Hydrocarbons and Alkanes

This lesson introduces hydrocarbons, crude oil and the alkane homologous series as required by the Edexcel GCSE Chemistry specification (1CH0), Topic 9: Separate Chemistry 2. You need to know what hydrocarbons are, how crude oil formed, and how to name, draw and recognise the first four alkanes. This is the foundation for all of the organic chemistry and earth science content that follows.

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What Is a Hydrocarbon?

A hydrocarbon is a compound that contains only hydrogen and carbon atoms. No other elements are present. Hydrocarbons are the simplest organic compounds.

Feature	Detail
Elements present	Carbon (C) and hydrogen (H) only
Type of bonding	Covalent bonds (atoms share pairs of electrons)
Examples	Methane (CH₄), ethane (C₂H₆), propene (C₃H₆)

Exam Tip: If a molecule contains any element other than carbon and hydrogen — for example oxygen or nitrogen — it is not a hydrocarbon. This is a common one-mark definition question on the exam.

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Crude Oil

Crude oil is a fossil fuel. It is a complex mixture of many different hydrocarbons.

How Crude Oil Formed

1. Millions of years ago, tiny marine organisms (plankton and algae) died and sank to the sea floor.
2. Their remains were buried under layers of sediment (mud and sand).
3. Over millions of years, heat and pressure from the overlying rock converted the organic matter into crude oil and natural gas.
4. The oil migrated through porous rock and became trapped under impermeable cap rock, forming underground reservoirs.

Crude oil is a finite (non-renewable) resource — once it is used up, it cannot be replaced within a human timescale.

Property	Detail
Origin	Ancient marine organisms buried under sediment
Timescale	Formed over millions of years
Composition	Mixture of many hydrocarbons
Resource type	Finite (non-renewable)

Exam Tip: Always describe crude oil as a mixture of hydrocarbons, not a single compound. Because it is a mixture, it can be separated by physical methods (fractional distillation).

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Alkanes — The Simplest Hydrocarbons

Alkanes are a family (homologous series) of hydrocarbons that contain only single covalent bonds between carbon atoms. Because every carbon atom forms the maximum number of bonds to hydrogen, alkanes are described as saturated hydrocarbons.

General Formula

The general formula for alkanes is:

CₙH₂ₙ₊₂

where n is the number of carbon atoms. For example, if n = 3 then the formula is C₃H₈ (propane).

The First Four Alkanes

Name	Molecular formula	Number of C atoms	Number of H atoms	State at room temperature
Methane	CH₄	1	4	Gas
Ethane	C₂H₆	2	6	Gas
Propane	C₃H₈	3	8	Gas
Butane	C₄H₁₀	4	10	Gas

Exam Tip: Learn the names and formulae of the first four alkanes. A helpful mnemonic is Monkeys Eat Peanut Butter (methane, ethane, propane, butane).

Drawing Displayed Formulae

A displayed formula shows every atom and every bond in the molecule. In a displayed formula:

• Each carbon atom forms four bonds.
• Each hydrogen atom forms one bond.
• All bonds between carbon atoms in alkanes are single bonds (shown as a single line: C–C).

Methane (CH₄): Each of the four hydrogen atoms is bonded to the single carbon atom: H–C(–H)(–H)–H arranged tetrahedrally (drawn flat on paper as a cross shape).

Ethane (C₂H₆): Two carbon atoms are bonded together (C–C), with three hydrogen atoms bonded to each carbon.

Propane (C₃H₈): A chain of three carbon atoms (C–C–C), with hydrogen atoms filling the remaining bonds on each carbon (3 H on each end carbon, 2 H on the middle carbon).

Butane (C₄H₁₀): A chain of four carbon atoms (C–C–C–C), with hydrogen atoms filling the remaining bonds (3 H on each end carbon, 2 H on each middle carbon).

Exam Tip: When drawing displayed formulae, count the bonds on each carbon — it must always be exactly four. If a carbon has fewer than four bonds, you have missed a hydrogen.

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Homologous Series

A homologous series is a family of compounds that:

1. Have the same general formula (e.g. CₙH₂ₙ₊₂ for alkanes).
2. Differ from one member to the next by CH₂ (one carbon and two hydrogens).
3. Show a gradual trend in physical properties (e.g. boiling point increases as chain length increases).
4. Have similar chemical properties because they share the same functional group (or, in the case of alkanes, the same type of bonding throughout).
5. Can be prepared by the same general methods.

Examples

Member	Formula	Differs from previous by
Methane	CH₄	—
Ethane	C₂H₆	+CH₂
Propane	C₃H₈	+CH₂
Butane	C₄H₁₀	+CH₂
Pentane	C₅H₁₂	+CH₂

The concept of a homologous series applies to other families too — alkenes (CₙH₂ₙ), alcohols (CₙH₂ₙ₊₁OH), and carboxylic acids all form their own homologous series.

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Worked Example — Using the General Formula

Question: An alkane has 6 carbon atoms. What is its molecular formula? [1 mark]

Answer: Using the general formula CₙH₂ₙ₊₂ with n = 6:

• H = 2(6) + 2 = 14
• The molecular formula is C₆H₁₄ (hexane).

Question: A hydrocarbon has the molecular formula C₅H₁₂. Show that it is an alkane. [2 marks]

Answer:

• If it were an alkane with n = 5, the formula would be CₙH₂ₙ₊₂ = C₅H₁₂.
• The actual formula matches the general formula for alkanes, so it is an alkane.

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Key Definitions to Learn

Term	Definition
Hydrocarbon	A compound containing only hydrogen and carbon
Crude oil	A finite resource; a mixture of hydrocarbons formed from ancient marine organisms
Alkane	A saturated hydrocarbon with the general formula CₙH₂ₙ₊₂
Saturated	Contains only single C–C bonds
Homologous series	A family of compounds with the same general formula and similar chemical properties, each member differing by CH₂
Displayed formula	A diagram showing every atom and every covalent bond in a molecule
Fossil fuel	A fuel formed from the remains of dead organisms over millions of years (e.g. crude oil, coal, natural gas)

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Summary

• Hydrocarbons contain only carbon and hydrogen.
• Crude oil is a fossil fuel — a mixture of hydrocarbons formed over millions of years from ancient marine organisms.
• Alkanes are the simplest homologous series: saturated hydrocarbons with the general formula CₙH₂ₙ₊₂.
• The first four alkanes are methane, ethane, propane and butane.
• You must be able to name, draw displayed formulae for, and write molecular formulae for the first four alkanes.
• Members of a homologous series share the same general formula, differ by CH₂, and show similar chemical properties.