Alkanes and their general formula
Alkanes are saturated hydrocarbons forming a homologous series — same general formula, same functional group, similar chemical properties, each member differing by CH₂. The general formula (spec 4.19) is CnH2n+2: substitute n, e.g. n=1 methane CH₄, n=5 pentane C₅H₁₂ (full series in the note). Boiling and melting points show a trend — they rise as molecules get bigger, not stay the same.
Why alkanes are saturated
A hydrocarbon contains hydrogen and carbon only. Alkanes are saturated (spec 4.20) because every carbon–carbon bond is a single covalent bond (C–C) — there are no C=C double bonds. "Saturated" means the molecule holds the maximum number of hydrogen atoms. This contrasts with alkenes, which are unsaturated (they contain a C=C). Because alkanes have no C=C, they do not decolourise bromine water — alkenes do.
Alkane reactions: combustion, substitution
Alkanes are fairly unreactive, but two reactions are examinable. (1) Combustion: in plenty of oxygen they give CO₂ and water, e.g. CH₄ + 2O₂ → CO₂ + 2H₂O; in limited oxygen, CO and/or carbon (soot). (2) Substitution with halogens (spec 4.22): under UV light a hydrogen is replaced by a halogen, e.g. CH₄ + Cl₂ → CH₃Cl + HCl. This is a photochemical reaction, limited to mono-substitution; the mechanism is not required.
Drawn from real examiner reports.
Displayed formula: show every bond
A displayed formula must show every atom and bond as lines. The common error is a missing bond — writing CH₃ as a group instead of three separate C–H bonds, or forgetting a C–C bond. Check each carbon has exactly 4 bonds and each hydrogen 1. A condensed structural formula is allowed only where asked; a displayed formula needs every bond drawn.
June 2024 Paper 2C displayed-formula question: candidates lost marks for not showing every bond; examiners stressed each carbon must show 4 bonds and each hydrogen 1, with all C–H and C–C bonds drawn as lines.
Homologous series: "similar", not "same"
Examiners credit "similar chemical properties", "same general formula" and "same functional group" — but "same chemical properties" is not creditworthy, since reactivity changes slightly down a series. Two more traps: the same general (not molecular) formula — each member differs by CH₂; and boiling/melting points form a trend, not equal values.
June 2024 Paper 2C homologous-series question: "same chemical properties" was not creditworthy (use "similar"); November 2024 Paper 2C: a mark was lost for confusing "same general formula" with "same molecular formula", and boiling/melting points were noted as a trend rather than equal values.
It is substitution, not addition
The alkane + halogen reaction is substitution (a hydrogen is replaced by a halogen), not addition (the alkene reaction). Three slips: (1) omitting the condition — it needs UV light; (2) forgetting the second product — the equation must include HCl, e.g. CH₄ + Cl₂ → CH₃Cl + HCl; (3) adding more than one halogen when only mono-substitution is asked.
Grounded in the spec 4.22 wording (mono-substitution, ultraviolet radiation) and the general organic-chemistry equation-balancing errors flagged across the June 2024 Paper 2C report, where unbalanced or incomplete organic equations were a recurring cause of lost marks.
Saturated vs unsaturated
Saturated (alkanes) means only single C–C bonds and the maximum number of hydrogen atoms. Unsaturated (alkenes) means the molecule contains a C=C double bond, so more atoms could be added across it. Do not say "saturated means full of hydrogen" without mentioning single bonds / no C=C — the single-bond point is what earns the mark.
(common Edexcel chemistry error)
Bromine water: alkanes do not react
With bromine water, an alkane (saturated, no C=C) leaves it orange/brown — no change; an alkene (unsaturated) turns it colourless (decolourised). Candidates often give the colour change the wrong way round, or claim the alkane decolourises it. Note: UV light belongs with alkane substitution, NOT with the bromine-water test.
(common Edexcel chemistry error)
Complete vs incomplete combustion
Complete combustion (plenty of oxygen) gives carbon dioxide and water. Incomplete combustion (limited oxygen) gives toxic carbon monoxide CO and/or carbon (soot), plus water. Writing CO₂ for incomplete combustion, or CO for complete, loses the mark; "limited" oxygen is not the same as "no" oxygen.
(common Edexcel chemistry error)
Build the alkane, then balance C to H to O
Write the molecular formula from CnH2n+2 first. To balance a combustion equation, work in a fixed order: carbon (one CO₂ per C), then hydrogen (one H₂O per 2 H), then oxygen as O₂ last. If the oxygen count is odd, double every coefficient to clear the fraction.
Balancing a substitution equation
For an alkane + halogen substitution, swap one hydrogen for one halogen atom and add HCl as the second product so the atoms balance — e.g. C₂H₆ + Cl₂ → C₂H₅Cl + HCl. Then state the condition (UV light), which often carries its own mark.
State the reaction conditions
Conditions frequently score their own mark, so always name them: UV light / sunlight for halogen substitution, and plenty of oxygen for complete combustion. Missing the condition is a common way to drop an easy mark even when the equation itself is right.
| Name | Molecular formula | ||
|---|---|---|---|
| 1 | Methane | 16 | |
| 2 | Ethane | 30 | |
| 3 | Propane | 44 | |
| 4 | Butane | 58 | |
| 5 | Pentane | 72 |
( = relative formula mass, using the 4CH1 data sheet values , .)
Define hydrocarbon.
Hexane is the alkane with six carbon atoms (). Using the general formula , work out the molecular formula of hexane and its relative formula mass . (: , .)
