Paper 2 essay guide: Carbon Compounds

This guide shows how Carbon Compounds is examined in the longer parts of Paper 2 and how to build an answer that earns every mark. It is a method, not an essay to memorise, the same planning steps and skeletons work whatever the exact wording of the question. Use it with the revision notes and key terms for this chapter, and rehearse the skeletons until you can reproduce them without notes.

How Carbon Compounds appears in Paper 2

Paper 2 has Section A (structured), Section B (limited-response) and Section C (open-response). Carbon Compounds can appear in any of them. In the structured questions you meet short parts, name a compound, write an equation, state an observation. In the longer limited-response and open-response questions you are asked to bring several ideas together, for example to:

• compare two homologous series (such as alkanes and alkenes) in structure, bonding and reactivity;
• explain a set of reactions of alcohols or carboxylic acids, with conditions and equations;
• describe how to prepare or test a named compound (ethanol, ethene, an ester);
• describe and explain esterification, or the difference between fats and oils; and
• interpret information about an unfamiliar carbon compound using the general formulae and functional groups you know.

A planning approach

Before writing, spend a moment planning so nothing is left out:

• Read the command word and the marks. They tell you how much and what kind of detail is wanted.
• List the marking points. Jot down the equations, conditions, observations and terms you intend to include, one line each.
• Order them logically. Definition or general point first, then the specific chemistry (reagent, condition, equation, observation), then a conclusion or comparison.
• Check the chemistry before you expand it. A balanced equation and the correct condition are worth more than extra sentences.

Model-answer skeleton: “Compare alkanes and alkenes”

Use a skeleton like this, then write it out in full sentences in the exam. Each bullet is a marking point, not a memorised line.

• Define both series. Alkanes are saturated hydrocarbons (single C–C bonds, general formula CₙH₂ₙ₊₂); alkenes are unsaturated (a C=C double bond, general formula CₙH₂ₙ).
• Give an example of each with formula (ethane C₂H₆; ethene C₂H₄).
• Compare reactivity. Alkanes are relatively unreactive; alkenes are more reactive because of the double bond.
• Support with a reaction. Alkenes undergo addition (ethene decolourises bromine water at room temperature); alkanes undergo substitution only in ultraviolet light.
• State a distinguishing test with observations for both. Bromine water is decolourised quickly by the alkene, unchanged by the alkane.
• Conclude by linking the difference in reactivity back to the presence or absence of the double bond.

Model-answer skeleton: “Explain the reactions of ethanol”

• Name the functional group, the hydroxyl group –OH.
• Combustion, burns completely to carbon dioxide and water.
• Oxidation, acidified potassium dichromate(VI) (orange to green) oxidises ethanol to ethanoic acid; give the equation with [O].
• Dehydration, hot aluminium oxide (or concentrated sulfuric acid) removes water to give ethene.
• Esterification, with ethanoic acid and a concentrated sulfuric acid catalyst, forms ethyl ethanoate and water.
• Finish with one use or one link (for example, ethanol as a fuel or solvent).

Model-answer skeleton: “Describe esterification and the uses of esters”

• Define esterification, the reaction of a carboxylic acid with an alcohol to form an ester and water.
• State the catalyst, a few drops of concentrated sulfuric acid.
• Give a named example with an equation, ethanoic acid + ethanol give ethyl ethanoate: CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O.
• Describe the observation, a sweet or fruity smell is produced.
• Explain the naming, the alkyl part comes from the alcohol and the -anoate part from the acid.
• Link to fats and oils, these are natural esters of glycerol and fatty acids; fats are saturated and solid, oils unsaturated and liquid; hydrogenating an oil makes a solid fat such as margarine.
• End with a use, esters as flavourings or in perfumes.

Command words and what each demands

The verb in the question tells you exactly what to do. Answer the command word that is used:

• Define, give the precise meaning. “Define a homologous series” needs the shared general formula, functional group and the CH₂ difference, not an example alone.
• State / name, give a short, exact answer with no explanation (a name, a formula, an observation).
• Describe, say what happens, in order. “Describe how to prepare ethanol by fermentation” needs the steps, the conditions and the equation.
• Explain, give reasons. “Explain why alkenes are more reactive than alkanes” needs the cause (the C=C double bond), not just the fact.
• Compare, give matched points about both, ideally with the word “whereas”. Cover the same properties for each so the comparison is balanced.
• Deduce / predict, apply the general formula or a trend to a new member you have not met.

Common ways marks are lost in the longer questions

• Writing everything you know about alcohols when the command word asked only for oxidation, answer the question set.
• Giving a reaction without its condition or catalyst.
• Describing a test but stating the observation for only one substance.
• Leaving equations unbalanced.
• In a “compare” answer, describing one series fully and the other barely.

Turning the skeleton into marks

Practise by taking a past-style question, writing the skeleton from memory, and only then expanding it into sentences. Mark yourself against the bullets: each equation, condition, observation and defined term is a point. A one-to-one teacher can mark a full-length answer and show you where a marking point was implied but not clearly stated, the commonest reason a strong answer scores less than it should across the SPM Chemistry papers.