Fuels, Alkanes and Alkenes: Revision Notes

Condensed for revision. For the full explanation, use the Petroleum, Alkanes and Alkenes study guide, then test yourself with the practice questions.

Tier note: items marked (0620 Extended, 5070 required) are in the Supplement column of the 0620 syllabus (11.4 outcomes 3–4, 11.5 outcomes 5–6). Everything else is 0620 Core. 5070 has no tiers, so O Level candidates need all of it.

Syllabus: Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028, subtopics 11.3 Fuels, 11.4 Alkanes, 11.5 Alkenes.

Fuels (11.3)

• Fossil fuels: coal, natural gas and petroleum.
• Natural gas is mainly methane, CH₄.
• Hydrocarbon: a compound that contains hydrogen and carbon only.
• Petroleum is a mixture of hydrocarbons, so it can be separated.

Fractional distillation of petroleum

Petroleum is heated so that it vaporises and enters a fractionating column, which is hot at the bottom and cooler at the top. The vapours rise; each fraction condenses at the height where the temperature falls below its boiling point, and is drawn off there.

From the bottom to the top of the column, the fractions have:

Property	Change going up the column
Chain length	decreases
Volatility	higher
Boiling point	lower
Viscosity	lower

Fractions and their uses (learn the syllabus list)

Fraction	Use
Refinery gas	gas for heating and cooking
Gasoline / petrol	fuel for cars
Naphtha	chemical feedstock
Kerosene / paraffin	jet fuel
Diesel oil / gas oil	fuel for diesel engines
Fuel oil	fuel for ships and home heating systems
Lubricating oil	lubricants, waxes and polishes
Bitumen	making roads

Refinery gas is drawn off at the top of the column and bitumen at the bottom.

Alkanes (11.4)

• General formula CₙH₂ₙ₊₂ (methane CH₄, ethane C₂H₆, propane C₃H₈).
• All bonds are single covalent bonds, so alkanes are saturated hydrocarbons.
• Alkanes are generally unreactive, except for combustion and substitution by chlorine.

Complete combustion (plenty of oxygen) gives carbon dioxide and water:

CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion (limited oxygen) also gives carbon monoxide and carbon (soot).

(0620 Extended, 5070 required) Substitution: one atom or group of atoms is replaced by another atom or group of atoms. With chlorine, the reaction is photochemical: ultraviolet light provides the activation energy, Eₐ. At this level, only monosubstitution is needed:

CH₄ + Cl₂ → CH₃Cl + HCl        (UV light)

The products are chloromethane and hydrogen chloride. In the displayed formula of CH₃Cl, one of the four H atoms around the carbon is replaced by Cl.

Alkenes (11.5)

• General formula CₙH₂ₙ (ethene C₂H₄, propene C₃H₆).
• They contain a C=C double covalent bond, so they are unsaturated hydrocarbons.

Cracking

Cracking breaks larger alkane molecules into smaller alkanes, alkenes and hydrogen, using a high temperature and a catalyst. For example:

C₁₀H₂₂ → C₈H₁₈ + C₂H₄
C₄H₁₀ → C₄H₈ + H₂

Why crack?

• There is more demand for the shorter-chain fractions (such as petrol) than petroleum supplies, and less demand for the longer ones. Cracking converts the surplus.
• It makes alkenes, which are used to make polymers and other chemicals, and hydrogen.

Test for unsaturation

Shake the hydrocarbon with aqueous bromine (bromine water, orange):

Hydrocarbon	Result
Alkene (unsaturated)	decolourises: orange to colourless
Alkane (saturated)	no change: stays orange

Addition reactions (0620 Extended, 5070 required)

In an addition reaction, only one product is formed. The C=C double bond opens and the new atoms join the two carbon atoms.

Reagent	Conditions	Product from ethene
Bromine (or aqueous bromine)	room temperature	1,2-dibromoethane, CH₂BrCH₂Br
Hydrogen	nickel catalyst	ethane, C₂H₆
Steam	acid catalyst (300 °C, 6000 kPa / 60 atm; see 11.6)	ethanol, C₂H₅OH

C₂H₄ + Br₂ → C₂H₄Br₂
C₂H₄ + H₂  → C₂H₆
C₂H₄ + H₂O → C₂H₅OH

In displayed formulae, the product has single bonds only: each carbon keeps its hydrogens and gains one of the added atoms or groups.

Exam traps

• “Hydrocarbons contain carbon and hydrogen” misses the word only.
• Up the column, boiling point goes down and volatility goes up. Students often reverse one of them.
• Cracking always gives at least one alkene, plus a smaller alkane and/or hydrogen. An alkane cannot crack into alkenes only, so an equation like that cannot balance.
• The bromine test result is “decolourised” or “orange to colourless”. “Clear” is not a colour and loses the mark.
• (0620 Extended, 5070 required) Substitution with chlorine needs UV light, not a catalyst or heat.

Self-test

1. Name the main constituent of natural gas.
2. State two properties that change from the bottom to the top of the fractionating column, and how.
3. Which fraction is used for jet fuel, and which for making roads?
4. Write the equation for the complete combustion of methane.
5. Give two reasons for cracking large alkane molecules.
6. Describe the test to show that a hydrocarbon is unsaturated, with the result.
7. (0620 Extended, 5070 required) What provides the activation energy for the reaction of methane with chlorine? Name both products.
8. (0620 Extended, 5070 required) Name the catalyst for adding hydrogen to an alkene, and the product from ethene.

Answers:

1. Methane.
2. Any two: chain length decreases, volatility increases, boiling point decreases, viscosity decreases.
3. Kerosene / paraffin; bitumen.
4. CH₄ + 2O₂ → CO₂ + 2H₂O.
5. Demand for shorter-chain fractions such as petrol is greater than supply; cracking also makes alkenes (for polymers and chemicals) and hydrogen.
6. Add aqueous bromine: an alkene turns it from orange to colourless; an alkane leaves it orange.
7. Ultraviolet light; chloromethane and hydrogen chloride.
8. Nickel; ethane.

These are original notes written for revision. Check the full syllabus wording in the official 0620 syllabus and official 5070 syllabus.