Hydrocarbons and the five formula types
A hydrocarbon contains hydrogen and carbon only — any other element (e.g. oxygen in an alcohol) means it is not one. Five ways to represent a molecule, each tested separately: empirical = simplest whole-number ratio (ethane = CH₃); molecular = actual atom count (C₂H₆); general = pattern for a series (alkanes CnH2n+2); structural = arrangement, bonds implied (CH₃CH₂OH); displayed = every atom and bond drawn out.
Homologous series and functional group
A homologous series shares the same general formula and same functional group, each member differing by CH₂. Members have similar (not identical) chemical properties (they share the functional group) and a trend in physical properties (boiling point rises with chain length). A functional group gives the series its characteristic reactions, e.g. –OH in alcohols. Key wording: SIMILAR not same; same GENERAL not molecular formula.
Isomers and the three reaction types
Isomerism: two or more compounds with the same molecular formula but different structural formulae. C₄H₁₀ is butane (unbranched) or methylpropane (branched), so a molecular formula alone cannot identify a compound. The three reaction types (4.6, no mechanism): substitution — one atom/group is replaced by another (e.g. alkane + halogen in UV light); addition — two molecules join into one with nothing left over (at a C=C); combustion — burning in oxygen.
Drawn from real examiner reports.
Homologous series: similar, not same
Two definition traps. (1) "Same chemical properties" is rejected — members have only similar chemical properties. (2) "Same molecular formula" is rejected — successive members differ by CH₂; what is shared is the same general formula. Creditworthy: same general formula, same functional group, similar chemical properties, a physical-property trend.
June 2024 Paper 2C Q5(a): "same general formula" and "same functional group" scored, but "same chemical properties" (must be SIMILAR) and "same molecular formula" (should be the same GENERAL formula) lost the mark.
Displayed formula: draw every bond
A displayed formula shows all atoms AND all bonds as lines. The commonest error is a missing bond — e.g. writing O–H with no line between O and H, or a carbon with only 3 bonds. Count before finishing: 4 bonds to each C, 2 to each O, 1 to each H. A C=O or C=C double bond counts as two of that carbon's four bonds.
June 2024 Paper 2C Q5(b)(iii): a mark was lost because there was no bond drawn between the O and the H; examiner advice was to "show all the bonds" and check 4 bonds to each C, 2 to each O and 1 to each H.
Molecular formula can not name an isomer
A molecular formula does not distinguish isomers: the same CnHmOp could be an ester or a carboxylic acid. So when a question names a specific compound, giving only the molecular formula scores nothing — you must give its structural (or displayed) formula, which shows the actual arrangement of atoms and so pins down one isomer.
June 2024 Paper 2C Q5(c): a molecular formula could not show whether the ester or the acid was meant, so it scored no mark.
Empirical given when molecular is asked
When a question asks for the molecular formula (actual atom count), candidates wrongly give the empirical formula (simplest ratio) or only part of a structural formula. A recorded slip was C₅H₁₂O₂ instead of C₆H₁₂O₂ — a miscount of carbons. Read the command word: empirical = simplest ratio; molecular = actual count.
June 2024 Paper 2C Q5(d)(ii): a common error was C5H12O2 instead of C6H12O2, and a few gave the empirical formula or part of a structural formula instead of the molecular formula.
Substitution vs addition reaction
In a substitution reaction one atom or group is replaced by another, so a second product leaves (alkane + halogen in UV light gives a haloalkane + HX). In an addition reaction two molecules join into ONE with nothing left over — only at a C=C double bond. Do not label an alkane reaction "addition" or an alkene addition "substitution".
(common Edexcel chemistry error)
Drawing isomers: valency and duplicates
When drawing isomers of a molecular formula, two errors recur: (1) a carbon with 3 or 5 bonds instead of exactly 4; (2) a "new" isomer that is really the same molecule at a different angle — rotating a chain does not make a new isomer. Every carbon needs 4 bonds and every hydrogen 1, and each structure must be a genuinely different arrangement of atoms.
(common Edexcel chemistry error)
Decode the formula-type command word
Match the exact formula type asked. "Empirical" → simplify the ratio (C₆H₁₂O₆ → CH₂O). "Molecular" → real atom counts. "Structural" → the compact arrangement. "Displayed" → every atom and bond drawn. The wrong type scores nothing even if the chemistry is right.
Molecular formula from the general one
To find a molecular formula fast, substitute the number of carbons into the general formula: an alkane with 4 carbons has 2(4)+2 = 10 hydrogens, giving C₄H₁₀. This also lets you sanity-check a formula you worked out from reacting masses.
Tackling isomer questions
For "draw the isomers" questions, fix the molecular formula first, then redraw the carbon skeleton in every distinct way. Check each carbon has 4 bonds, and discard any structure that is just an earlier one rotated. Count distinct structures against the marks available.
Define a hydrocarbon.
Butane is the alkane with the molecular formula .
Calculate its relative formula mass (). Use : , .
