 2H₂O, and check the hydrogen and oxygen atoms on both sides. An unbalanced equation loses the balancing mark even when the products are right.
19. Saying fermentation needs air. Because combustion needs oxygen, students assume fermentation does too. Correct: fermentation is anaerobic; air is excluded so that yeast enzymes convert glucose to ethanol and carbon dioxide rather than oxidising it further.
20. Forgetting the enzyme or the temperature idea in fermentation. Answers name only “yeast”. Correct: it is the enzymes in yeast that catalyse the change, and the mixture is kept warm, too hot destroys the enzymes, too cold slows them.
21. Muddling the functional groups. Students place –OH on a carboxylic acid or –COOH on an alcohol. Correct: an alcohol carries –OH (hydroxyl); a carboxylic acid carries –COOH (carboxyl). The whole –COOH group is one functional group.
22. Giving the ester link as –O– only. Students shorten the ester linkage. Correct: the ester functional group is –COO– (an oxygen double-bonded to carbon and a single-bonded oxygen), formed between the acid and the alcohol.
23. Assuming addition polymerisation changes the empirical formula. Students think poly(ethene) has a different formula ratio from ethene. Correct: in addition polymerisation the monomer units simply join, so the repeating unit keeps the same atoms; nothing is added or removed.
Why precision pays here
Carbon Compounds packs a great deal of markable detail into short answers, which is why a careless phrase can turn a correct idea into a lost mark. Build the four-part check into every answer, correct formula, stated condition or catalyst, balanced equation, and both results in any test, and rehearse the reaction map (alkane, alkene, alcohol, acid, ester) until each reagent, condition and colour change is automatic. Work through the practice questions for this chapter, then compare your wording against the corrections above; the gap between “nearly right” and “full marks” in this topic is almost always one of these details.
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