AQA A-Level Chemistry: Alkanes (7405)

This guide teaches section 3.3.2 Alkanes of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards. It covers all four sub-sections: 3.3.2.1 Fractional distillation of crude oil, 3.3.2.2 Modification of alkanes by cracking, 3.3.2.3 Combustion of alkanes and 3.3.2.4 Chlorination of alkanes. All of it is AS content, so nothing here is A-level only. The specification places organic sections 3.3.1 to 3.3.6 on AS Paper 2 and all of section 3.3 on A-level Paper 2, and A-level Paper 3 can test any content.

For quick recall, use the alkanes revision notes. To test yourself, try the alkanes practice questions. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics show where your gaps are. Van der Waals forces are taught in bonding, and enthalpy of combustion in energetics.

What this topic covers

Spec section	What you must be able to do
3.3.2.1 Fractional distillation of crude oil	Know that alkanes are saturated hydrocarbons; know that petroleum is a mixture consisting mainly of alkanes that can be separated by fractional distillation
3.3.2.2 Modification of alkanes by cracking	Know that cracking breaks C–C bonds; know the conditions and main products of thermal and catalytic cracking (mechanisms not required); explain the economic reasons for cracking
3.3.2.3 Combustion of alkanes	Know alkanes are used as fuels; write equations for complete and incomplete combustion; know the pollutants from the internal combustion engine and their removal by catalytic converters; know that sulfur in fuels forms SO₂; explain why CaO or CaCO₃ removes SO₂ from flue gases
3.3.2.4 Chlorination of alkanes	Explain the reaction of methane with chlorine as free-radical substitution with initiation, propagation and termination steps

The specification lists no required practical in 3.3.2. It suggests fractional distillation of a crude oil substitute as an opportunity for skills development.

Alkanes and crude oil (3.3.2.1)

An alkane is a saturated hydrocarbon. Hydrocarbon means it contains hydrogen and carbon only. Saturated means it has only single C–C bonds, so no more hydrogen can be added. The general formula of the non-cyclic alkanes is CₙH₂ₙ₊₂: methane CH₄, ethane C₂H₆, propane C₃H₈, butane C₄H₁₀ and so on.

Petroleum (crude oil) is a mixture consisting mainly of alkanes, with a wide range of chain lengths. It is separated into fractions by fractional distillation. Each fraction is a mixture of hydrocarbons with similar boiling points.

How the column works:

1. The crude oil is heated until most of it vaporises.
2. The vapour enters near the bottom of a tall fractionating column.
3. The column has a temperature gradient: hottest at the bottom, coolest at the top.
4. Vapour rises. Each hydrocarbon condenses when it reaches a level that is below its boiling point, and is collected there.
5. Short-chain alkanes have low boiling points, so they rise furthest and leave near the top (some as gases). Long-chain alkanes have high boiling points and condense low down. The heaviest residue does not vaporise and leaves at the bottom.

Why chain length controls boiling point. Only van der Waals forces act between alkane molecules. A longer chain has more electrons and more surface contact, so these forces are stronger and more energy is needed to separate the molecules. Boiling does not break C–C or C–H bonds, so fractional distillation is a physical process. That is the key contrast with cracking.

Cracking (3.3.2.2)

Cracking breaks C–C bonds in long-chain alkanes to make shorter molecules. If the products are all hydrocarbons, there is not enough hydrogen for every product molecule to be an alkane, so at least one product is an alkene.

	Thermal cracking	Catalytic cracking
Pressure	high	slight pressure
Temperature	high	high
Catalyst	none	zeolite
Main products	a high percentage of alkenes	motor fuels and aromatic hydrocarbons

You do not need a mechanism for either type. The specification describes the conditions in words, as in the table.

Worked example 1: cracking equations

Dodecane, C₁₂H₂₆, is thermally cracked. One molecule forms one molecule of octane and ethene only. Write the equation.

Octane  = C8H18 (alkane, CnH2n+2)
Carbon left for ethene: 12 − 8 = 4, so 2 × C2H4
Check H: 18 + 2 × 4 = 26  ✓

C12H26 → C8H18 + 2C2H4

Write the alkane product first, then share what is left between the alkenes (CₙH₂ₙ).

Economic reasons for cracking

You must be able to explain these, not just list them.

• Fractional distillation gives more of the long-chain fractions than the market wants, and less of the short-chain fractions such as petrol than the market wants. Supply and demand do not match.
• Short-chain fractions are in higher demand, so they are worth more. Cracking turns low-value, surplus long-chain fractions into high-value products.
• Cracking makes alkenes, which are more reactive than alkanes and are valuable starting materials for making polymers and other chemicals.
• Catalytic cracking makes motor fuels and aromatic hydrocarbons, which are valuable products.

Combustion of alkanes (3.3.2.3)

Alkanes are used as fuels because their combustion releases a lot of energy. Combustion of alkanes and other organic compounds can be complete or incomplete.

• Complete combustion (plenty of oxygen): the products are carbon dioxide and water.
• Incomplete combustion (limited oxygen): the products include carbon monoxide and/or carbon (soot), plus water.

Worked example 2: combustion equations for octane

Write equations for the combustion of octane, C₈H₁₈, to form (a) CO₂, (b) CO and (c) C, with water in each case.

Balance C first, then H, then O last.
(a) 8 CO2 + 9 H2O needs 16 + 9 = 25 O atoms = 12½ O2
    C8H18 + 12½O2 → 8CO2 + 9H2O
(b) 8 CO + 9 H2O needs 8 + 9 = 17 O atoms = 8½ O2
    C8H18 + 8½O2 → 8CO + 9H2O
(c) 8 C + 9 H2O needs 9 O atoms = 4½ O2
    C8H18 + 4½O2 → 8C + 9H2O

Halves are fine; double every coefficient if you need whole numbers. The same method works for other organic compounds: ethanol burns completely as C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (remember to count the oxygen already in the fuel).

Worked example 3: amounts from a combustion equation

(a) Calculate the mass of carbon dioxide formed when 1.00 kg of octane burns completely. (Ar: C = 12.0, H = 1.0, O = 16.0)

Mr(C8H18) = 8 × 12.0 + 18 × 1.0 = 114.0
n(C8H18)  = 1000 ÷ 114.0 = 8.772 mol
n(CO2)    = 8 × 8.772 = 70.18 mol        (1 : 8 ratio)
mass      = 70.18 × 44.0 = 3088 g = 3.09 kg (3 s.f.)

(b) Calculate the volume of oxygen, in dm³, needed to burn 0.250 mol of propane completely at 100 kPa and 298 K. (R = 8.31 J K⁻¹ mol⁻¹)

C3H8 + 5O2 → 3CO2 + 4H2O
n(O2) = 5 × 0.250 = 1.25 mol
V = nRT / p = 1.25 × 8.31 × 298 ÷ 100 000 = 0.0310 m³ = 31.0 dm³

Use p in Pa to get V in m³; multiply by 1000 for dm³.

Pollutants from the internal combustion engine

The internal combustion engine produces several gaseous pollutants:

Pollutant	How it forms	Why it matters
NOₓ (NO and NO₂)	Nitrogen and oxygen from the air react at the high temperature in the engine: N₂ + O₂ → 2NO; then 2NO + O₂ → 2NO₂	contributes to acid rain and photochemical smog
CO	incomplete combustion	toxic
carbon (soot particles)	incomplete combustion	harmful when breathed in
unburned hydrocarbons	fuel that has not burned	contributes to photochemical smog

Catalytic converters

These gaseous pollutants can be removed using a catalytic converter fitted to the exhaust. The catalyst speeds up reactions that turn the pollutants into less harmful gases: carbon dioxide, nitrogen and water.

2CO + 2NO → 2CO2 + N2
C8H18 + 25NO → 8CO2 + 9H2O + 12½N2

CO or the hydrocarbon is oxidised, and NO is reduced to N₂.

Sulfur dioxide and flue gases

Some fuels, especially coal and some crude oil fractions, contain sulfur compounds. When they burn, the sulfur forms sulfur dioxide, which causes air pollution (it contributes to acid rain):

S + O2 → SO2

Power stations remove SO₂ from their flue gases (waste gases) using calcium oxide or calcium carbonate. You must explain why this works:

• SO₂ is an acidic oxide.
• CaO is a basic oxide, and CaCO₃ also reacts with acids.
• So they react in a neutralisation-type reaction, and the SO₂ is trapped as a calcium salt.

CaO + SO2 → CaSO3
CaCO3 + SO2 → CaSO3 + CO2

The flue-gas mass calculation for CaO and CaCO₃ is worked through on the Group 2 study guide, so it is not repeated here.

Chlorination of methane (3.3.2.4)

Methane reacts with chlorine in ultraviolet (UV) light. A hydrogen atom is replaced by a chlorine atom, so it is a substitution reaction. It happens through radicals, so it is called free-radical substitution.

Overall: CH4 + Cl2 → CH3Cl + HCl

A radical is a species with an unpaired electron. The specification represents the unpaired electron by a dot, and does not require curly arrows for radical mechanisms (see introduction to organic chemistry). You must write balanced equations for each step.

The three stages

Initiation — UV light breaks the Cl–Cl bond. Each chlorine atom keeps one electron of the bonding pair, so two radicals form:

Cl2 → 2Cl•

Propagation — a radical reacts with a molecule to make a new radical. Two steps form a chain that repeats many times:

Cl• + CH4 → •CH3 + HCl
•CH3 + Cl2 → CH3Cl + Cl•

The Cl• made in the second step repeats the first step, so the chain continues.

Termination — two radicals join to form a molecule. This removes radicals and stops the chain:

Cl• + Cl• → Cl2
•CH3 + Cl• → CH3Cl
•CH3 + •CH3 → C2H6

Worked example 4: further substitution

Explain how dichloromethane, CH₂Cl₂, forms when methane reacts with chlorine, and write the propagation steps.

A chlorine radical can remove a hydrogen atom from chloromethane instead of from methane:

Cl• + CH3Cl → •CH2Cl + HCl
•CH2Cl + Cl2 → CH2Cl2 + Cl•

Further substitution can continue to CHCl₃ and CCl₄, so the product is a mixture. A large excess of methane makes it more likely that Cl• collides with CH₄ rather than with CH₃Cl, which increases the proportion of chloromethane. A large excess of chlorine favours further substitution.

Check: each propagation step has one radical in and one out; each termination step has two in and none out.

Common errors

• Saying that fractional distillation breaks bonds. It only separates by boiling point; cracking breaks C–C bonds.
• Explaining the boiling point trend without mentioning stronger van der Waals forces.
• Swapping the cracking conditions: thermal is high pressure; catalytic is slight pressure with a zeolite.
• Giving “to make petrol” as the whole economic reason.
• Unbalanced combustion equations, usually from forgetting the oxygen in water or in the fuel.
• Saying the nitrogen in NOₓ comes from the fuel.
• Saying CaO “absorbs” SO₂ without the acid–base explanation.
• Putting the dot on the wrong atom (•CH₃ has the unpaired electron on carbon) or writing propagation steps that do not regenerate a radical.
• Forgetting UV light as the condition for initiation.

Where to go next

Use the alkanes revision notes and alkanes practice questions. Chloromethane is a halogenoalkane, so continue with halogenoalkanes. For mole calculations, see the mole revision notes; for a wider plan, the exam preparation guide.

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.2 Alkanes (3.3.2.1 Fractional distillation of crude oil, 3.3.2.2 Modification of alkanes by cracking, 3.3.2.3 Combustion of alkanes and 3.3.2.4 Chlorination of alkanes).