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This lesson introduces crude oil and hydrocarbons as required by the AQA GCSE Chemistry specification (5.8.1). Crude oil is a finite resource found in the Earth's crust and is the starting point for manufacturing fuels and feedstock chemicals. Understanding the composition of crude oil and the nature of hydrocarbons is essential for the rest of the organic chemistry topic.
Crude oil is a fossil fuel formed over millions of years from the remains of ancient marine organisms (plankton and algae) that were buried under layers of sediment. Heat and pressure gradually converted this organic matter into a complex mixture of hydrocarbons.
Key facts about crude oil:
| Property | Detail |
|---|---|
| Origin | Formed from the remains of ancient marine organisms over millions of years |
| Location | Found trapped in porous rock beneath impermeable cap rock in the Earth's crust |
| Type of resource | Finite (non-renewable) — once used up, it cannot be replaced on a human timescale |
| Composition | A complex mixture of many different hydrocarbons |
| State | A thick, dark liquid in its raw form |
Crude oil is extracted by drilling wells into underground reservoirs. It has no practical use in its raw form and must be separated into useful fractions before it can be used.
Exam Tip: The key phrase is "a complex mixture of hydrocarbons." In exam answers, always describe crude oil as a mixture, not a compound. A mixture can be separated by physical methods; a compound cannot.
A hydrocarbon is a molecule that contains only hydrogen and carbon atoms. No other elements are present. Hydrocarbons are the simplest type of organic compound.
The two most important families of hydrocarbons for GCSE are:
| Family | General Formula | Bonding | Saturation |
|---|---|---|---|
| Alkanes | CnH2n+2 | Single C–C bonds only | Saturated |
| Alkenes | CnH2n | Contains at least one C=C double bond | Unsaturated |
Alkanes are hydrocarbons with the general formula CnH2n+2 where n is the number of carbon atoms. They contain only single covalent bonds between carbon atoms, which is why they are described as saturated — every carbon atom forms bonds with the maximum possible number of hydrogen atoms.
The first four alkanes are:
| Name | Molecular Formula | Number of Carbons |
|---|---|---|
| Methane | CH4 | 1 |
| Ethane | C2H6 | 2 |
| Propane | C3H8 | 3 |
| Butane | C4H10 | 4 |
Exam Tip: You must be able to recognise, name, and draw the displayed formulae of the first four alkanes. Use the mnemonic Monkeys Eat Peanut Butter to remember methane, ethane, propane, butane in order.
There are several ways to represent the structure of a hydrocarbon molecule:
| Representation | Description | Example (Methane) |
|---|---|---|
| Molecular formula | Shows the number and type of each atom | CH4 |
| Displayed formula | Shows all bonds between all atoms | H–C–H with two additional H atoms bonded to C |
| Structural formula | Shows how atoms are grouped | CH4 |
For larger molecules the displayed formula becomes more important, as it shows the arrangement of atoms and bonds clearly.
All bonds in hydrocarbons are covalent bonds — atoms share pairs of electrons to achieve a full outer shell of electrons.
graph TD
A["Hydrocarbons"] --> B["Alkanes"]
A --> C["Alkenes"]
B --> D["Single bonds only"]
B --> E["Saturated"]
B --> F["General formula: CnH2n+2"]
C --> G["Contains C=C double bond"]
C --> H["Unsaturated"]
C --> I["General formula: CnH2n"]
style A fill:#2c3e50,color:#fff
style B fill:#2980b9,color:#fff
style C fill:#27ae60,color:#fff
Crude oil is a mixture, not a single compound. This is an important distinction:
| Property | Mixture | Compound |
|---|---|---|
| Composition | Two or more substances not chemically bonded | Two or more elements chemically bonded in fixed proportions |
| Separation | Can be separated by physical methods | Can only be separated by chemical reactions |
| Properties | Properties of individual substances are retained | Properties differ from the constituent elements |
| Ratio | Components can be present in any ratio | Elements are present in a fixed ratio |
Because crude oil is a mixture, its hydrocarbon components can be separated by physical methods — specifically, fractional distillation. The hydrocarbons in crude oil have different chain lengths and different boiling points, which is what makes separation possible.
Exam Tip: If asked why crude oil can be separated by fractional distillation, say: "Crude oil is a mixture of hydrocarbons with different boiling points, so they can be separated by a physical process." Do not say it is a compound — this is a common error.
A homologous series is a family of compounds that share the same general formula, have similar chemical properties, and show a gradual trend in physical properties (such as boiling point) as the chain length increases.
The alkanes form a homologous series. Each member differs from the next by a CH2 unit.
| Member | Formula | Boiling Point (°C) |
|---|---|---|
| Methane | CH4 | -162 |
| Ethane | C2H6 | -89 |
| Propane | C3H8 | -42 |
| Butane | C4H10 | -1 |
| Pentane | C5H12 | 36 |
| Hexane | C6H14 | 69 |
As the number of carbon atoms increases, the boiling point increases because the molecules are larger and have stronger intermolecular forces (London dispersion forces) between them.
Crude oil is one of the most economically important natural resources. It provides:
However, crude oil is a finite resource, meaning supplies will eventually run out. This makes it essential to develop alternative fuels and to use crude oil fractions efficiently.
The extraction and use of crude oil has significant environmental impacts:
These issues are explored further in the lesson on combustion of hydrocarbons.
Exam Tip: When writing about crude oil, always use precise language: it is a "finite resource," a "mixture of hydrocarbons," and it is separated by "fractional distillation." These key phrases will pick up marks in extended-response questions.