What you'll learn
This revision guide covers the chemistry of fuels, crude oil, and Earth's atmosphere as specified in the Edexcel GCSE Chemistry curriculum. You'll understand how hydrocarbons are extracted, processed and used as fuels, the chemical reactions involved in combustion, and the environmental impacts of burning fossil fuels. This topic connects organic chemistry with environmental issues relevant to your exam.
Key terms and definitions
Hydrocarbon — a compound containing only hydrogen and carbon atoms
Fractional distillation — the separation of crude oil into fractions based on different boiling points
Homologous series — a family of organic compounds with the same general formula and similar chemical properties
Complete combustion — burning a fuel in excess oxygen to produce carbon dioxide and water only
Cracking — the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules
Alkane — a saturated hydrocarbon with the general formula CₙH₂ₙ₊₂ containing only single carbon-carbon bonds
Greenhouse effect — the process by which certain gases in the atmosphere trap heat radiated from Earth's surface
Carbon footprint — the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event
Core concepts
Crude oil and hydrocarbons
Crude oil is a finite resource found in rocks beneath the Earth's surface. It formed over millions of years from the remains of ancient marine organisms subjected to high temperature and pressure. Crude oil is a mixture of many different hydrocarbons, most of which are alkanes.
The alkane homologous series includes:
- Methane (CH₄)
- Ethane (C₂H₆)
- Propane (C₃H₈)
- Butane (C₄H₁₀)
As the carbon chain length increases in alkanes:
- Boiling point increases
- Viscosity increases (the liquid becomes thicker)
- Flammability decreases
- Colour becomes darker
These trends are crucial for understanding how crude oil is separated and which fractions are most useful as fuels.
Fractional distillation of crude oil
Crude oil is separated into useful fractions by fractional distillation in a fractionating column. The process works as follows:
- Crude oil is heated to approximately 350°C, vaporising most hydrocarbons
- Vapours enter the fractionating column, which is hotter at the bottom and cooler at the top
- Hydrocarbons rise up the column and condense when they reach their boiling point
- Different fractions are collected at different levels
The main fractions from top to bottom are:
- Refinery gases (bottled gas for camping/heating) — shortest chains, lowest boiling points
- Petrol (gasoline) — fuel for cars
- Naphtha — chemical feedstock for plastics
- Kerosene (paraffin) — jet fuel, heating
- Diesel oil — fuel for cars, lorries, trains
- Fuel oil — fuel for ships, power stations
- Bitumen — surfacing roads, roofing — longest chains, highest boiling points
The fractions with shorter hydrocarbon chains are more in demand as fuels because they ignite more easily and burn more cleanly.
Cracking hydrocarbons
Long-chain hydrocarbons from crude oil are less useful than shorter chains. Cracking breaks down these large molecules into smaller, more valuable ones. There are two types you need to know:
Catalytic cracking:
- Long-chain hydrocarbons are heated to vaporise them
- Vapours are passed over a hot aluminium oxide or silica catalyst at approximately 600-700°C
- Thermal decomposition occurs, breaking C-C bonds
Steam cracking:
- Hydrocarbons are mixed with steam
- Heated to very high temperatures (over 800°C)
- No catalyst used
Cracking produces:
- Shorter-chain alkanes (useful as fuels like petrol)
- Alkenes (unsaturated hydrocarbons used to make polymers and other chemicals)
Example cracking equation: C₁₀H₂₂ → C₈H₁₈ + C₂H₄ (decane → octane + ethene)
The alkene product (ethene) can be identified by adding bromine water, which turns from orange to colourless. Alkanes do not decolourise bromine water.
Combustion of hydrocarbon fuels
When hydrocarbons burn in oxygen, combustion reactions occur. The products depend on the oxygen supply.
Complete combustion occurs when there is excess oxygen:
Hydrocarbon + oxygen → carbon dioxide + water
Example: CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion releases the maximum energy from the fuel and produces a blue flame.
Incomplete combustion occurs when oxygen is limited:
Hydrocarbon + oxygen → carbon monoxide + water (+ carbon)
Example: 2CH₄ + 3O₂ → 2CO + 4H₂O
Incomplete combustion may also produce carbon (soot) as solid particles, visible as smoke. The flame is typically yellow or orange.
Carbon monoxide is a toxic gas that:
- Is colourless and odourless, making it difficult to detect
- Binds to haemoglobin in red blood cells, reducing oxygen transport
- Can cause death by preventing oxygen reaching body tissues
- Is produced by faulty gas appliances with insufficient ventilation
Environmental impacts of combustion
Burning fossil fuels produces several pollutants with serious environmental consequences:
Carbon dioxide (CO₂):
- Main greenhouse gas contributing to climate change
- Produced by complete combustion of all carbon-containing fuels
- Levels have increased from approximately 280 ppm pre-industrial revolution to over 420 ppm today
- Enhanced greenhouse effect leads to global warming, causing ice cap melting, sea level rise, and extreme weather events
Sulfur dioxide (SO₂):
- Produced when sulfur impurities in fossil fuels burn: S + O₂ → SO₂
- Dissolves in rainwater to form sulfurous acid: SO₂ + H₂O → H₂SO₃
- Can be further oxidised to sulfuric acid, causing acid rain
- Acid rain damages limestone buildings, kills aquatic life, and harms trees
- Can be reduced by removing sulfur from fuels before burning or using flue gas desulfurisation (scrubbing with calcium oxide/limestone)
Nitrogen oxides (NOₓ):
- Formed when nitrogen and oxygen from air react at high temperatures in engines: N₂ + O₂ → 2NO
- Further oxidised to NO₂ in the atmosphere
- Contribute to acid rain and photochemical smog
- Can be reduced using catalytic converters in vehicle exhausts
Particulates (carbon/soot):
- Solid particles from incomplete combustion
- Cause respiratory problems and global dimming
- Blacken buildings
The Earth's atmosphere
The current composition of Earth's atmosphere is approximately:
- 78% nitrogen
- 21% oxygen
- 0.04% carbon dioxide
- Small amounts of water vapour and noble gases (mainly argon)
Evolution of the atmosphere:
Early atmosphere (4.6 billion years ago):
- Intense volcanic activity released gases including water vapour, carbon dioxide, methane, and ammonia
- Little or no oxygen present
- Similar to atmospheres of Mars and Venus today
Development of oxygen (from 2.7 billion years ago):
- Water vapour condensed to form oceans
- Primitive algae and plants evolved, carrying out photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Oxygen levels gradually increased over billions of years
- Enabled evolution of aerobic organisms
Reduction of carbon dioxide:
- Dissolved in oceans
- Used in photosynthesis by plants
- Locked up in fossil fuels formed from dead organisms
- Formed sedimentary rocks (limestone/chalk) through:
- Marine organisms incorporating dissolved CO₂ into shells as calcium carbonate
- Shells forming sediments when organisms died
- Compression over millions of years: CaCO₃ deposits
This understanding of atmospheric evolution is based on evidence from ice cores, ancient rocks, and comparison with other planets, though uncertainty remains about precise early conditions.
Worked examples
Example 1: Balance the equation for the complete combustion of propane (C₃H₈). [2 marks]
Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [1 mark for correct products, 1 mark for correct balancing]
Working: Each propane molecule contains 3 carbon atoms (forming 3 CO₂) and 8 hydrogen atoms (forming 4 H₂O). This requires 10 oxygen atoms total (3×2 + 4×1 = 10), meaning 5 O₂ molecules.
Example 2: A student performs cracking on liquid paraffin in a school laboratory. She collects a gas that turns bromine water from orange to colourless.
(a) What type of hydrocarbon is the gas? [1 mark] (b) Explain why cracking is performed industrially. [2 marks]
Answer: (a) Alkene [1 mark]
(b) To break down long-chain hydrocarbons [1 mark] into shorter, more useful molecules for fuels or to produce alkenes for making plastics/polymers [1 mark]
Example 3: Sulfur dioxide is produced when coal is burned in power stations. Describe how sulfur dioxide causes environmental damage and suggest one method to reduce this damage. [4 marks]
Answer: Sulfur dioxide dissolves in rainwater/clouds [1 mark] to form sulfurous acid or sulfuric acid (acid rain) [1 mark]. This damages limestone buildings/statues, kills fish in lakes/rivers, or damages trees/crops [1 mark]. It can be reduced by removing sulfur from coal before burning/using flue gas desulfurisation/scrubbing with calcium oxide or calcium carbonate [1 mark].
Common mistakes and how to avoid them
Confusing complete and incomplete combustion products: Remember complete combustion needs excess oxygen and produces only CO₂ and H₂O. Incomplete combustion (limited oxygen) produces CO and/or C (soot). Don't write "CO₂ and CO" as products together unless specifically asked about both types.
Forgetting to balance combustion equations properly: Always count atoms systematically. Start with carbon, then hydrogen, then oxygen. Remember O₂ molecules provide two oxygen atoms each.
Mixing up fractional distillation with cracking: Fractional distillation is physical separation of existing molecules by boiling point. Cracking is chemical breaking of bonds to make new, smaller molecules. They achieve different purposes.
Incorrectly stating how fractions are ordered: Remember shorter chains have lower boiling points and come off at the top of the fractionating column. Long chains (like bitumen) have high boiling points and exit at the bottom. The column is hotter at the bottom.
Writing vague environmental impacts: Instead of "sulfur dioxide is bad for the environment," specify: "sulfur dioxide dissolves in rainwater to form acid rain, which damages limestone buildings and harms aquatic life." Examiners reward specific detail.
Confusing the test for alkenes: Bromine water turns from orange/brown to colourless with alkenes (decolourisation). Don't say it "turns clear" — clear is not a colour. Alkanes do not react with bromine water in normal conditions.
Exam technique for "Fuels and Earth Science"
Command words matter: "State" requires a brief answer without explanation (1 mark). "Explain" requires reasoning and mechanisms (usually 2+ marks). "Describe" needs sequential steps or observations. "Suggest" indicates an unfamiliar context where you apply your knowledge.
Extended response questions: When asked to discuss atmospheric evolution or environmental impact of fuels (6-mark questions), structure your answer in clear paragraphs. Include multiple points for each aspect, use scientific terminology precisely, and link ideas logically. Aim for 2 marks per well-developed point.
Equation questions: Always check your balancing. Write state symbols if asked (g, l, s, aq). For combustion, ensure you know whether complete or incomplete combustion is required based on the oxygen availability mentioned in the question.
Practical context questions: You may be asked about industrial processes (fractional distillation, catalytic cracking) or laboratory demonstrations (cracking liquid paraffin, testing for alkenes). Know the conditions (temperatures, catalysts), equipment, and safety considerations for common practicals.
Quick revision summary
Crude oil contains hydrocarbon mixtures separated by fractional distillation based on boiling points. Shorter chains have lower boiling points and make better fuels. Cracking breaks long hydrocarbons into useful shorter alkanes and alkenes. Complete combustion of hydrocarbons in excess oxygen produces CO₂ and H₂O; incomplete combustion produces toxic CO and soot. Burning fossil fuels releases CO₂ (greenhouse gas causing climate change), SO₂ (causes acid rain), and NOₓ. Earth's early atmosphere contained mainly CO₂ and water vapour; oxygen increased through photosynthesis while CO₂ decreased through dissolution in oceans and incorporation into sedimentary rocks.