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Pearson Edexcel ChemistryOrganic chemistry

Hydrocarbons and crude oil

Name and draw alkanes and alkenes and explain how crude oil is processed.

Start here

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.

Equation to know

alkanes: CₙH₂ₙ₊₂ | alkenes: CₙH₂ₙ

Hydrocarbons And Crude Oil
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Use the labels to explain the scientific relationship shown.

Revision notes

The bit that matters

Keep the idea tight, then use the worked example to practise the exact exam wording.

1

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.

2

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).

3

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.

4

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.

Key terms

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.

Worked example

Explain how fractional distillation separates crude oil into useful fractions.

1

Crude oil is heated and vaporised at the bottom of the column.

2

Vapours rise through the column, which is cooler at the top.

3

Each fraction condenses at its boiling point and is collected.

4

Short-chain hydrocarbons (low boiling points) reach the top; long-chain ones condense near the bottom.

Final answer

Fractions are separated by their different boiling points — shorter chains condense higher up.

Exam habit

Always state the general formula when naming a homologous series.For combustion equations, balance C, then H, then O atoms in that order.

Watch out

Do not say the oil is separated by different densities. Separation is by boiling point (chain length).

Examiner tips

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.
Practice questions

Try these yourself

Open each answer only after you have explained the full chemical process.

1Give the general formula for alkanes.[1 mark]
Mark scheme
  1. 1.Recall the homologous series formula.
CₙH₂ₙ₊₂ (1).
2State the name and formula of the alkane with 3 carbon atoms.[1 mark]
Mark scheme
  1. 1.Recall the name prefix for 3 carbons.
Propane, C₃H₈ (1).
3State what is meant by a saturated hydrocarbon.[1 mark]
Mark scheme
  1. 1.Describe the bonding between carbon atoms.
A hydrocarbon containing only single C–C bonds (1).
4Describe a test to distinguish between an alkane and an alkene.[3 marks]
Mark scheme
  1. 1.Use bromine water.
Add bromine water (1); an alkene decolourises the orange bromine water (1); an alkane causes no colour change (1).
5Explain why short-chain hydrocarbons make better fuels than long-chain ones.[3 marks]
Mark scheme
  1. 1.Link chain length to viscosity and flammability.
Shorter chains have lower boiling points so evaporate easily (1), are less viscous (1), and ignite more easily (1).
6Describe cracking and explain why it is carried out.[3 marks]
Mark scheme
  1. 1.Link demand for fuels to the process.
Cracking is the thermal or catalytic decomposition of long-chain hydrocarbons into shorter, more useful ones (1); it is used because there is a greater demand for shorter-chain fractions (e.g. petrol) than for longer-chain ones (1); it also produces alkenes used in polymer production (1).
7Write a balanced equation for the complete combustion of propane (C₃H₈).[2 marks]
Mark scheme
  1. 1.Balance C, then H, then O.
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O (1) — fully balanced (1).
8Explain the environmental and health problems caused by burning fossil fuels.[4 marks]
Mark scheme
  1. 1.Name specific pollutants and their effects.
Complete combustion produces CO₂, which is a greenhouse gas contributing to climate change (1); incomplete combustion produces CO, which is toxic (1); sulfur impurities produce SO₂, which causes acid rain (1); nitrogen oxides are produced at high temperatures and also cause acid rain and respiratory problems (1).
Official exam-board sources
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