Organic chemistry: crude oil, hydrocarbons and alkanes

What you'll learn

This revision guide covers the AQA GCSE Chemistry specification content on crude oil, hydrocarbons and alkanes. You'll understand how crude oil is formed, separated and used, learn the properties and structure of alkanes, and apply this knowledge to exam-style questions. This topic forms the foundation of organic chemistry at GCSE level.

Key terms and definitions

Hydrocarbon — a compound containing only hydrogen and carbon atoms

Alkane — a saturated hydrocarbon with the general formula CₙH₂ₙ₊₂, containing only single carbon-carbon bonds

Fractional distillation — the separation process that uses differences in boiling points to separate crude oil into fractions

Fraction — a mixture of hydrocarbons with similar boiling points, separated from crude oil

Homologous series — a family of compounds with the same general formula and similar chemical properties

Viscosity — a measure of how easily a liquid flows; high viscosity means thick and sticky

Finite resource — a resource that cannot be replaced once used, such as fossil fuels

Complete combustion — burning in excess oxygen to produce carbon dioxide and water only

Core concepts

What is crude oil?

Crude oil is a finite resource found in rocks. It is the remains of ancient biomass, mostly plankton, that was buried in mud millions of years ago. Over time, heat and pressure converted this organic material into crude oil.

Formation of crude oil:

• Ancient marine organisms (mainly plankton) died and fell to the seabed
• Layers of sediment buried the organic matter
• High pressure and temperature, combined with absence of oxygen, prevented complete decay
• Over millions of years, the organic material transformed into crude oil
• Oil migrated through porous rocks and became trapped under impermeable rock layers

Crude oil is a mixture of many different hydrocarbons, mostly alkanes. Because it is a mixture rather than a pure substance, crude oil does not have a fixed boiling point. The different compounds in crude oil can be separated by fractional distillation because they have different boiling points.

Fractional distillation of crude oil

Fractional distillation separates crude oil into useful fractions. This process takes place in a fractionating column at an oil refinery.

The process:

• Crude oil is heated to approximately 350°C, causing most hydrocarbons to vaporise
• The vaporised oil enters the fractionating column, which is hotter at the bottom and cooler at the top
• Vapours rise up the column through trays or bubble caps
• As vapours rise and cool, different hydrocarbons condense at different heights
• Hydrocarbons with higher boiling points condense near the bottom
• Hydrocarbons with lower boiling points condense near the top
• Very long hydrocarbons with very high boiling points do not vaporise and are removed as bitumen at the bottom
• Very short hydrocarbons with very low boiling points do not condense and are collected as gases at the top

Main fractions from bottom to top:

• Bitumen (residue) — roads, waterproofing
• Fuel oil / heavy fuel oil — ships, power stations
• Diesel oil / gas oil — diesel engines, heating
• Kerosene (paraffin) — jet fuel, heating
• Petrol (gasoline) — car fuel
• Refinery gases (LPG) — bottled gas for heating and cooking

Properties of hydrocarbons and how they change

The properties of hydrocarbons depend on the size of their molecules. As the length of the hydrocarbon chain increases, several physical properties change in predictable ways.

Trends as molecular size increases:

Why these trends matter:

• Smaller hydrocarbons (like petrol) are runny, easy to ignite and burn with clean flames — ideal for fuels
• Larger hydrocarbons (like bitumen) are thick, difficult to ignite and produce smoky flames — better for industrial uses

These trends explain why different fractions are suited to different uses. The high demand for smaller hydrocarbons like petrol has led to the development of cracking (covered in a separate topic).

Structure and naming of alkanes

Alkanes are saturated hydrocarbons, meaning they contain only single covalent bonds between carbon atoms. They form a homologous series with the general formula CₙH₂ₙ₊₂.

First ten alkanes:

1. Methane — CH₄
2. Ethane — C₂H₆
3. Propane — C₃H₈
4. Butane — C₄H₁₀
5. Pentane — C₅H₁₂
6. Hexane — C₆H₁₄
7. Heptane — C₇H₁₆
8. Octane — C₈H₁₈
9. Nonane — C₉H₂₀
10. Decane — C₁₀H₂₂

Structural features:

• Each carbon atom forms four single covalent bonds
• Carbon atoms bond to other carbon atoms forming chains (or rings, though not required at GCSE)
• Remaining bonds are with hydrogen atoms
• All C-C bonds are single bonds (saturated)

Displayed formula vs molecular formula:

• Molecular formula shows the number of each type of atom (e.g., C₃H₈)
• Displayed formula shows all atoms and bonds drawn out
• Structural formula shows arrangement without showing all bonds (e.g., CH₃CH₂CH₃)

Combustion of hydrocarbons

Hydrocarbon fuels are burned to release energy. The type of combustion depends on the oxygen supply.

Complete combustion:

• Occurs when there is sufficient (excess) oxygen
• Produces carbon dioxide and water only
• Releases maximum energy
• Produces a blue flame

Word equation: hydrocarbon + oxygen → carbon dioxide + water

Example (methane): CH₄ + 2O₂ → CO₂ + 2H₂O

Incomplete combustion:

• Occurs when oxygen supply is limited
• Produces carbon monoxide and/or carbon (soot) and water
• Releases less energy than complete combustion
• Produces a yellow/orange, smoky flame

Equation (incomplete combustion producing CO): 2CH₄ + 3O₂ → 2CO + 4H₂O

Equation (incomplete combustion producing carbon): CH₄ + O₂ → C + 2H₂O

Hazards:

• Carbon monoxide is a toxic gas that prevents blood from carrying oxygen properly; it has no colour or smell, making it particularly dangerous
• Carbon (soot) causes breathing problems and blackens buildings
• Carbon dioxide contributes to climate change (though produced in both complete and incomplete combustion)

Uses of crude oil fractions

The many fractions obtained from crude oil have different uses based on their properties.

Fuels: Most fractions are used as fuels in combustion reactions. The choice of fuel depends on properties like ease of ignition, energy released per gram, and cleanliness of burning.

Petrochemicals: Some fractions serve as feedstock for the petrochemical industry. Through chemical reactions, these can be converted into:

• Polymers (plastics)
• Solvents
• Lubricants
• Detergents
• Pharmaceuticals

The petrochemical industry is vital to modern life, producing materials for clothing, packaging, construction and medicine. However, crude oil is a finite resource, so its use for making materials (which can be recycled or last a long time) may be more sustainable than burning it as fuel.

Worked examples

Example 1: Applying the general formula

Question: An alkane contains 7 carbon atoms. What is its molecular formula? [2 marks]

Answer:

• Use the general formula for alkanes: CₙH₂ₙ₊₂ [1 mark]
• When n = 7: H = 2(7) + 2 = 16
• Molecular formula = C₇H₁₆ [1 mark]

Examiner note: Always show your working. If you make an arithmetic error, you can still gain the method mark.

Example 2: Fractional distillation

Question: Describe how fractional distillation separates crude oil into fractions. Explain why this process works. [6 marks]

Mark scheme answer:

• Crude oil is heated/vaporised [1 mark]
• Vapour enters the fractionating column [1 mark]
• The column is hotter at the bottom and cooler at the top / temperature gradient [1 mark]
• Different hydrocarbons condense at different temperatures/heights [1 mark]
• Hydrocarbons with higher boiling points condense lower down [1 mark]
• This works because crude oil contains hydrocarbons with different boiling points [1 mark]

Examiner note: For 6-mark questions, ensure you cover both description (the process) and explanation (why it works). Use scientific terminology precisely.

Example 3: Combustion equation balancing

Question: Balance the equation for the complete combustion of propane (C₃H₈). [2 marks]

Answer: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O [2 marks for correctly balanced equation]

Working:

• 3 carbons on left → need 3CO₂ on right
• 8 hydrogens on left → need 4H₂O on right
• Right side has (3×2) + (4×1) = 10 oxygen atoms
• Left side needs 10 oxygen atoms, so 5O₂

Examiner note: Award 1 mark if one error is present but the equation is otherwise correctly balanced. Always check your final answer by counting atoms on both sides.

Common mistakes and how to avoid them

• Confusing molecular and displayed formulae — Remember that C₃H₈ is a molecular formula showing composition, while a displayed formula shows the arrangement of all atoms and bonds. You may be asked to write either, so read the question carefully.

• Getting the general formula wrong — The alkane general formula is CₙH₂ₙ₊₂, not CₙH₂ₙ (that's alkenes). Write it down at the start of a question if needed and double-check your substitution.

• Thinking crude oil is a compound — Crude oil is a mixture of many different hydrocarbons. Mixtures can be separated by physical methods (like fractional distillation), while compounds require chemical reactions to break down.

• Reversing the temperature gradient — Students often think the fractionating column is cooler at the bottom. Remember: hot at bottom, cool at top. Smaller molecules (lower boiling points) condense higher up where it's cooler.

• Incomplete combustion equations with carbon dioxide — In incomplete combustion, you should produce either CO or C (or both), NOT CO₂. Carbon dioxide is only produced in complete combustion or alongside CO/C in incomplete combustion.

• Not linking properties to uses — Exam questions often ask you to explain why a fraction is suitable for a particular use. Always link the property (e.g., "low boiling point," "runny/low viscosity," "easily vaporised") to the use (e.g., "good fuel for cars").

Exam technique for "Organic chemistry: crude oil, hydrocarbons and alkanes"

• Command word "describe" requires you to state what happens without explanation. Command word "explain" requires you to give reasons why something happens. In a 6-mark question, you typically need both description and explanation.

• When drawing displayed formulae, show ALL atoms and ALL bonds clearly. Use straight lines for bonds and write element symbols clearly. Common mistake: forgetting hydrogen atoms bonded to carbon atoms.

• For calculation questions (e.g., using CₙH₂ₙ₊₂), always show your working step-by-step. Even if your final answer is wrong, you can gain method marks. State the formula you're using, substitute the values, then calculate.

• Comparison questions about properties or fractions typically require you to make statements about BOTH things being compared. Don't just describe one — you must make comparative statements (e.g., "smaller molecules are more flammable than larger molecules" rather than just "smaller molecules are flammable").

Quick revision summary

Crude oil is a finite resource formed from ancient marine organisms over millions of years. It contains a mixture of hydrocarbons, mostly alkanes. Fractional distillation separates crude oil into useful fractions based on boiling point differences — the column is hotter at the bottom and cooler at the top. Alkanes are saturated hydrocarbons with the general formula CₙH₂ₙ₊₂. As hydrocarbon chain length increases, boiling point and viscosity increase while flammability decreases. Hydrocarbons undergo complete combustion in excess oxygen (producing CO₂ and H₂O) or incomplete combustion in limited oxygen (producing CO and/or C).