Hydrocarbon and the five formula types
Hydrocarbon = a compound of hydrogen and carbon only ("only" is the marking point; ethanol has oxygen too). The five representations: empirical (simplest ratio, ethane ); molecular (actual atoms, ); general (series in , alkanes ); structural (); displayed (every atom and bond drawn). Give exactly the one asked for.
Homologous series: four shared features
A homologous series is a family sharing: (1) the same general formula; (2) the same functional group — the reactive part that sets the chemistry; (3) similar chemical properties; (4) a gradual trend in physical properties (boiling point rises up the series). Successive members differ by ( +14); the chemical properties stay similar while the physical ones trend — never swap these. Isomers share a molecular formula but have different structural formulae.
Substitution, addition or combustion
Classify a reaction as one of three types (no mechanism examined). Substitution — one atom is replaced, giving two products (alkane + halogen in UV light haloalkane + hydrogen halide). Addition — a molecule adds across a double bond to give one product only (alkenes only, no UV). Combustion — burning in oxygen ( + water; incomplete gives carbon monoxide/soot). Clue: two products = substitution; one = addition; oxygen a reactant = combustion.
Drawn from real examiner reports.
Chemical stays similar, physical trends
In a homologous series the chemical properties stay similar (shared functional group) while the physical properties (boiling point, density) show a gradual trend up the series. Candidates reverse these and score zero. A second wrong answer, "same empirical formula", is false — the real feature is the same general formula.
Nov24 1C Q10b: examiner noted recurring confusion between similar chemical properties and a trend in physical properties, and that "same empirical formula" was a common wrong answer for homologous-series features.
Every carbon needs four bonds
When drawing isomers, every carbon must have exactly four bonds — a carbon with three or five bonds is not a real molecule and earns nothing. The other frequent error is redrawing the same molecule rotated or flipped, or confusing but-1-ene with but-2-ene. Before counting one, check four bonds per carbon and a genuinely different structure.
Nov24 1C Q10biii: drawing isomers exposed carbons drawn with 3 or 5 bonds, and structures identical to the stem merely drawn at a different angle.
Hydrocarbon means H and C only
The definition of a hydrocarbon must contain the word only: a compound of hydrogen and carbon only. "A compound containing hydrogen and carbon" is not enough, because that also fits ethanol and other compounds that additionally contain oxygen. The single word "only" is the difference between one mark and zero.
Don't repeat a term from the stem
Read what the question already gives you. Asked for a feature of a homologous series, some candidates simply wrote "general formula" — but the general formula was already printed in the stem, so it could not be credited again. Always offer a new point the stem has not already supplied.
Nov24 1C Q10bi: some candidates repeated "general formula" as their answer, which had already been given in the question stem.
Empirical is not molecular formula
The empirical formula is the simplest whole-number ratio of atoms (ethane is ); the molecular formula is the actual count in one molecule (ethane is ). Do not treat "same empirical formula" as a feature of a homologous series, and do not give an empirical formula when the molecular formula is asked for.
Draw isomers systematically
To list every isomer without repeats: draw the longest chain first, then shorten it by one carbon and attach that carbon as a branch at each position, checking four bonds per carbon. gives exactly two: butane and methylpropane.
Two asked? Give exactly two
If a question asks for two features or two answers, give exactly two. Adding a third, wrong answer can cancel out a correct one, so an extra "safety" answer costs marks rather than gaining them.
Name or formula — read the command
Check whether the command asks for a name or a formula and answer in that form only. Giving a formula when the name was required (or the reverse) scores nothing even when the chemistry is correct.
Check which formula is asked
Empirical, molecular, general, structural and displayed formulae are five different answers. A displayed-formula question is not satisfied by a molecular formula. Underline which one the command wants before you write, and give exactly that representation.
Organic chemistry is the chemistry of carbon compounds. This topic covers the language of organic chemistry: what a hydrocarbon is, the five ways to represent a molecule, homologous series, isomers, IUPAC naming, and how to classify reactions.
Learn these in the exact mark-scheme form — each needs the underlined element.
Define the term hydrocarbon.
Propane has the molecular formula .
Calculate the relative formula mass () of propane. (: C = 12, H = 1.)
