AQA A-Level Chemistry: Alkanes (7405) – Revision Notes

These notes condense 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: 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. It is examined on AS Paper 2 and A-level Paper 2, and A-level Paper 3 can test any content.

For full explanations and worked examples, read the alkanes study guide first. Then test yourself with the alkanes practice questions. The course hub is AQA A-Level Chemistry, and the printable checklist lists every specification point. The free 10-minute diagnostics help you find weak spots. Flue-gas calculations are in the Group 2 revision notes, and van der Waals forces in bonding.

Key definitions

Term	Meaning
Hydrocarbon	compound of hydrogen and carbon only
Saturated	only single C–C bonds
Alkane	saturated hydrocarbon, general formula CₙH₂ₙ₊₂
Petroleum (crude oil)	mixture consisting mainly of alkane hydrocarbons
Fraction	mixture of hydrocarbons with similar boiling points
Cracking	breaking C–C bonds in alkanes to make shorter molecules
Radical	species with an unpaired electron, shown by a dot
Free-radical substitution	substitution in which the chain is carried by radicals

Fractional distillation (3.3.2.1)

Method in steps:

1. Heat crude oil to vaporise it.
2. Vapour enters the bottom of the column.
3. Column is hot at the bottom, cool at the top.
4. Each hydrocarbon condenses at the level where the temperature falls below its boiling point.
5. Short chains (low boiling point) collect near the top; long chains (high boiling point) near the bottom.

Boiling point answer in steps:

1. Alkanes have only van der Waals forces between molecules.
2. Longer chain = more electrons and more surface contact.
3. Stronger van der Waals forces.
4. More energy needed to overcome them, so higher boiling point.

Physical separation only: no covalent bonds break.

The specification lists no required practical in 3.3.2. Fractional distillation of a crude oil substitute is suggested as a skills opportunity.

Cracking (3.3.2.2)

	Thermal	Catalytic
Pressure	high	slight
Temperature	high	high
Catalyst	none	zeolite
Main products	high percentage of alkenes	motor fuels and aromatic hydrocarbons
Mechanism	not required	not required

Writing a cracking equation:

1. Write the reactant alkane.
2. Write the named alkane product (CₙH₂ₙ₊₂).
3. Subtract; share what is left between alkenes (CₙH₂ₙ).
4. Check C and H both balance.

Worked reminder: C₁₄H₃₀ → C₁₀H₂₂ + 2C₂H₄ (C: 14 = 10 + 4; H: 30 = 22 + 8).

Economic reasons (explain, don’t just list):

• Distillation gives a surplus of long-chain fractions and too little of the short-chain fractions.
• Demand is higher for short-chain fractions such as petrol, so they are worth more.
• Cracking converts low-value surplus into high-value products.
• It makes alkenes, valuable starting materials for polymers and other chemicals.
• Catalytic cracking makes motor fuels and aromatic hydrocarbons.

Combustion (3.3.2.3)

Type	Oxygen supply	Carbon product
Complete	plenty	CO₂
Incomplete	limited	CO and/or C (soot)

Water forms in every case. Balance C, then H, then O. Half-coefficients for O₂ are allowed.

Worked reminder: incomplete combustion of heptane to CO: C₇H₁₆ + 7½O₂ → 7CO + 8H₂O (O atoms: 7 + 8 = 15, so 7½ O₂).

Combustion calculation in steps:

1. Write and balance the equation.
2. Moles of fuel = mass ÷ Mr.
3. Use the equation ratio to find moles of O₂, CO₂ or H₂O.
4. Convert: mass = n × Mr, or V = nRT ÷ p (p in Pa, V in m³; × 1000 for dm³).
5. For gases at the same temperature and pressure, volumes are in the same ratio as moles, so you can use the equation ratio directly on volumes.

Worked reminder: 10 cm³ of methane burning completely needs 20 cm³ of oxygen and forms 10 cm³ of CO₂ (CH₄ + 2O₂ → CO₂ + 2H₂O), with all volumes at the same temperature and pressure.

Finding a formula from combustion data: moles of C = moles of CO₂; moles of H = 2 × moles of H₂O. Find the simplest ratio, then check against CₙH₂ₙ₊₂ for an alkane.

Engine pollutants

Pollutant	Source	Equation
NO	N₂ and O₂ from air at high engine temperature	N₂ + O₂ → 2NO
NO₂	NO reacting with more oxygen	2NO + O₂ → 2NO₂
CO	incomplete combustion	e.g. CH₄ + 1½O₂ → CO + 2H₂O
carbon	incomplete combustion	—
unburned hydrocarbons	fuel that did not burn	—

Catalytic converters

Convert CO, NOₓ and unburned hydrocarbons into CO₂, N₂ and H₂O.

2CO + 2NO → 2CO2 + N2
C7H16 + 22NO → 7CO2 + 8H2O + 11N2

Sulfur dioxide

• Fuels containing sulfur form SO₂ on combustion: S + O₂ → SO₂. SO₂ causes air pollution.
• SO₂ is an acidic oxide; CaO is basic (CaCO₃ also reacts with acids).
• So they react and remove SO₂ from flue gases: CaO + SO₂ → CaSO₃ and CaCO₃ + SO₂ → CaSO₃ + CO₂.

Chlorination of methane (3.3.2.4)

Condition: UV light. Type: free-radical substitution.

Stage	Equation(s)	Radical count
Initiation	Cl₂ → 2Cl•	0 in, 2 out
Propagation 1	Cl• + CH₄ → •CH₃ + HCl	1 in, 1 out
Propagation 2	•CH₃ + Cl₂ → CH₃Cl + Cl•	1 in, 1 out
Termination	Cl• + Cl• → Cl₂	2 in, 0 out
Termination	•CH₃ + Cl• → CH₃Cl	2 in, 0 out
Termination	•CH₃ + •CH₃ → C₂H₆	2 in, 0 out

No curly arrows are required for radical mechanisms. The dot goes on the atom with the unpaired electron (the carbon in •CH₃).

Further substitution gives CH₂Cl₂, CHCl₃ and CCl₄. Excess methane favours CH₃Cl; excess chlorine favours more substitution. Overall for full substitution: CH₄ + 4Cl₂ → CCl₄ + 4HCl. The products are halogenoalkanes, the next topic.

Must-know distinctions

• Distillation vs cracking: separating by boiling point (physical) vs breaking C–C bonds (chemical).
• Thermal vs catalytic: high pressure, alkenes vs slight pressure, zeolite, motor fuels and aromatics.
• Complete vs incomplete: CO₂ vs CO or C.
• Where NOₓ comes from: air, not fuel. Where SO₂ comes from: sulfur impurities in the fuel.
• Propagation vs termination: radical regenerated vs radicals removed.

Quick self-test

1. Calculate Mr of decane, C₁₀H₂₂. (Ar: C = 12.0, H = 1.0)
2. Decane is cracked to form hexane and one other product only. Write the equation.
3. Which type of cracking gives a high percentage of alkenes?
4. Name the catalyst used in catalytic cracking.
5. Write the equation for complete combustion of hexane.
6. How many moles of O₂ are needed to burn 0.200 mol of heptane completely?
7. Calculate the mass of CO₂ formed when 1.44 g of pentane, C₅H₁₂, burns completely. (Ar: C = 12.0, H = 1.0, O = 16.0)
8. State why calcium carbonate removes SO₂ from flue gases.
9. Write the initiation step for chlorination of methane and give the condition.
10. Write a termination step that forms ethane.
11. Write an equation for the reaction of CO with NO in a catalytic converter.
12. Explain why hexane has a higher boiling point than propane.

Answers

1. 10 × 12.0 + 22 × 1.0 = 142.0
2. C₁₀H₂₂ → C₆H₁₄ + C₄H₈
3. Thermal cracking.
4. Zeolite.
5. C₆H₁₄ + 9½O₂ → 6CO₂ + 7H₂O
6. C₇H₁₆ + 11O₂ → 7CO₂ + 8H₂O, so 0.200 × 11 = 2.20 mol
7. Mr(C₅H₁₂) = 72.0; n = 1.44 ÷ 72.0 = 0.0200 mol; n(CO₂) = 5 × 0.0200 = 0.100 mol; mass = 0.100 × 44.0 = 4.40 g
8. SO₂ is an acidic oxide and CaCO₃ reacts with acids, so the SO₂ reacts to form calcium sulfite: CaCO₃ + SO₂ → CaSO₃ + CO₂
9. Cl₂ → 2Cl•, in UV light.
10. •CH₃ + •CH₃ → C₂H₆
11. 2CO + 2NO → 2CO₂ + N₂
12. Hexane has a longer chain, so more electrons and more surface contact; stronger van der Waals forces need more energy to overcome.

Where marks are usually lost

• Saying covalent bonds break during fractional distillation or boiling.
• Comparing boiling points by “size” without naming van der Waals forces.
• Swapping thermal and catalytic conditions, or giving catalytic cracking a high pressure.
• Listing “makes petrol” without linking surplus, demand and value.
• Cracking equations where H does not balance because the alkene is written as an alkane.
• Combustion equations with the wrong number of O₂ molecules.
• Saying the nitrogen in NOₓ comes from the fuel.
• “CaO absorbs SO₂” with no mention of acidic and basic oxides.
• Missing UV light, or writing Cl₂ → Cl⁻ + Cl⁺ (ions, not radicals) for initiation.
• Propagation steps that form H• instead of HCl; the first step gives •CH₃ + HCl.

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).