Crude Oil, Hydrocarbons and Alkanes

This lesson introduces organic chemistry as covered in the AQA GCSE Combined Science Trilogy (8464) specification. You will learn about crude oil as a finite resource, what hydrocarbons are, the alkane homologous series, and how the properties of hydrocarbons change with chain length.

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What Is Crude Oil?

Crude oil is a fossil fuel formed over millions of years from the remains of ancient marine organisms (plankton) that were buried under layers of sediment. Heat and pressure transformed these remains into a complex mixture of hydrocarbons.

Crude oil is:

• A finite (non-renewable) resource — it is being used up faster than it forms.
• A mixture of many different hydrocarbons.
• The basis of the modern petrochemical industry.

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What Are Hydrocarbons?

A hydrocarbon is a compound made up of hydrogen and carbon atoms only.

Most of the hydrocarbons in crude oil are alkanes.

Key Definition: A hydrocarbon contains only hydrogen and carbon atoms bonded together.

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The Alkane Homologous Series

The alkanes are a family (homologous series) of hydrocarbons with the general formula:

$C_nH_{2n+2}$Cn​H2n+2​

where n is the number of carbon atoms.

Name	Molecular Formula	Number of Carbons	Structure
Methane	CH₄	1	Single carbon with 4 hydrogens
Ethane	C₂H₆	2	Two carbons bonded together
Propane	C₃H₈	3	Three-carbon chain
Butane	C₄H₁₀	4	Four-carbon chain
Pentane	C₅H₁₂	5	Five-carbon chain
Octane	C₈H₁₈	8	Eight-carbon chain

Features of a Homologous Series

All members of a homologous series:

• Have the same general formula
• Differ by CH₂ from one member to the next
• Show a gradual trend in physical properties
• Have similar chemical properties

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Properties of Alkanes and Chain Length

As the number of carbon atoms in an alkane increases, the following trends are observed:

Property	Short chain (e.g. methane)	Long chain (e.g. octane and above)
Boiling point	Low	High
Viscosity	Low (runny)	High (thick/viscous)
Flammability	High (burns easily)	Low (harder to ignite)
State at room temperature	Gas	Liquid (or solid for very long chains)

These trends are due to intermolecular forces (forces between molecules). Longer chains have stronger intermolecular forces because there are more points of contact between molecules, so more energy is needed to separate them.

graph LR
    A["Short carbon chain"] --> B["Weak intermolecular forces"]
    B --> C["Low boiling point"]
    B --> D["Low viscosity"]
    B --> E["High flammability"]
    F["Long carbon chain"] --> G["Strong intermolecular forces"]
    G --> H["High boiling point"]
    G --> I["High viscosity"]
    G --> J["Low flammability"]

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Crude Oil as a Mixture

Crude oil straight from the ground is not very useful because it is a mixture of hundreds of different hydrocarbons. To make it useful, it must be separated into fractions — groups of hydrocarbons with similar chain lengths and boiling points. This is done by fractional distillation (covered in the next lesson).

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Complete Combustion of Hydrocarbons

When hydrocarbons are burned in a plentiful supply of oxygen, complete combustion occurs:

$\text{hydrocarbon} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$hydrocarbon+oxygen→carbon dioxide+water

For methane:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$CH4​+2O2​→CO2​+2H2​O

This releases energy and is exothermic.

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Incomplete Combustion

If there is a limited supply of oxygen, incomplete combustion occurs. This produces:

• Carbon monoxide (CO) — a toxic, colourless, odourless gas
• Carbon (soot) — solid particles
• Water (H₂O)

$\text{CH}_4 + \text{O}_2 \rightarrow \text{CO} + 2\text{H}_2\text{O}$CH4​+O2​→CO+2H2​O

or

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$2CH4​+3O2​→2CO+4H2​O

Exam Tip: Incomplete combustion is dangerous because carbon monoxide binds to haemoglobin in red blood cells, reducing the blood's ability to carry oxygen. This can be fatal.

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Worked Example: Using the General Formula

Question: Decane has 10 carbon atoms. Use the general formula to work out its molecular formula.

$C_nH_{2n+2} \quad \text{where } n = 10$Cn​H2n+2​where n=10

$C_{10}H_{(2 \times 10) + 2} = C_{10}H_{22}$C10​H(2×10)+2​=C10​H22​

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Common Mistakes

• Confusing hydrocarbons with carbohydrates (carbohydrates contain oxygen too).
• Forgetting that alkanes contain only single covalent bonds between carbon atoms (they are "saturated").
• Writing incomplete combustion equations that are not balanced.

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Summary

• Crude oil is a finite resource — a mixture of hydrocarbons formed from ancient marine organisms
• Hydrocarbons contain only carbon and hydrogen
• Alkanes are the main hydrocarbons in crude oil, with general formula CₙH₂ₙ₊₂
• As chain length increases: boiling point and viscosity increase; flammability decreases
• Complete combustion produces CO₂ and H₂O; incomplete combustion produces CO and/or C (soot)
• Trends in properties are explained by stronger intermolecular forces in longer chains

Exam Tip: AQA questions often ask you to use the general formula to predict the formula of an alkane, or to explain property trends using intermolecular forces. Make sure you can do both confidently.

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Extended Worked Examples and Exam Technique

Worked Example 1: Using the General Formula

Question: Deduce the molecular formula of an alkane that contains 10 carbon atoms.

Solution: Using CₙH₂ₙ₊₂ with n = 10:

• H = 2(10) + 2 = 22
• Formula: C₁₀H₂₂ (decane)

Worked Example 2: Balancing Complete Combustion

Question: Write a balanced equation for the complete combustion of propane (C₃H₈).

Solution: $\text{C}_3\text{H}_8 + 5\text{O}_2 \rightarrow 3\text{CO}_2 + 4\text{H}_2\text{O}$C3​H8​+5O2​→3CO2​+4H2​O

Check: C: 3 = 3 ✓ H: 8 = 4 × 2 = 8 ✓ O: 5 × 2 = 10; right side: (3 × 2) + (4 × 1) = 10 ✓

A systematic method for balancing combustion equations:

1. Balance carbons first: one CO₂ per C in the hydrocarbon.
2. Balance hydrogens next: one H₂O per 2 H atoms.
3. Count oxygens on the right and halve — that is the number of O₂ molecules on the left.
4. If the O₂ coefficient is a fraction, multiply the entire equation by 2.

Worked Example 3: Incomplete Combustion

Question: A Bunsen burner on a yellow flame produced a black deposit on the underside of a beaker. Explain what this tells you about combustion conditions.