OxfordAQA A-Level Chemistry: Alkanes (9620) – Revision Notes

Below is a condensed version of section 3.3.2 Alkanes from the OxfordAQA International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. The four points are crude oil distillation (3.3.2.1), cracking (3.3.2.2), combustion (3.3.2.3) and chlorinating methane (3.3.2.4). Every point is International AS; none is International A-level only. It is assessed in Unit 2 (Organic 1 and Physical 1), and Unit 5 can draw on any content.

Full explanations and worked examples are in the alkanes study guide. Once these notes are secure, try the alkanes practice questions. The course hub lists every 9620 topic, the printable checklist lets you tick off each point, and a free 10-minute diagnostic shows where to focus. Intermolecular forces are covered in shapes, polarity and intermolecular forces.

Definitions to quote

Word	Say this
Hydrocarbon	contains carbon and hydrogen atoms only
Saturated	all C–C bonds are single bonds
Alkane	a saturated hydrocarbon; chains fit CₙH₂ₙ₊₂
Petroleum	a mixture consisting mainly of alkane hydrocarbons
Fraction	part of the distillate with boiling points in a narrow range
Cracking	breaking C–C bonds in alkanes to give smaller molecules
Radical	a species with an unpaired electron, written with a dot
Homolytic fission	a covalent bond breaks and each atom keeps one electron

3.3.2.1 Fractional distillation

• Crude oil is heated and fed into the base of a column that is hot at the bottom, cool at the top.
• Vapour rises and cools. Each compound condenses at the height where the temperature drops below its boiling point.
• Short chains: low boiling point, collected high up. Long chains: high boiling point, collected low down.

Boiling point chain of reasoning (four links):

1. Only van der Waals forces act between alkane molecules.
2. A longer chain has more electrons and a bigger contact area.
3. So the van der Waals forces are stronger.
4. So more energy is needed to separate the molecules: higher boiling point.

Distillation is physical. No covalent bond breaks.

Crude oil is both a source of fuels and an important raw material for the chemical industry.

3.3.2.2 Cracking

Type	Pressure	Temperature	Catalyst	Main use or product
Thermal	high	high	none	a high percentage of alkenes
Catalytic	slight	high	zeolite	motor fuels and aromatic hydrocarbons

Mechanisms: not required for either.

Method: balancing a cracking equation

1. Write the alkane being cracked.
2. Write any product you are given.
3. Subtract carbons and hydrogens separately to find what is left.
4. Check each product against CₙH₂ₙ₊₂ (alkane) or CₙH₂ₙ (alkene).

Reminder: undecane to pentane and propene: C₁₁H₂₄ → C₅H₁₂ + 2C₃H₆ (H: 12 + 12 = 24).

Economic reasons, as a chain:

• Distillation gives a mix of fractions that does not match demand.
• Heavy, long-chain fractions are in surplus; lighter fractions such as motor fuels are in short supply.
• Higher demand means higher value, so cracking upgrades low-value surplus into high-value products.
• Alkenes from cracking react more readily than alkanes, so they are valuable for making polymers.
• The catalytic process gives motor fuels and aromatic hydrocarbons.

3.3.2.3 Combustion

Oxygen supply	Carbon ends up as	Also formed
Plenty (complete)	CO₂	H₂O
Limited (incomplete)	CO and/or C (soot)	H₂O

Method: balancing any combustion equation

1. Carbon atoms in fuel → that many CO₂ (or CO, or C).
2. Hydrogen atoms ÷ 2 → number of H₂O.
3. Count oxygen atoms in the products. Subtract any oxygen in the fuel. Halve to get O₂.
4. A half coefficient for O₂ is fine; double everything if you prefer whole numbers.

Reminder: butane burning to soot: C₄H₁₀ + 2½O₂ → 4C + 5H₂O (only the 5 H₂O need oxygen).

Method: combustion amounts

1. Balanced equation.
2. n(fuel) = mass ÷ Mr.
3. Multiply by the mole ratio.
4. Mass = n × Mr, or V = nRT ÷ p with p in Pa and V in m³ (× 1000 for dm³).

Engine pollutants

Pollutant	Origin
NOₓ	N₂ and O₂ from the air combine at high engine temperature: N₂ + O₂ → 2NO, then 2NO + O₂ → 2NO₂
CO	incomplete combustion; toxic
C (carbon particles)	incomplete combustion
Unburned hydrocarbons	fuel that leaves the engine unburnt

Catalytic converters remove the gaseous pollutants, giving CO₂, N₂ and H₂O. CO and hydrocarbons are oxidised; NO is reduced.

2CO + 2NO → 2CO2 + N2

For a hydrocarbon with NO: balance C, then H, then let NO supply every O atom; N₂ is half the NO.

Sulfur dioxide

• Sulfur in fuels burns to SO₂ (S + O₂ → SO₂), an air pollutant.
• Key idea: an acidic oxide (SO₂) meets a base (CaO is a basic oxide; CaCO₃ reacts with acids).
• The calcium compound neutralises the gas, trapping it as calcium sulfite in the power station:

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

Flue-gas masses are practised in the Group 2 revision notes.

3.3.2.4 Chlorination of methane

Condition: UV light. Mechanism: free-radical substitution. Dots for unpaired electrons; curly arrows are not required (section 3.3.1.2).

Stage	Equation	Radicals: left → right
Initiation	Cl₂ → 2Cl•	0 → 2
Propagation 1	CH₄ + Cl• → HCl + •CH₃	1 → 1
Propagation 2	•CH₃ + Cl₂ → CH₃Cl + Cl•	1 → 1
Termination	2Cl• → Cl₂	2 → 0
Termination	Cl• + •CH₃ → CH₃Cl	2 → 0
Termination	2•CH₃ → C₂H₆	2 → 0

Further substitution: Cl• can remove H from any chloromethane still holding C–H bonds, giving CH₂Cl₂, CHCl₃ and CCl₄. The product is a mixture.

• More methane relative to chlorine → mainly CH₃Cl.
• More chlorine relative to methane → more highly substituted products.
• Two-carbon products (C₂H₆, CH₃CH₂Cl) can only come from termination steps.

Must-know distinctions

• Distillation vs cracking: separates by boiling point vs breaks C–C bonds.
• Thermal vs catalytic: high pressure giving mostly alkenes, against a zeolite at slight pressure giving motor fuels and aromatic compounds.
• Complete vs incomplete: CO₂ against CO and/or C.
• NOₓ vs SO₂: nitrogen comes from air; sulfur comes from the fuel.
• Catalytic converter vs flue-gas treatment: engine exhaust (CO, NOₓ, hydrocarbons) vs power-station SO₂ with CaO or CaCO₃.
• Propagation vs termination: a radical is passed on vs radicals are used up.

Quick self-test

1. Work out Mr for nonadecane, C₁₉H₄₀, using H 1.0 and C 12.0.
2. Name the catalyst in catalytic cracking, and give the two main types of product it is used for.
3. Octadecane is cracked to give one molecule of hexadecane and one other molecule only. Identify it.
4. Write an equation for the complete combustion of methanol, CH₃OH.
5. What amount of O₂, in mol, does 0.400 mol of undecane, C₁₁H₂₄, use when it burns completely?
6. Find the mass of CO₂ produced by complete combustion of 7.10 g of decane, C₁₀H₂₂. Relative atomic masses: H 1.0, O 16.0, C 12.0.
7. Name four pollutants from the internal combustion engine.
8. Give the reason why calcium oxide removes SO₂ from flue gases.
9. Write the two propagation steps that convert trichloromethane into tetrachloromethane.
10. Which stage of the chlorination mechanism removes radicals? Give one equation for it.
11. Explain why distilling crude oil does not change the alkanes in it.

Answers

1. 19 × 12.0 + 40 × 1.0 = 268.0
2. Zeolite; motor fuels and aromatic hydrocarbons.
3. C₁₈H₃₈ − C₁₆H₃₄ leaves C₂H₄: ethene.
4. CH₃OH + 1½O₂ → CO₂ + 2H₂O (products hold 4 O atoms; the fuel has 1, so 3 come from O₂).
5. C₁₁H₂₄ + 17O₂ → 11CO₂ + 12H₂O, so 0.400 × 17 = 6.80 mol.
6. Mr(C₁₀H₂₂) = 142.0; n = 7.10 ÷ 142.0 = 0.0500 mol; n(CO₂) = 10 × 0.0500 = 0.500 mol; mass = 0.500 × 44.0 = 22.0 g.
7. NOₓ, CO, carbon (soot) and unburned hydrocarbons.
8. CaO, a basic oxide, neutralises SO₂, an acidic oxide: CaO + SO₂ → CaSO₃.
9. CHCl₃ + Cl• → HCl + •CCl₃ and •CCl₃ + Cl₂ → CCl₄ + Cl•
10. Termination, for example 2Cl• → Cl₂ (any two radicals combining).
11. Boiling only overcomes the van der Waals forces that hold molecules together; every covalent bond survives, so each alkane leaves the column unchanged.

Where marks are usually lost

• Calling distillation “breaking down” crude oil, or saying C–C bonds break on boiling.
• Stating that larger alkanes have higher boiling points without naming van der Waals forces.
• Mixing up the conditions: high pressure is thermal; slight pressure plus zeolite is catalytic.
• Explaining cracking with “to make petrol” alone, with no link between surplus, demand and value.
• Treating an alkene product as CₙH₂ₙ₊₂, so the cracking equation will not balance for hydrogen.
• Forgetting the oxygen already in an alcohol or ketone when balancing its combustion.
• Leaving the pressure in kPa in pV = nRT, giving a volume 1000 times too large.
• Tracing the N in NOₓ to the fuel instead of to air.
• Writing that CaO or CaCO₃ “absorbs” SO₂ with no acidic oxide and base reasoning.
• Initiation written as ions (Cl⁺ and Cl⁻), or no mention of UV light.

Official syllabus

OxfordAQA, International AS and A-level Chemistry (9620) specification, Version 5.3, for International AS and A-level exams May/June 2020 onwards. Section 3.3.2 Alkanes, points 3.3.2.1 to 3.3.2.4, headed: Fractional distillation of crude oil; Modification of alkanes by cracking; Combustion of alkanes; Chlorination of alkanes.