3.2Organic Chemistry

Alkanes — Oxford AQA International A Level Chemistry Revision Notes & Topical Questions

5 min read 15 parts Lesson 6 of 55

📚 Oxford AQA International A Level Chemistry Hub / Topical Revision

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Lesson summary

Complete PDF with figures and original page layoutPage 1 Oxford AQA International Your notes A Level (IAL) Chemistry Alkanes Contents Fractional Distillation of Crude Oil Modification of Alkanes By Cracking Combustion of Alkanes Chlorination of Alkane…

Part 1 of 9Crude oil and fractional distillation

Crude oil

Crude oil a mixture of mostly alkanes and the world’s main source of organic chemicals

Alkanes

Alkanes saturated hydrocarbons with only single C–C bonds, made of hydrogen and carbon only, as straight or branched chains

Fractions

Fractions the mixture is separated into fractions of similar chain length and boiling point, each with its own uses

Link

Link the size of the molecule decides the fraction it enters, and it relates to the number of carbon and hydrogen atoms

Part 2 of 9The fractionating column

Temperature gradient
- the column is hot at the bottom and cooler at the top

Crude oil is heated and vaporises.

The vapours enter the column.

Hydrocarbons with high boiling points condense at the bottom.

Those with lower boiling points rise and condense at the top.

1Other compounds

Other compounds crude oil contains small amounts of sulfur compounds, which burn to sulfur dioxide, a cause of acid rain

2Physical process

Physical process no covalent bonds break, and only intermolecular forces are overcome as the oil is heated

3Syllabus note

Syllabus note the fractions need not be memorised, but the principle and its purpose must be understood

Part 3 of 9Economics of cracking

Supply

Supply how much of a fraction fractional distillation produces

Demand

Demand how much customers want to buy

Imbalance

Imbalance petrol, kerosene and diesel are in high demand but short supply, whereas fuel oil and bitumen are in low demand but high supply
- (surplus fractions)

Solution

Solution some surplus fractions are broken down into smaller, more useful compounds

Part 4 of 9Cracking

Cracking = an endothermic process that breaks surplus long-chain alkanes into smaller, more useful compounds

Mechanism

Mechanism single C–C covalent bonds break in long-chain alkanes

Products

Products a smaller alkane and one or more alkenes with lower M_r

Uses

Uses low-M_r alkanes make good fuels, and alkenes are useful starting materials

Example

Example decane cracks into octane and ethene

Part 5 of 9Thermal and catalytic cracking

	Thermal cracking	Catalytic cracking
Temperature	High, up to 1000 °C	Lower, about 450 °C
Pressure	High, up to 70 atm	Slight pressure
Catalyst	None	Zeolite
Products	Alkanes and a high percentage of alkenes	Motor fuels and aromatic hydrocarbons

Part 6 of 9Complete and incomplete combustion

Use

Use alkanes are burnt on a large scale as fuels

Complete combustion

Complete combustion excess oxygen oxidises all carbon and hydrogen to carbon dioxide and water, as in octane

Incomplete combustion

Incomplete combustion limited oxygen oxidises all hydrogen to water but only some carbon, which becomes carbon monoxide

Soot

Soot with very little oxygen, carbon forms

Car engines

Car engines incomplete combustion often occurs because oxygen is limited

Part 7 of 9Pollutants: carbon monoxide and oxides of nitrogen

Exhaust pollutants

Exhaust pollutants carbon monoxide, nitrogen oxides (NO_x), unburned hydrocarbons and sulfur dioxide,
- which have serious environmental and health effects

Carbon monoxide

2C₈H₁₈ + 17O₂ → 16CO + 18H₂O
It is toxic and odourless, causing dizziness, loss of consciousness and death.
It binds to haemoglobin, reducing the transport of oxygen and removal of carbon dioxide.

Oxides of nitrogen

Nitrogen is normally unreactive, but high temperature and pressure in an engine oxidise it: N₂ + O₂ → 2NO and N₂ + 2O₂ → 2NO₂.
They dissolve and react with water and oxygen to form nitric acid, causing acid rain.
Acid rain corrodes buildings, harms plant and aquatic life, and damages human health.

Part 8 of 9Pollutants: hydrocarbons and sulfur dioxide

Pollutants: hydrocarbons and sulfur dioxide

Unburned hydrocarbons

Unburned hydrocarbons nitrogen oxides react with them in air to form photochemical smog, and they harm lungs, eyes and plants

Sulfur dioxide

Sulfur dioxide sulfur impurities oxidise, S(s) + O₂(g) → SO₂(g), and SO₂ forms sulfuric acid in water, causing acid rain

Sulfur scrubbing (flue gas desulfurisation)

Sulfur scrubbing (flue gas desulfurisation) a slurry of calcium oxide and calcium carbonate is sprayed into waste gases
CaO(s) + 2H₂O(l) + SO₂(g) + ½O₂(g) → CaSO₄·2H₂O(s), forming gypsum via calcium sulfite.
CaCO₃ + ½O₂ + SO₂ → CaSO₄ + CO₂.
It removes 90–95% of the sulfur dioxide.

Part 9 of 9Catalytic converters

Design

Design precious metals such as platinum are coated on a honeycomb for a large surface area

Reactions

Oxidation of CO2CO + O₂ → 2CO₂.
Reduction of NO2CO + 2NO → 2CO₂ + N₂.
Oxidation of unburnt hydrocarbons, such as octane to CO₂ and H₂O.

Pollutant	Formation	Consequence	Catalytic removal
Carbon monoxide	Incomplete combustion in car engines	Toxic gas	Oxidation to CO₂
Oxides of nitrogen	Oxidation of nitrogen in car engines	Acid rain	Reduction to N₂
Unburned hydrocarbons	Unburned fuel	Smog with nitrogen oxides	Oxidation to CO₂ and H₂O
Sulfur dioxide	Oxidation of sulfur impurities	Acid rain	Oxidation to calcium sulfate (scrubbing)

Carbon dioxide
- it is not toxic, but it is a pollutant that causes global warming

Key terms

Term	Meaning
Fractional distillation	Separation of crude oil into fractions by boiling point.
Cracking	Breaking long-chain alkanes into shorter alkanes and alkenes.
Incomplete combustion	Burning in limited oxygen, producing CO or soot.
Catalytic converter	A device with precious metal catalysts that removes pollutants from exhaust.
Flue gas desulfurisation	Removing sulfur dioxide from waste gases using calcium compounds.

Avoid these mistakes

Saying fractional distillation breaks covalent bonds — it only overcomes intermolecular forces.

Saying cracking is exothermic — it is endothermic.

Saying nitrogen oxides form from fuel nitrogen — they form from air nitrogen in hot engines.

Forgetting carbon dioxide as a pollutant — it causes climate change.

Last updated: 29 October 2026