The key idea
Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. Alkenes are unsaturated with a C=C double bond.Crude oil is fractionally distilled to separate hydrocarbons by boiling point.
alkanes: CₙH₂ₙ₊₂ | alkenes: CₙH₂ₙ
Use the labels to explain the scientific relationship shown.
The bit that matters
Keep the idea tight, then use the worked example to practise the exact exam wording.
Hydrocarbons and crude oil
Crude oil is a finite resource that is a mixture of hydrocarbons — compounds containing only carbon and hydrogen.The main types are alkanes (saturated, single bonds only) and alkenes (unsaturated, containing at least one C=C double bond).Crude oil is separated into fractions by fractional distillation, exploiting the fact that longer hydrocarbon chains have higher boiling points.
Alkanes
Alkanes have the general formula CₙH₂ₙ₊₂ and are saturated.The first four are methane (CH₄), ethane (C₂H₆), propane (C₃H₈) and butane (C₄H₁₀).They burn in excess oxygen (complete combustion) to produce carbon dioxide and water.In limited oxygen, incomplete combustion produces carbon monoxide (toxic) and carbon (soot).
Alkenes and addition reactions
Alkenes have the general formula CₙH₂ₙ and contain a C=C double bond, making them more reactive than alkanes.They undergo addition reactions, where the double bond opens and adds atoms across it.Bromine water is decolourised by alkenes (but not alkanes), providing a simple test.Alkenes can also undergo addition polymerisation.
Cracking
Cracking is the decomposition of large hydrocarbon molecules into smaller, more useful ones.Thermal cracking uses high temperatures and pressures; catalytic cracking uses a zeolite catalyst at a lower temperature.Cracking always produces at least one alkene.It is economically important because there is greater demand for short-chain fuels (petrol, diesel) than for the heavy fractions produced in distillation.
Definitions to learn
Hydrocarbon
A compound containing only carbon and hydrogen atoms.
Alkane
A saturated hydrocarbon with only single C–C bonds, general formula CₙH₂ₙ₊₂.
Alkene
An unsaturated hydrocarbon containing a C=C double bond, general formula CₙH₂ₙ.
Fractional distillation
Separation of crude oil into fractions by differences in boiling point in a fractionating column.
Cracking
The thermal or catalytic decomposition of long-chain hydrocarbons into shorter, more useful molecules.
Complete combustion
Burning in excess oxygen to produce only CO₂ and H₂O.
Explain how fractional distillation separates crude oil into useful fractions.
Crude oil is heated and vaporised at the bottom of the column.
Vapours rise through the column, which is cooler at the top.
Each fraction condenses at its boiling point and is collected.
Short-chain hydrocarbons (low boiling points) reach the top; long-chain ones condense near the bottom.
Fractions are separated by their different boiling points — shorter chains condense higher up.
Always state the general formula when naming a homologous series.For combustion equations, balance C, then H, then O atoms in that order.
Do not say the oil is separated by different densities. Separation is by boiling point (chain length).
How to score full marks
- 1Alkanes are SATURATED (all single bonds); alkenes are UNSATURATED (at least one C=C double bond) — state which clearly.
- 2Bromine water test: alkene → decolourises (orange to colourless); alkane → no change.
- 3Fractional distillation separates by BOILING POINT (related to chain length), not by density.
Try these yourself
Open each answer only after you have explained the full chemical process.
1Give the general formula for alkanes.[1 mark]
- 1.Recall the homologous series formula.
2State the name and formula of the alkane with 3 carbon atoms.[1 mark]
- 1.Recall the name prefix for 3 carbons.
3State what is meant by a saturated hydrocarbon.[1 mark]
- 1.Describe the bonding between carbon atoms.
4Describe a test to distinguish between an alkane and an alkene.[3 marks]
- 1.Use bromine water.
5Explain why short-chain hydrocarbons make better fuels than long-chain ones.[3 marks]
- 1.Link chain length to viscosity and flammability.
6Describe cracking and explain why it is carried out.[3 marks]
- 1.Link demand for fuels to the process.
7Write a balanced equation for the complete combustion of propane (C₃H₈).[2 marks]
- 1.Balance C, then H, then O.
8Explain the environmental and health problems caused by burning fossil fuels.[4 marks]
- 1.Name specific pollutants and their effects.