What you'll learn
This topic examines the chemistry of crude oil, fossil fuels, and Earth's atmospheric composition. Edexcel GCSE Chemistry papers regularly test fractional distillation, hydrocarbon properties, combustion reactions, cracking processes, and the environmental impact of burning fuels. Understanding these concepts is essential for both Paper 1 and Paper 2, particularly in questions worth 4-6 marks that require extended explanations.
Key terms and definitions
Hydrocarbon — a compound containing only hydrogen and carbon atoms, typically derived from crude oil.
Fractional distillation — the separation of a mixture into fractions based on different boiling points, used to separate crude oil into useful products.
Cracking — the thermal decomposition of long-chain hydrocarbons into shorter, more useful molecules including alkenes.
Homologous series — a family of compounds with the same general formula and similar chemical properties that show a gradual change in physical properties.
Complete combustion — burning in excess oxygen to produce carbon dioxide and water only, releasing maximum energy.
Incomplete combustion — burning in limited oxygen producing carbon monoxide, carbon (soot), and water, releasing less energy than complete combustion.
Fossil fuels — natural fuels formed from the remains of living organisms over millions of years, including coal, crude oil, and natural gas.
Alkanes — saturated hydrocarbons with single carbon-carbon bonds, following the general formula CₙH₂ₙ₊₂.
Core concepts
Crude oil composition and formation
Crude oil is a finite resource found in rocks. It formed over millions of years from the remains of ancient marine organisms (mainly plankton) that were buried under layers of sediment. High temperature and pressure converted the organic material into crude oil and natural gas.
Crude oil is a mixture of hydrocarbons, predominantly alkanes. The mixture contains molecules with varying chain lengths:
- Short-chain hydrocarbons (1-4 carbons): gases at room temperature
- Medium-chain hydrocarbons (5-12 carbons): liquids
- Long-chain hydrocarbons (13+ carbons): thick liquids or solids
Because crude oil is a mixture rather than a pure substance, it does not have a sharp boiling point. Different fractions vaporise at different temperatures, making fractional distillation possible.
Fractional distillation process
Crude oil undergoes fractional distillation at an oil refinery to separate it into useful fractions. The process works because different hydrocarbons have different boiling points.
The industrial process:
- Crude oil is heated to approximately 350°C in a furnace until most of it vaporises
- The vaporised mixture enters the fractionating column, which is hottest at the bottom and cooler at the top
- Hydrocarbon vapours rise up the column through bubble caps or trays
- As vapours rise and cool, they condense at different heights depending on their boiling points
- Fractions are collected at different levels: gases at the top, bitumen at the bottom
Main fractions from top to bottom:
- Refinery gases (LPG): used for heating and cooking, bottled gas
- Gasoline (petrol): used as fuel for cars
- Naphtha: used for making chemicals
- Kerosene (paraffin): used as aircraft fuel
- Diesel oil: used as fuel for diesel engines, central heating
- Fuel oil: used for ships and power stations
- Bitumen: used for road surfacing and roofing
Properties and trends in hydrocarbons
As the carbon chain length increases in alkanes, several properties change predictably:
Boiling point: increases with chain length because longer molecules have stronger intermolecular forces (London dispersion forces) requiring more energy to overcome.
Viscosity: increases with chain length. Longer molecules become more tangled, making the liquid thicker and less runny.
Flammability: decreases with chain length. Shorter hydrocarbons ignite more easily and burn with cleaner flames.
Colour: fractions become darker (more brown/black) as chain length increases.
These trends explain why different fractions suit different purposes. Petrol (short chains) ignites easily in car engines, while bitumen (very long chains) is thick enough to resurface roads.
Combustion of hydrocarbon fuels
When hydrocarbon fuels burn, they undergo oxidation reactions. The products depend on the oxygen supply.
Complete combustion occurs with plentiful oxygen:
Hydrocarbon + oxygen → carbon dioxide + water
Example: Methane burning in air
CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion produces blue flames and releases maximum energy. All carbon atoms are fully oxidised to carbon dioxide.
Incomplete combustion occurs with limited oxygen supply:
Hydrocarbon + oxygen → carbon monoxide + carbon + water
Example: Methane burning with insufficient oxygen
2CH₄ + 3O₂ → 2CO + 4H₂O
Incomplete combustion produces yellow/orange flames and solid carbon particles (soot). Carbon monoxide is a toxic, colourless, odourless gas that binds to haemoglobin in blood, preventing oxygen transport. This is why proper ventilation is critical for gas appliances.
Cracking processes
Longer hydrocarbon chains from crude oil are less useful than shorter ones. Cracking breaks long-chain alkanes into shorter, more useful molecules including alkenes.
Two industrial cracking methods feature in Edexcel GCSE Chemistry:
Catalytic cracking:
- Long-chain hydrocarbons are vaporised
- Vapours pass over a hot aluminium oxide or zeolite catalyst (approximately 600-700°C)
- Moderate pressure used
- Produces alkanes and alkenes
Steam cracking:
- Vaporised hydrocarbons are mixed with steam
- Heated to very high temperatures (800-900°C)
- No catalyst required
- Produces alkenes (particularly ethene for polymer production)
Example cracking equation:
Decane → octane + ethene
C₁₀H₂₂ → C₈H₁₈ + C₂H₄
The products are more valuable than the starting material. Short-chain alkanes are used as fuels (high demand for petrol). Alkenes are used to make polymers, which are unreactive alkanes cannot do because they lack a C=C double bond.
Environmental issues with fossil fuels
Burning fossil fuels causes several environmental problems regularly examined in Edexcel papers:
Carbon dioxide and climate change: CO₂ is a greenhouse gas that contributes to global warming. Increased atmospheric CO₂ from burning fossil fuels enhances the greenhouse effect, causing average global temperatures to rise. This leads to climate change effects including ice cap melting, sea level rise, and extreme weather events.
Sulfur dioxide and acid rain: Fossil fuels contain sulfur impurities. When burned, sulfur dioxide forms:
S + O₂ → SO₂
Sulfur dioxide dissolves in atmospheric water droplets forming sulfurous acid, which oxidises to sulfuric acid:
SO₂ + H₂O → H₂SO₃
2H₂SO₃ + O₂ → 2H₂SO₄
Acid rain (pH below 5) damages buildings (especially limestone and marble), harms aquatic ecosystems by lowering water pH, and damages plant leaves.
Nitrogen oxides and acid rain: High temperatures in engines cause nitrogen and oxygen from air to react:
N₂ + O₂ → 2NO
2NO + O₂ → 2NO₂
Nitrogen dioxide also contributes to acid rain and causes respiratory problems.
Particulates: Incomplete combustion releases solid carbon particles (soot) and unburnt hydrocarbons. These particulates cause respiratory problems, worsen asthma, and contribute to global dimming by reflecting sunlight.
Earth's early and current atmosphere
Edexcel specifications require understanding of atmospheric evolution:
Early atmosphere (first billion years):
- Volcanic activity released gases including water vapour, carbon dioxide, nitrogen, methane, and ammonia
- Very little or no oxygen present
- Similar to atmospheres of Mars and Venus today
Evolution of oxygen:
- Water vapour condensed to form oceans (approximately 4 billion years ago)
- Primitive photosynthetic organisms (algae and cyanobacteria) evolved around 2.7 billion years ago
- Photosynthesis removed CO₂ and produced O₂:
6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
- Oxygen levels gradually increased, enabling aerobic life forms to evolve
- Carbon dioxide decreased as it dissolved in oceans and was locked in sedimentary rocks and fossil fuels
Current atmosphere composition:
- Nitrogen: 78%
- Oxygen: 21%
- Argon: 0.9%
- Carbon dioxide: 0.04%
- Water vapour: variable
- Trace amounts of other noble gases
Worked examples
Example 1: A student investigates the distillation of crude oil using a fractionating column in the laboratory. Explain why different fractions are collected at different heights in the column. (3 marks)
Model answer:
- Crude oil contains hydrocarbons with different boiling points (1)
- The column is hotter at the bottom and cooler at the top (1)
- Molecules with lower boiling points rise higher before condensing / molecules with higher boiling points condense lower down (1)
Example 2: Propane (C₃H₈) is used as a fuel. Write a balanced symbol equation for the complete combustion of propane. (2 marks)
Model answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
- Correct products (1)
- Correctly balanced (1)
Example 3: Long-chain hydrocarbons can be broken down into shorter, more useful molecules by cracking.
(a) Name the type of reaction that occurs during cracking. (1 mark)
(b) Write a balanced equation to show the cracking of dodecane (C₁₂H₂₆) to produce octane (C₈H₁₈) and one other product. Name the other product. (3 marks)
Model answer:
(a) Thermal decomposition (1)
(b) C₁₂H₂₆ → C₈H₁₈ + C₄H₈ (2)
[or accept C₁₂H₂₆ → C₈H₁₈ + 2C₂H₄ or other valid combinations]
But-1-ene / butene (1) [or ethene if equation shows that product]
Common mistakes and how to avoid them
Mistake: Writing incomplete combustion always produces carbon monoxide only. Correction: Incomplete combustion can produce carbon monoxide, solid carbon (soot), or both, plus water. The exact products depend on how limited the oxygen supply is.
Mistake: Stating that cracking requires a catalyst in all cases. Correction: Catalytic cracking uses a catalyst (aluminium oxide), but steam cracking uses high temperatures without a catalyst. Both methods are acceptable answers unless the question specifies which type.
Mistake: Confusing viscosity and flammability trends—thinking longer hydrocarbons are more flammable. Correction: Longer hydrocarbons are more viscous (thicker) but less flammable. Shorter hydrocarbons ignite more easily and are more flammable.
Mistake: Forgetting that cracking always produces at least one alkene. Correction: Cracking breaks C-C bonds, producing shorter alkanes AND alkenes. The presence of alkenes is the key difference from the original long-chain alkanes. At least one product must be unsaturated.
Mistake: Writing that crude oil is a compound. Correction: Crude oil is a mixture of hydrocarbons. Compounds have fixed compositions and properties; mixtures have variable compositions and can be separated by physical methods like distillation.
Mistake: Placing fractions at the wrong position in the fractionating column. Correction: Remember: gases and low-boiling-point fractions (like petrol) exit near the top where it's cooler. Heavy fractions (like bitumen) remain at the hot bottom. Match boiling points to column position.
Exam technique for Fuels and Earth Science
Command word awareness: Questions using "explain" require reasons or mechanisms (worth 3-6 marks). Simply stating facts without linking cause and effect loses marks. For example, explaining why long-chain hydrocarbons are cracked requires stating both that they're less useful AND that shorter chains are in higher demand for fuels or that alkenes are needed for polymers.
Equation writing: When asked to write balanced equations for combustion or cracking, check that atoms balance on both sides. Examiners award marks separately for correct products and correct balancing. State symbols are only required if the question specifically requests them.
Describing processes: Questions asking you to describe fractional distillation or cracking typically award 1 mark per valid point up to 4-6 marks. Use the numbered steps structure given in this guide, and include specific details like temperature ranges or catalyst names where possible.
Environmental chemistry: Extended response questions (6 marks) on pollution often require you to name the pollutant, state its source, describe how it's formed (with equation if relevant), and explain its environmental impact. Structure answers with clear paragraphs for each pollutant.
Quick revision summary
Crude oil is a mixture of hydrocarbons separated by fractional distillation based on boiling point differences. Short-chain hydrocarbons are more flammable, less viscous, and have lower boiling points. Complete combustion produces CO₂ and H₂O; incomplete combustion produces CO and C. Cracking breaks long chains into shorter alkanes and alkenes using catalysts or high temperatures. Burning fossil fuels releases CO₂ (climate change), SO₂ (acid rain), and NOₓ (acid rain, respiratory issues). Earth's atmosphere evolved from volcanic gases to its current composition (78% N₂, 21% O₂) through photosynthesis.