# Crude Oil, Hydrocarbons and Alkanes
Crude oil is one of the most important natural resources on Earth. It fuels our cars, heats our homes, and provides the raw materials for thousands of products — from plastics to medicines. In this guide, we'll explore what crude oil is, how it's separated, and the chemistry of alkanes.
1. What Is Crude Oil?
Crude oil is a finite (non-renewable) resource found trapped in rocks underground. It is a complex mixture of hydrocarbons — molecules containing only hydrogen and carbon atoms.
Crude oil was formed over millions of years from the remains of ancient marine organisms buried under layers of sediment, subjected to high pressure and temperature.
2. Hydrocarbons
A hydrocarbon is a compound containing only carbon and hydrogen atoms.
Alkanes
The simplest hydrocarbons are alkanes. They are a homologous series — a family of compounds with:
- The same general formula:
- Similar chemical properties
- A gradual trend in physical properties
- Each member differs by from the next
| Name | Formula | Structure | State at RT |
|---|---|---|---|
| Methane | C bonded to 4 H | Gas | |
| Ethane | C−C chain, 6 H | Gas | |
| Propane | C−C−C chain | Gas | |
| Butane | 4-C chain | Gas | |
| Pentane | 5-C chain | Liquid | |
| Octane | 8-C chain | Liquid |
Alkanes contain only single covalent bonds between carbon atoms. They are saturated hydrocarbons.
3. Fractional Distillation
Crude oil is separated into useful fractions by fractional distillation.
How It Works
- Crude oil is heated in a furnace until most of it vaporises
- The vapour enters a fractionating column — a tall tower that is hot at the bottom and cool at the top
- Vapours rise up the column and condense at different heights depending on their boiling point
- Hydrocarbons with high boiling points condense near the bottom (long chain molecules)
- Hydrocarbons with low boiling points rise to the top before condensing (short chain molecules)
The Fractions
| Fraction | Approx. C atoms | Boiling Point | Uses |
|---|---|---|---|
| Gases (LPG) | C₁ – C₄ | < 25°C | Heating, cooking fuel |
| Petrol (gasoline) | C₅ – C₈ | 25–75°C | Car fuel |
| Naphtha | C₈ – C₁₂ | 75–180°C | Chemical feedstock |
| Kerosene | C₁₂ – C₁₅ | 180–250°C | Jet fuel |
| Diesel | C₁₅ – C₂₀ | 250–350°C | Lorry/bus fuel, trains |
| Fuel oil | C₂₀ – C₃₅ | 350–500°C | Ships, power stations |
| Bitumen | C₃₅+ | > 500°C | Roads, roofing |
4. Properties and Chain Length
As the chain length increases (more carbon atoms):
| Property | Trend | Reason |
|---|---|---|
| Boiling point | Increases | Longer chains have stronger intermolecular forces |
| Viscosity | Increases | Longer chains tangle more, resist flow |
| Flammability | Decreases | Harder to ignite |
| Colour | Darker |
Short-chain hydrocarbons are more useful as fuels because they ignite easily, flow well, and burn with a cleaner flame.
5. Combustion of Hydrocarbons
Complete Combustion
When hydrocarbons burn in plenty of oxygen, they undergo complete combustion:
Complete combustion produces a blue flame and releases lots of energy.
Incomplete Combustion
When there is insufficient oxygen, incomplete combustion occurs:
Products include:
- Carbon monoxide (CO) — a toxic, colourless, odourless gas that binds to haemoglobin
- Soot (carbon particles) — causes breathing problems, global dimming
- Water
- Less energy released
Pollutants from Burning Fuels
| Pollutant | Source | Problem |
|---|---|---|
| CO₂ | Complete combustion | Greenhouse gas → climate change |
| CO | Incomplete combustion | Toxic — prevents O₂ transport in blood |
| Soot (C) | Incomplete combustion | Respiratory problems, global dimming |
| SO₂ | Sulfur impurities in fuel | Acid rain → damages buildings, lakes, forests |
| NO_x | N₂ + O₂ at high engine temperatures | Acid rain, photochemical smog, respiratory problems |
| Particulates | Incomplete combustion | Lung disease, global dimming |
Worked Example: General Formula
Question: An alkane has 6 carbon atoms. What is its molecular formula?
where : (This is hexane.)
Worked Example: Balanced Combustion Equation
Question: Write a balanced equation for the complete combustion of propane ().
Worked Example: Fractional Distillation
Question: Explain why petrol is collected higher up the fractionating column than diesel.
Petrol has shorter hydrocarbon chains than diesel, so it has a lower boiling point. It remains as a vapour for longer and rises higher up the column before condensing.
7. Practice Questions
- What is a hydrocarbon? What is the general formula of alkanes?
- Name the first four alkanes and give their molecular formulae.
- Explain why fractional distillation can separate crude oil into fractions.
- Describe the trend in viscosity as chain length increases. Explain why.
- Write balanced equations for the complete combustion of: (a) methane, (b) ethane ().
Want to check your answers and get step-by-step solutions?
8. Common Misconceptions
| Misconception | Reality |
|---|---|
| Crude oil is a single substance | It's a mixture of many hydrocarbons |
| Fractional distillation is a chemical reaction | It's a physical separation technique |
| Burning always gives CO₂ | Only complete combustion gives CO₂; incomplete gives CO and/or C |
| CO₂ is toxic | CO₂ is not toxic (we breathe it out). CO (carbon monoxide) is the toxic gas |
9. Exam Tips
- Learn the order of fractions and their uses
- Know the difference between complete and incomplete combustion
- For combustion equations, balance C first, then H, then O last
- Remember: crude oil is finite (non-renewable)
- When describing fractional distillation, mention the temperature gradient in the column
Summary
- Crude oil is a mixture of hydrocarbons separated by fractional distillation
- Alkanes: general formula , saturated hydrocarbons
- As chain length increases: boiling point ↑, viscosity ↑, flammability ↓
- Complete combustion → CO₂ + H₂O
- Incomplete combustion → CO + C (soot) + H₂O
- Burning fuels produces pollutants: CO₂, CO, SO₂, NOₓ, particulates