Your revision route
What you’ll learn
- Describe crude oil as a finite hydrocarbon mixture formed from ancient biomass.
- Explain fractional distillation using evaporation, condensation and different boiling points.
- Relate hydrocarbon size to boiling point, viscosity and flammability and balance combustion equations.
- Explain catalytic and steam cracking and use bromine water to test for alkenes.
- Separate Chemistry: recognise alkenes, alcohols and carboxylic acids and describe their specified reactions.
- Separate Chemistry: explain addition polymerisation and identify monomers and repeating units.
- Separate Chemistry Higher Tier: explain condensation polymers, amino acids and polypeptides.
- Separate Chemistry: identify DNA, proteins, starch and cellulose as natural polymers and name their monomer types.
Build the big picture
Key ideas
Crude oil is a chemical library, not one substance
Buried ancient biomass became a finite mixture whose shelves are crowded mainly with hydrocarbons.
- Rock reservoirs hold this finite material, produced over geological time after mainly plankton biomass was buried with mud.
- Carbon can build related families, which is why nature and industry contain so many different carbon compounds.
- Every molecule classed as a hydrocarbon is built exclusively from carbon and hydrogen.
- Most hydrocarbons in crude oil are alkanes with general formula CnH₂n+₂. The required alkane sequence starts with methane, then moves to ethane, propane and butane.
The punchline: Mixture, finite resource, ancient biomass, mostly hydrocarbons: keep all four ideas distinct.
A fractionating column sorts by boiling range
The column is a temperature staircase: long chains generally condense low down; shorter chains rise farther and condense nearer the top, while some lowest-boiling ones leave as gases.
- Heat vaporises much of the crude oil before vapours enter the column.
- The column is hottest at the bottom and cooler towards the top.
- Fractions contain molecules with similar numbers of carbon atoms and similar boiling points.
- Fuel fractions include LPG, petrol, kerosene, diesel oil and heavy fuel oil. Petrochemical feedstocks supply solvents, lubricants, polymers and detergents.
The punchline: Describe evaporation, upward cooling and condensation at different heights.
Molecular size controls the handling of a fuel
Lengthen the carbon chain and the molecules grip one another more strongly, changing when they boil, flow and ignite.
- As molecular size increases, boiling point and viscosity increase while flammability decreases.
- Complete combustion oxidises carbon and hydrogen to carbon dioxide and water while releasing energy.
- Balance combustion equations from the given hydrocarbon formula, including oxygen last.
- Incomplete combustion can produce carbon monoxide and carbon particles when oxygen is limited.
The punchline: For the required trend, stick to boiling point, viscosity and flammability.
From ancient biomass to useful fractions
Crude oil is separated physically, not cracked, in a fractionating column. Hydrocarbon vapours generally condense where the temperature falls below their boiling points; some lowest-boiling hydrocarbons leave as gases.
- Crude oilA finite mixture found in rocks, formed from ancient biomass and containing mostly hydrocarbons.
- VaporiseHeat the mixture so much of it enters the column as vapour.
- Cool upwardsThe column is hot at the bottom and cooler near the top.
- Condense in fractionsHydrocarbons with similar boiling points condense at similar heights and are collected together.
Cracking cuts unwieldy molecules into useful pieces
Long hydrocarbons are plentiful but less demanded; cracking reshapes the supply into shorter fuels and reactive alkenes.
- Cracking is thermal decomposition that produces smaller molecules, including alkanes and alkenes.
- Catalytic cracking vaporises hydrocarbons and passes them over a hot catalyst; steam cracking mixes vapour with steam at high temperature.
- Alkenes are generally more reactive than alkanes. Modern life depends on hydrocarbon fuels and feedstocks for solvents, lubricants, detergents, polymers and other chemicals.
- Alkenes decolourise orange bromine water; use this observation as the test for unsaturation.
The punchline: Fractional distillation separates; cracking reacts and changes molecular formulae.
Separate Chemistry: the C=C bond is an invitation
The carbon–carbon double bond can open, allowing new atoms to add without ejecting part of the molecule.
- Members of the acyclic alkene homologous series contain one C=C bond and have general formula CnH₂n.
- Required alkene names run from ethene through propene and butene to pentene; no others need be recalled.
- For AQA's acyclic alkene series, recognise an alkene from a name ending -ene, a molecular formula fitting CnH₂n or a displayed C=C; displayed formulae resolve wider structural ambiguity.
- Compared with the alkane of equal carbon number, an alkene has two fewer hydrogen atoms.
The punchline: Recognise the AQA alkene series by -ene, CnH₂n or displayed C=C; use displayed structure to resolve wider ambiguity.
Separate Chemistry: addition converts C=C into C–C
Hydrogen, steam or a halogen adds across the double bond, turning one unsaturated molecule into one product molecule.
- Hydrogen adds over a heated nickel catalyst to make an alkane; steam adds over an acid catalyst at high temperature and pressure to make an alcohol.
- Chlorine, bromine or iodine adds at room temperature to form a dihaloalkane, with one halogen joining each carbon.
- Their flames are often sootier because burning in air is more likely to be incomplete.
- Draw every bond in the first four alkenes and their specified addition products.
The punchline: Open the double bond, add one new group to each carbon, then check carbon valency.
Make the model move
Interactive checkpoint
Touch the science. Change a state, build a route or test a relationship.
Open the double bond
Separate Chemistry: what crosses ethene's C=C?
Choose a reagent to see its conditions and the fully determined product pattern when it adds to ethene.
Industrial condition, not a learner procedure: hydrogen contacts a heated nickel catalyst. C=C becomes C–C and one H joins each former double-bond carbon.
1 of 5 states explored
Keep ethene's two-carbon skeleton, change C=C to C–C, attach one incoming atom or group to each carbon, then show every atom and bond and check carbon valency.
Spot the reactive patch
Separate Chemistry: match family to clue
Pair each family or structure with the feature that most directly identifies it.
Functional groups are structural evidence. Use them before relying on a remembered name ending.
Separate, do not react
Order the fractional-distillation journey
Arrange the physical steps that separate crude oil into fractions.
Choose the first step below.
The hydrocarbons keep their molecular identities. Separation relies on different molecular boiling points in a temperature gradient.
Separate Chemistry: –OH changes the carbon chain's behaviour
Add an –OH group and a hydrocarbon skeleton becomes an alcohol with a recognisable family of reactions.
- Required alcohol names run from methanol through ethanol and propanol to butanol; no others need be recalled.
- They burn in air; complete combustion forms carbon dioxide and water. With sodium they release hydrogen and form sodium alkoxides; oxidation makes carboxylic acids.
- They form neutral solutions in water; solubility decreases along the first four as the hydrocarbon part grows.
- Across methanol through butanol, common uses include fuels and solvents; ethanol is also present in alcoholic drinks. Yeast ferments a sugar solution at about 30 °C when air is excluded.
The punchline: For this section, balanced equations are expected only for combustion.
Separate Chemistry: –COOH supplies acidic chemistry
A carboxylic acid contains the –COOH functional group; dissolving it in water produces an acidic solution.
- The required acid sequence begins with methanoic and ethanoic acid, then continues to propanoic and butanoic acid; each forms an acidic solution in water.
- They react with carbonates to form a salt, water and carbon dioxide.
- A carboxylic acid reacts with an alcohol to form an ester and water; only ethyl ethanoate must be named here.
- Separate Chemistry HT: partial ionisation makes carboxylic acids weak acids with higher pH than equal-concentration strong acids.
The punchline: Descriptions are required for these reactions; balanced equations are not.
Separate Chemistry: a double bond becomes a chain link
When an alkene's C=C opens, each monomer gains two single-bond connections and becomes part of a long carbon backbone.
- Opening ethene or propene double bonds produces poly(ethene) or poly(propene), respectively.
- During addition polymerisation, every atom from each alkene remains in the chain; the reaction produces no separate small molecule.
- To recover the monomer from a repeating unit, restore the C=C bond between the two backbone carbons.
- Use brackets and an outside n to show that the repeating unit occurs many times.
The punchline: Open C=C for monomer-to-polymer; restore it for polymer-to-monomer.
Chain length tunes three assessed properties
As hydrocarbon molecules become larger, intermolecular attractions become stronger and the behaviour of the fraction changes.
- Shorter chainsLower boiling point, lower viscosity and greater flammability.
- Longer chainsHigher boiling point, higher viscosity and lower flammability.
- UseProperty patterns help match fractions to fuels, lubricants and other feedstocks.
Separate Chemistry: three organic families plus addition polymerisation
Recognise each family's functional group before predicting its characteristic chemistry, then follow an alkene double bond into an addition polymer.
- Alkene: C=CUnsaturated; addition reactions open the double bond and bromine water is decolourised.
- Alcohol: –OHThe first four react with sodium, burn, dissolve to differing extents and can be oxidised.
- Carboxylic acid: –COOHAcidic; reacts with carbonates and alcohols, and Higher Tier explains weak ionisation.
- Addition-polymer monomerAn alkene C=C opens so many monomers join without forming another small molecule.
Separate Chemistry: structure and repeating-unit sketchbook
Read each structure from left to right. The bond and bracket changes reveal addition reactions and polymer formation.
- Fully displayed etheneSeparate Chemistry: H H │ │ H–C=C–H In words: the two carbons share a double bond, and each carbon has single bonds to two hydrogen atoms.
- Fully displayed propene + bromineAll bonds: C(–H)(–H)(–H)–C(–H)=C(–H)(–H) + Br–Br → C(–H)(–H)(–H)–C(–H)(–Br)–C(–H)(–H)(–Br). Text: CH₃–CH=CH₂→CH₃–CHBr–CH₂Br; per C L→R, H 3,1,2→3,1,2 and Br 0,0,0→0,1,1.
- Poly(ethene)Separate Chemistry: n H₂C=CH₂ → [–CH₂–CH₂–]ₙ. The repeat has a C–C single-bonded backbone, two H on each carbon and one bond crossing each bracket edge.
- Fully displayed hydrogen and steam productsSeparate Chemistry: +H₂, ethane +steam, ethanol H H H H │ │ │ │ H–C–C–H H–C–C–O–H │ │ │ │ H H H H In words: C=C becomes C–C; add H/H or H/OH.
- Polyester route (schematic)Separate Chemistry HT: R is the carbon-containing part of the diol; R′ is that of the dicarboxylic acid. In [–O–R–O–C(=O)–R′–C(=O)–]ₙ, these parts alternate between ester links.
- Glycine routeSeparate Chemistry HT, schematic: glycine units form [–NH–CH₂–C(=O)–]ₙ while water is released. Each peptide link is –C(=O)–NH–; bonds cross both bracket edges.
Separate Chemistry HT: two functional groups build both ways
A monomer with a reactive group at each end can join the chain at both ends as each new link usually forms with the loss of a small molecule such as water.
- Each condensation monomer needs a reactive group at both ends so chain growth can continue.
- Two suitable monomer types can form a polyester, such as a diol with a dicarboxylic acid.
- Joining usually eliminates a small molecule such as water.
- Trace each monomer's end groups into the linkage that repeats along the chain.
The punchline: Two functional groups per monomer allow chain growth at both ends.
Separate Chemistry HT: amino acids carry two different handles
Its two reactive ends let an amino-acid unit link repeatedly, release a small molecule at each join and build a polypeptide chain.
- Every amino acid carries both an amino group and a carboxylic acid group.
- Glycine, H₂NCH₂COOH, can polymerise by condensation to form a polypeptide.
- Changing the choice and order of amino-acid units gives different protein chains.
- A small molecule is lost as each new link forms.
The punchline: Identify both functional groups before explaining polypeptide formation.
Separate Chemistry: life writes with polymers
DNA stores instructions in a monomer sequence, while proteins, starch and cellulose turn biological building blocks into working materials.
- Four kinds of nucleotide supply the units for a typical DNA molecule, whose paired polymer strands coil into a double helix.
- The order of those nucleotides carries information used in the development and operation of organisms and viruses.
- A protein chain is assembled from amino-acid units.
- Simple sugars provide the building units for both starch and cellulose.
The punchline: Match DNA to nucleotides, proteins to amino acids, and starch and cellulose to sugars.
Words worth knowing
Key definitions
- hydrocarbon
- A compound whose atoms are exclusively carbon and hydrogen.
- alkane
- A saturated hydrocarbon in the homologous series with general formula CnH₂n+₂.
- fraction
- A mixture of hydrocarbons with similar numbers of carbon atoms and boiling points.
- cracking
- Thermal decomposition of long hydrocarbons into smaller molecules, including alkanes and alkenes.
- Separate Chemistry: alkene
- A member of the acyclic alkene homologous series containing one C=C bond, with general formula CnH₂n.
- Separate Chemistry: functional group
- The atom or group of atoms responsible for a homologous series' characteristic reactions.
- Separate Chemistry: alcohol
- A member of the alcohol homologous series containing the –OH functional group.
- Separate Chemistry: carboxylic acid
- An organic compound containing the –COOH functional group.
- Separate Chemistry: monomer
- A small molecule that can join with many others to form a polymer.
- Separate Chemistry: polymer
- A very large molecule made from many joined monomer units.
- Separate Chemistry: repeating unit
- The section of a polymer structure that repeats along the chain.
- Separate Chemistry Higher Tier: amino acid
- An organic molecule with both amino and carboxylic acid functional groups.
Calculate with confidence
Equations
Alkane general formula
CnH₂n+₂
A saturated open-chain alkane has two more hydrogen atoms than twice its carbon count.
| Symbol | Meaning | Unit |
|---|---|---|
| n | number of carbon atoms | none (dimensionless whole-number count) |
Exam tip: Shared content: recall the alkane names in order from methane through ethane and propane to butane; later names are outside this requirement.
Separate Chemistry: alkene general formula
CnH₂n
A member of the acyclic alkene homologous series with one C=C bond has twice as many hydrogen atoms as carbon atoms.
| Symbol | Meaning | Unit |
|---|---|---|
| n | number of carbon atoms | none (dimensionless whole-number count) |
Exam tip: Recall the required alkene sequence as ethene, then propene and butene, ending with pentene.
Complete combustion example
CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion oxidises carbon to carbon dioxide and hydrogen to water.
Exam tip: For any given hydrocarbon, balance carbon, then hydrogen, then oxygen.
Worked complete combustion example
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
One propane contains 3 C and 8 H, so use 3CO₂ and 4H₂O; those products contain 10 O atoms, requiring 5O₂.
Exam tip: Balance C first, H second, then count O on the product side.
Separate Chemistry: ethanol combustion
C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
Complete combustion of ethanol forms carbon dioxide and water; the balance includes the oxygen atom already present in ethanol.
Exam tip: Balance C and H first, then remember that the alcohol molecule already contains oxygen.
Follow it step by step
Processes to remember
How to explain fractional distillation
- Begin by identifying crude oil as hydrocarbons spread across a range of boiling points.
- Heat it so much of the mixture vaporises.
- Describe the hot-bottom, cool-top temperature gradient.
- Explain that molecules condense at different heights and similar molecules form fractions.
Exam tip: No covalent bonds are broken: this is physical separation.
How to balance complete combustion
- Write the given hydrocarbon plus oxygen forming carbon dioxide and water.
- Balance carbon atoms using CO₂ coefficients.
- Balance hydrogen atoms using H₂O coefficients.
- Balance oxygen last and multiply all coefficients if a fraction appears.
Exam tip: With enough oxygen, account for every carbon in CO₂ and every hydrogen in H₂O.
Separate Chemistry: how to draw an addition polymer
- Draw the alkene monomer with its C=C double bond and all attached groups.
- Open the double bond to a single bond.
- Extend one backbone bond through each bracket edge.
- Place brackets around the repeating unit and write n outside.
Exam tip: Check atom accounting after C=C opens: the chain should neither gain nor lose atoms.
Separate Chemistry: how to draw alkene addition products
- Copy the displayed alkene exactly, keeping its carbon skeleton and every original atom.
- Replace C=C with C–C. Each former double-bond carbon now needs one new single bond.
- For hydrogen, add one H to each carbon. For steam, add H to one carbon and OH to the other.
- For chlorine, bromine or iodine, add one Cl, Br or I atom to each carbon respectively.
- Expand every condensed group so every atom and bond is shown, then check that each carbon has four bonds.
Exam tip: For ethene: H₂ gives ethane, steam gives ethanol, and X₂ gives CH₂X–CH₂X where X is Cl, Br or I. Draw every bond in the final answer.
Separate Chemistry: how to identify a family
- Inspect the displayed or structural formula for the pattern of bonds and atoms that forms the functional group.
- For AQA's acyclic alkene series, a name ending -ene or molecular formula fitting CnH₂n can identify an alkene; a displayed formula shows C=C and resolves wider structural ambiguity.
- Use C=C for alkene, –OH for alcohol and –COOH for carboxylic acid.
- Check the carbon count against the required first-four naming pattern.
- Predict only reactions assigned to that functional group.
Exam tip: A name ending helps, but the structure is stronger evidence.
See the thinking
Worked example
Worked example: complete a cracking equation
Balance this cracking equation using the supplied formulae, then explain why the products are useful: C₁₆H₃₄ → C₁₂H₂₆ + __ C₂H₄. You need not recall an individual alkene formula.
- Count carbon: the supplied C₁₂H₂₆ product accounts for 12 of the 16 carbon atoms.
- Each supplied C₂H₄ molecule adds 2 carbon atoms, so its coefficient must be 2.
- Check hydrogen: 26 + 2(4) = 34, matching the reactant.
- The shorter alkane can be a useful fuel; the alkene can make polymers and other chemicals.
Answer: C₁₆H₃₄ → C₁₂H₂₆ + 2C₂H₄. The shorter alkane can be a useful fuel, while the alkene is a feedstock for polymers and other chemicals.
All formulae are supplied, as required for Combined Science. The coefficient 2 balances both carbon and hydrogen without asking you to recall an individual alkene formula.
Protect the marks
Common mistakes
Watch out: Calling crude oil one giant hydrocarbon.
Do this instead: It is a mixture of many compounds, mostly hydrocarbons.
Watch out: Saying fractional distillation breaks long molecules.
Do this instead: It physically separates by boiling point; cracking breaks molecules chemically.
Watch out: Reversing the viscosity trend.
Do this instead: Viscosity increases as hydrocarbon molecular size increases.
Watch out: Calling bromine water colourless before the test.
Do this instead: It starts orange and an alkene decolourises it.
Watch out: Separate Chemistry Higher Tier: saying weak carboxylic acid means dilute acid.
Do this instead: Separate Chemistry HT: weak means only partially ionised in water.
Watch out: Separate Chemistry: leaving the double bond inside an addition-polymer repeating unit.
Do this instead: The C=C opens to C–C as monomers join.
Watch out: Separate Chemistry: saying addition polymerisation produces water.
Do this instead: Separate Chemistry: addition makes no small molecule. Separate Chemistry Higher Tier: condensation usually releases one, such as water.
Try it before you move on
Quick check
Say your answer first, then open the card to check it.
Which formula fits an alkane with five carbon atoms?
Answer: C₅H₁₂
CnH₂n+₂ gives H = 2(5) + 2 = 12.
Separate Chemistry: which AQA organic family can C₃H₆ be recognised as, and what would a displayed formula confirm?
Answer: An alkene; a displayed formula would confirm C=C.
C₃H₆ fits CnH₂n, so AQA's acyclic alkene series identifies it as an alkene. A displayed formula confirms the double bond and resolves wider structural ambiguity.
Where do shorter-chain crude-oil hydrocarbons generally condense, and what may happen to the lowest-boiling ones?
Answer: They generally condense nearer the cooler top; some lowest-boiling hydrocarbons may leave as gases.
Shorter chains generally have lower boiling points, so they rise farther before condensing; the lowest-boiling hydrocarbons may not condense in the column.
How do viscosity and flammability change as hydrocarbon size increases?
Answer: Viscosity increases; flammability decreases.
Larger hydrocarbons are less volatile, so they generally form an ignitable vapour less readily.
What is the positive bromine-water result for an alkene?
Answer: Orange bromine water becomes colourless.
Alkenes react with bromine water, so the orange colour disappears.
Separate Chemistry: using the supplied fully displayed propene, redraw propene and then draw its bromine-addition product with every atom and bond shown.
Answer: The supplied propene has a three-carbon skeleton with C=C at one end. In the product that bond is C–C and each of those two carbons gains one Br; expand every C–H bond and check each carbon has four bonds.
Bromine addition replaces the double bond with a single bond and fills the two freed valencies with Br. The carbon skeleton and every original H stay in place.
Separate Chemistry: what happens to C=C during addition polymerisation?
Answer: It opens to a C–C single bond as monomers join.
The double bond supplies links into the growing chain.
Separate Chemistry: balance C₂H₅OH + O₂ → CO₂ + H₂O.
Answer: C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
2 C gives 2CO₂ and 6 H gives 3H₂O. The products contain 7 O atoms; ethanol supplies 1, so 3O₂ supplies the other 6.
Separate Chemistry: what monomer types make proteins and DNA?
Answer: Amino acids make proteins; nucleotides make DNA.
DNA's nucleotide order encodes genetic information; a protein's amino-acid order determines how it folds and functions.
Good questions, clear answers
Frequently asked questions
Is crude oil renewable?
No. It formed from ancient biomass over geological timescales and is used far faster than natural processes replace it, so it is treated as a finite resource.
What is the difference between a fraction and a pure compound?
A fraction is still a mixture, containing hydrocarbons with similar carbon numbers and boiling points. It is not one pure molecular substance.
Why is cracking useful?
It converts surplus long hydrocarbons into shorter fuel molecules in high demand and alkenes used to make polymers and other chemicals.
How does modern life depend on hydrocarbons?
Hydrocarbon fuels release useful energy for transport and heating. Hydrocarbons also supply petrochemical feedstocks used to make solvents, lubricants, detergents, polymers and many other chemicals, so both their energy and their carbon-based raw materials support everyday life.
Which organic content is shared with Combined Science?
Crude oil, alkanes, fractional distillation, size–property trends, combustion and cracking are shared. Cracking also covers alkene uses, reactivity and the bromine-water test. Individual alkene formulae, detailed alkene sections and later families are Separate Chemistry.
Separate Chemistry Higher Tier: why are carboxylic acids weak acids?
Separate Chemistry Higher Tier: their molecules ionise only partially in water, so equal-concentration solutions contain fewer hydrogen ions than a strong acid.
Separate Chemistry Higher Tier: how do addition and condensation polymers differ?
Separate Chemistry: addition opens alkene double bonds without a small by-product. Separate Chemistry Higher Tier: condensation joins two-functional-group monomers as each new link usually forms with the loss of a small molecule such as water.
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