Alkenes: unsaturated, CnH2n, C=C
Alkenes are unsaturated hydrocarbons with general formula (). Their functional group is the carbon–carbon double bond (C=C) — one bond can open so atoms add across it, which is why alkenes undergo addition while saturated alkanes do not. Key members: ethene (Mr 28), propene (42) and the butenes (56), which differ in where the double bond sits.
Bromine water test
The bromine water test distinguishes unsaturated from saturated hydrocarbons. Add orange-brown bromine water and shake: an alkene decolourises it (orange-brown to colourless) because bromine adds across the C=C to a colourless dibromo compound, e.g. . An alkane has no C=C, so no addition occurs and the colour is retained. Report the visible colour change, not the product name.
Cracking and (Extended) addition
Cracking breaks long-chain alkanes into smaller, more useful molecules — shorter alkanes and alkenes — using high temperature and a catalyst, e.g. . (Extended) Alkenes undergo addition across the C=C: hydrogen gives an alkane, bromine a dibromoalkane, HBr a bromoalkane, steam an alcohol. Each gives one saturated product with no by-product.
Drawn from real examiner reports.
Cracking ≠ distillation
Cracking is a chemical change — long-chain hydrocarbon molecules are broken down into smaller molecules, including alkenes. Describing it as "separating crude oil" or "distilling" is wrong: that is fractional distillation, a physical separation with no new substances. A definition that omits alkenes, or just says "breaks down compounds", is too vague.
W22 P31 Q1(b)(i); W22 P32 Q6(d)(i); S22 P31 Q6(d)(i) — cracking described as separation/distillation.
Cracking needs a catalyst, not pressure
The conditions for cracking are high temperature AND a catalyst (e.g. aluminium oxide). The most common error is writing high pressure instead of the catalyst — high pressure belongs to the Haber process, not cracking. "High temperature only" also drops a mark: both conditions are required. Oxygen is excluded so the hydrocarbon cracks rather than burns.
W22 P11 Q38; S22 P31 Q6(d)(ii) — high pressure given instead of catalyst.
Decolourised, not cloudy
For the bromine water test, the observation is that the orange-brown colour is decolourised (becomes colourless). Saying it "goes cloudy" is wrong — cloudiness means a precipitate (as in the silver nitrate test). Naming the product "1,2-dibromoethane" instead of the colour change also scores zero: examiners want the visible change you see, not the substance formed.
W22 P32 Q1(b); S22 P31 Q6(a)(i) — cloudy stated, or a process named instead of the test.
Ethene, not ethane, is hydrated
Ethanol is made by hydrating ethene (), an unsaturated alkene, not ethane (). Ethane is a saturated alkane with no C=C, so it cannot add steam. The "-ene" ending signals the double bond that makes addition possible. Writing "ethane + steam" is a frequently penalised slip.
W22 P31 Q7(b)(i)(ii); S23 P31 Q7(e) — ethane given for the addition route instead of ethene.
Addition forms one product only
(Extended) In an addition reaction the whole reagent adds across the C=C, giving one saturated product with no by-product. Writing "C2H4 + H2 → C2H6 + H" (a leftover atom) or treating it like substitution (which gives a by-product such as HCl) is wrong. Alkenes add; alkanes substitute — no atoms are lost.
Alkene CnH2n vs alkane CnH2n+2
Do not mix the general formulae: alkanes are (saturated), alkenes are (one C=C). Ethene is , not (that is ethane). Using the wrong formula makes every derived Mr and equation wrong, so check the hydrogen count against the ending: "-ane" has H, "-ene" has H.
Answer cracking in two parts
Answer in two parts: the definition (long-chain hydrocarbons broken into shorter molecules, including alkenes) and the conditions (high temperature AND a catalyst). Any equation balances by atom count — a long alkane gives a shorter alkane plus alkene(s).
Give observation and conclusion
For the bromine water test, write two separate points: the observation (decolourised to colourless for an alkene; retained for an alkane) and the conclusion (unsaturated / has C=C, or saturated). A conclusion with no observation, or vice versa, drops a mark.
Build addition products across C=C
(Extended) Join the alkene + reagent by a single arrow to one product, adding one atom or group to each carbon of the C=C: H+H gives an alkane, Br+Br a dibromoalkane, H+Br a bromoalkane, H+OH an alcohol. Count atoms on both sides to check it balances.
Check the formula against the ending
Before any alkene calculation, confirm the formula fits — the "-ene" ending means hydrogens and one C=C. A wrong formula spoils the Mr and later steps, so check the hydrogen count first: propene is , Mr 42.
Alkene — an unsaturated hydrocarbon with the general formula (). Alkenes contain a carbon–carbon double bond (), which is their functional group.
Unsaturated — a hydrocarbon that contains one or more double bonds; it can undergo addition reactions by adding atoms across the double bond.
Saturated — a hydrocarbon that contains only single bonds (e.g. alkanes, general formula ); it cannot undergo addition reactions.
Two-element mark-scheme check for "unsaturated":
Define unsaturated hydrocarbon and state which homologous series this describes.
Decane () is a large hydrocarbon that can be cracked.
(a) State the conditions required for cracking.
(b) Write a balanced equation for one possible cracking of decane to give octane () and one other product. Name the other product.
(c) Explain why cracking is industrially important.
