Four homologous series (up to C4)
The syllabus names four series up to four carbons, prefixed by carbon count: meth- (1C), eth- (2C), prop- (3C), but- (4C). Alkanes (-ane, ) are saturated with no functional group. Alkenes (-ene, ) have a C=C double bond; methene cannot exist. Alcohols (-ol) carry -OH; carboxylic acids (-anoic acid) carry -COOH. Examples: methane–butane, ethene–but-2-ene, methanol–butan-1-ol, methanoic–butanoic acid.
Displayed formulae — valency rules
A displayed formula shows every atom and every bond as a line. Fixed valencies must hold: carbon forms exactly 4 bonds, hydrogen exactly 1, oxygen exactly 2. A carbon with 5 bonds or a hydrogen with 2 is always wrong and loses the mark for that atom. In -COOH the carboxyl carbon has a double bond to one oxygen, a single bond to the -O-H oxygen, and one bond to the chain — four in total. After drawing, count the bonds at every atom.
Homologous series — mark-scheme definition
A homologous series is a family of compounds with the SAME functional group and the same general formula, whose consecutive members differ by -CH₂- (a mass difference of 14). The word must be "same", never "similar". Members share similar chemical reactions and show a gradual change in physical properties, such as rising boiling point down the series. It applies to alkanes, alkenes, alcohols AND carboxylic acids, not just alkanes and alkenes.
Drawn from real examiner reports.
-OH and -COOH confused
The alcohol group -OH (hydroxyl) and the acid group -COOH (carboxyl) are distinct. -OH is one oxygen bonded to hydrogen, with no C=O. -COOH contains BOTH a C=O double bond and an O-H on the same carbon. A useful check: the carboxyl group has its own carbon (the "C" in -COOH); the hydroxyl group does not. Alcohols end in -ol; carboxylic acids end in -anoic acid.
W22 Paper 31 Q7(a)(i); W22 Paper 41 Q6(c); W22 Paper 42 Q6(d) — alcohol and carboxyl functional groups consistently confused across three sittings.
Pentavalent C or divalent H
Displayed-formula drawings often show a carbon with five bonds or a hydrogen with two — both are impossible and score zero for that atom. Every carbon must show exactly 4 bonds, every hydrogen 1, every oxygen 2. The usual slip is adding a double bond or a group without removing a hydrogen. After drawing, count the lines at each atom and redraw any that break the rule.
W22 Paper 31 Q7(a)(i); W22 Paper 41 Q6(c); W22 Paper 42 Q6(d) — pentavalent carbon and divalent hydrogen are persistent displayed-formula errors across multiple sittings.
"Similar" instead of "same"
In the homologous series definition, the mark scheme requires the SAME functional group. Writing "similar functional group" is not credited and loses that element of the definition. Give the two-part answer every time: "same functional group" plus "same general formula" (or "consecutive members differ by -CH₂-"). One wrong word costs a mark at almost every sitting.
S23 Paper 31 Q7(b) — "similar" not credited; the word must be "same".
Formula types mixed up
Four formula types are examined and must not be swapped. Molecular gives the total count of each atom (ethanoic acid C₂H₄O₂). Structural is shorthand for the groups (CH₃COOH). Displayed shows every bond as a line. Empirical is the simplest whole-number ratio (CH₂O). Giving CH₃COOH when a molecular formula is asked for scores zero, and vice versa.
W22 Paper 42 Q6(c); W22 Paper 41 Q6(a)(ii) — structural formula given in place of molecular formula across sittings.
Alkene locants: but-1-ene vs but-2-ene
But-1-ene has its C=C between carbons 1 and 2; but-2-ene has it between carbons 2 and 3. They are different compounds (isomers) sharing the molecular formula C₄H₈, so the locant is essential from four carbons up. Propene needs no number — a three-carbon chain has only one possible position for the double bond. Omitting the locant for but-1-ene or but-2-ene loses marks.
Series is not just alkanes/alkenes
A homologous series is not limited to alkanes and alkenes. Alcohols and carboxylic acids each form their own homologous series, defined by their functional group (-OH, -COOH). Candidates who think only hydrocarbons form series miss marks when asked to identify or extend an alcohol or acid series. Any family sharing one functional group and a general formula qualifies.
S23 Paper 31 Q7(b) — candidates confuse homologous series with alkanes/alkenes only, not realising it applies to alcohols and carboxylic acids too.
Naming an organic compound
Three steps. Identify the functional group: none → alkane, C=C → alkene, -OH → alcohol, -COOH → acid. Count the carbons in the main chain for the prefix: 1 meth-, 2 eth-, 3 prop-, 4 but-. Add the suffix and any locant (e.g. but-2-ene, propan-1-ol).
Drawing a displayed formula
Draw the carbon chain first, then add bonds — single, or one C=C for an alkene. Attach the functional group (-OH or -COOH) at the stated carbon. Fill remaining bonds with hydrogen so each carbon has four bonds. Then check every atom: C = 4, H = 1, O = 2.
Give the exact formula type asked
Read whether the question wants a molecular, structural, displayed or empirical formula, and give only that one. A structural formula like CH₃COOH scores zero when the molecular formula C₂H₄O₂ is required, and vice versa. A displayed formula must show every bond as a line.
Homologous series: A family of organic compounds that have the same functional group, the same general formula, and differ by between consecutive members. Members show a gradual change in physical properties and similar chemical properties.
Definition-precision trap: The word must be "same" functional group — not "similar". Writing "similar" loses the mark.
Molecular formula: Shows the total number of each type of atom in one molecule (e.g. for ethanoic acid).
Displayed formula: Shows every atom and every bond as a line — no shorthand. Carbon always has 4 bonds; hydrogen always has 1 bond; oxygen always has 2 bonds.
Structural formula: A shorthand showing groups of atoms (e.g. ). Not the same as a molecular formula — do not give a structural formula when asked for a molecular formula.
Define the term "homologous series".
(a) Calculate the relative formula mass () of butane, .
(b) Calculate the relative formula mass () of ethanol, .
(: C = 12, H = 1, O = 16)
