C23: Hydrocarbons and Crude Oil
Crude oil as a finite resource, fractional distillation of crude oil, the alkane homologous series, and how properties of hydrocarbons change with chain length.
Crude Oil
Crude oil is a finite resource found in rocks. It was formed over millions of years from the remains of ancient marine organisms (plankton) that were buried under sediment and subjected to high pressure and temperature in the absence of oxygen.
Crude oil is a mixture of many different hydrocarbons. Most of the hydrocarbons in crude oil are alkanes.
A hydrocarbon is a compound of hydrogen and carbon atoms only.
Because crude oil is a mixture, it can be separated by fractional distillation into fractions with different boiling points. Each fraction contains hydrocarbons with a similar number of carbon atoms.
Crude oil is a finite (non-renewable) resource — it is being used up faster than it is being formed. This is why alternative energy sources and feedstocks are important.
Fractional Distillation of Crude Oil
In fractional distillation, crude oil is heated to vaporise it. The vapour enters a fractionating column which is hot at the bottom and cooler at the top. Fractions condense at different heights depending on their boiling points.
How the fractionating column works:
- Crude oil is heated to about 350 °C to vaporise it
- The vapour enters the fractionating column
- Hydrocarbons with high boiling points condense near the bottom where it is hot
- Hydrocarbons with low boiling points condense near the top where it is cooler
- Very short-chain hydrocarbons do not condense at all and are collected as gases at the top
| Fraction | Number of C atoms | Boiling point range | Uses |
|---|---|---|---|
| Petroleum gas | 1–4 | Below 25 °C | LPG, heating, camping gas |
| Petrol (gasoline) | 5–8 | 25–75 °C | Car fuel |
| Naphtha | 7–14 | 75–150 °C | Chemical feedstock |
| Kerosene (paraffin) | 11–18 | 150–250 °C | Aircraft fuel, heating |
| Diesel oil | 15–20 | 250–350 °C | Lorry, bus, train fuel |
| Fuel oil | 20–30 | 350–450 °C | Ship fuel, power stations |
| Lubricating oil | 30–40 | Above 450 °C | Lubricants, waxes |
| Bitumen | 40+ | Residue (does not vaporise) | Road surfacing, roofing |
Worked Example: Identifying fractions
Question: A hydrocarbon with 6 carbon atoms is distilled from crude oil. Which fraction does it belong to?
Answer: A hydrocarbon with 6 carbon atoms falls in the 5–8 range, so it belongs to the petrol (gasoline) fraction.
Alkanes
Alkanes are the simplest homologous series of hydrocarbons. They contain only single carbon-carbon bonds (C−C) and are described as saturated hydrocarbons.
The general formula for alkanes is:
CₙH₂ₙ₊₂
The first four members of the alkane homologous series:
| Name | Molecular formula | Displayed formula description |
|---|---|---|
| Methane | CH₄ | One carbon with four hydrogens |
| Ethane | C₂H₆ | Two carbons joined, each with three hydrogens |
| Propane | C₃H₈ | Three carbons in a chain, end carbons with three H each, middle with two H |
| Butane | C₄H₁₀ | Four carbons in a chain |
A homologous series is a family of compounds with the same functional group and similar chemical properties, where each member differs from the next by a CH₂ group.
Worked Example: Using the alkane general formula
Question: Write the molecular formula for the alkane with 8 carbon atoms.
Using CₙH₂ₙ₊₂ with n = 8:
C₈H(2×8+2) = C₈H₁₈
The alkane is octane (C₈H₁₈).
Worked Example: Checking if a formula is an alkane
Question: Is C₆H₁₄ an alkane?
Check against CₙH₂ₙ₊₂: n = 6, so 2(6) + 2 = 14. Yes, C₆H₁₄ is an alkane (hexane).
Question: Is C₅H₁₂ an alkane?
n = 5, so 2(5) + 2 = 12. Yes, C₅H₁₂ is an alkane (pentane).
Properties of Hydrocarbons and Chain Length
As the chain length of hydrocarbons increases (more carbon atoms):
- Boiling point increases — longer chains have stronger intermolecular forces, needing more energy to overcome
- Viscosity increases — longer chains are thicker and flow less easily
- Flammability decreases — longer chains are harder to ignite and burn less completely
- Volatility decreases — longer chains evaporate less easily
| Property | Short-chain hydrocarbons | Long-chain hydrocarbons |
|---|---|---|
| Boiling point | Low | High |
| Viscosity | Low (runny) | High (thick, sticky) |
| Flammability | High (easy to ignite) | Low (harder to ignite) |
| Volatility | High (evaporates easily) | Low (does not evaporate easily) |
| Intermolecular forces | Weaker | Stronger |
Worked Example: Comparing hydrocarbon properties
Question: Explain why pentane (C₅H₁₂) has a lower boiling point than decane (C₁₀H₂₂).
Answer: Pentane has a shorter carbon chain than decane. Shorter chains have weaker intermolecular forces between molecules. Less energy is needed to overcome these weaker forces, so pentane has a lower boiling point.
Worked Example: Predicting properties
Question: Which hydrocarbon is more viscous: C₃H₈ or C₁₆H₃₄?
Answer: C₁₆H₃₄ (hexadecane) has a much longer carbon chain, so it has stronger intermolecular forces and is more viscous. C₃H₈ (propane) is a gas at room temperature and has very low viscosity.
Combustion of Hydrocarbons
During complete combustion of a hydrocarbon, the carbon and hydrogen are oxidised to produce carbon dioxide and water. This releases a large amount of energy.
Hydrocarbon + oxygen → carbon dioxide + water
Worked Example: Balancing combustion equations
Complete combustion of methane:
CH₄ + 2O₂ → CO₂ + 2H₂O
Complete combustion of propane:
C₃H₈ + 5O₂ → 3CO₂ + 4H₂O
Complete combustion of ethane:
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
If there is insufficient oxygen, incomplete combustion occurs. This produces carbon monoxide (a toxic gas) and/or carbon (soot), as well as water.
Separating Mixtures — Key Points
Fractional distillation works because different hydrocarbons have different boiling points. The boiling point depends on the size of the molecule and the strength of the intermolecular forces.
Important terminology:
- Feedstock — raw material used in an industrial process (e.g. naphtha as a feedstock for making chemicals)
- Fuel — a substance that releases energy when burned
- Saturated — a hydrocarbon with only single carbon-carbon bonds
When explaining trends in hydrocarbon properties, always mention intermolecular forces. Longer chains have stronger intermolecular forces because there is more surface contact between adjacent molecules, leading to stronger London dispersion forces.
Practice Questions
1. Crude oil is a mixture of hydrocarbons. Explain what is meant by the term hydrocarbon. (2 marks)
A hydrocarbon is a compound that contains hydrogen and carbon atoms only.
2. Describe how fractional distillation separates crude oil into fractions. (4 marks)
Crude oil is heated and vaporised. The vapour enters a fractionating column which is hot at the bottom and cooler at the top. Fractions with high boiling points (longer chains) condense near the bottom. Fractions with low boiling points (shorter chains) condense near the top. Each fraction is collected at its condensation point.
3. Write the molecular formula for the alkane with 10 carbon atoms. (1 mark)
Using CₙH₂ₙ₊₂ with n = 10: C₁₀H₂₂ (decane)
4. Explain why the boiling point of octane (C₈H₁₈) is higher than that of butane (C₄H₁₀). (3 marks)
Octane has a longer carbon chain than butane. Longer chains have stronger intermolecular forces between molecules. More energy is needed to overcome these stronger forces, so octane has a higher boiling point.
5. Write a balanced equation for the complete combustion of ethane (C₂H₆). (2 marks)
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O
Maths Skills
Interpreting Boiling Point vs Chain Length Data
As the chain length of alkanes increases, the boiling point increases. This is because longer molecules have stronger intermolecular forces (London forces) that require more energy to overcome.
Interpreting data tables: When given a table of alkane names, formulae and boiling points, you should be able to:
- Identify the trend — boiling point increases with chain length.
- State that the increase is not linear — the boiling point rises more steeply for the first few members and levels off for longer chains.
- Explain the trend using intermolecular forces.
Example: Methane (CH₄) boils at −162 °C, ethane (C₂H₆) at −89 °C, propane (C₃H₈) at −42 °C, butane (C₄H₁₀) at −1 °C. The difference between methane and ethane is 73 °C, but the difference between propane and butane is only 41 °C.
Using graphs: Plot boiling point (y-axis) against number of carbon atoms (x-axis). The curve rises steeply at first then gradually levels off.
Common Misconceptions
Misconceptions About Crude Oil
Crude oil is made of one compound.
Crude oil is a mixture of many different hydrocarbons (mainly alkanes) with different chain lengths and boiling points. This is why it can be separated by fractional distillation — each fraction contains molecules with similar boiling points.
Fractional distillation separates crude oil into individual pure compounds.
Each fraction is still a mixture — it contains hydrocarbons with a similar range of boiling points, not a single pure compound. For example, the petrol fraction contains many different alkanes with 5–8 carbon atoms.
Shorter-chain alkanes have stronger intermolecular forces.
Shorter-chain alkanes have weaker intermolecular forces (fewer points of contact between molecules), which is why they have lower boiling points and are more volatile.
6-Mark Question
Explain how crude oil is separated into fractions by fractional distillation.
Crude oil is first heated in a furnace to vaporise it (turn it into a gas). The hot vapour enters a fractionating column, which is cooler at the top and hotter at the bottom. The vapour rises up the column through trays with bubble caps. As the vapour rises, it cools. When a hydrocarbon vapour reaches a height where the temperature is below its boiling point, it condenses back into a liquid. Fractions with low boiling points (short-chain hydrocarbons like refinery gases and petrol) condense near the top of the column where it is coolest. Fractions with high boiling points (long-chain hydrocarbons like diesel, lubricating oil and bitumen) condense near the bottom where it is hotter. The very longest chains (bitumen) do not vaporise and are collected at the base as a residue. Each fraction is collected at a different level through outlet pipes. The separation works because different hydrocarbons have different boiling points due to differences in intermolecular forces.
AO3: Analysis and Evaluation
Evaluating Uses of Crude Oil Fractions
The table below shows the properties of different crude oil fractions:
- Refinery gases (C₁–C₄): very low boiling point, highly flammable, viscous liquid? No — gas at room temperature.
- Petrol (C₅–C₁₀): low boiling point, fairly volatile, flows easily.
- Diesel (C₁₅–C₂₀): higher boiling point, less volatile, more viscous.
- Bitumen (C₃₀+): very high boiling point, not volatile, very viscous, solid-ish at room temperature.
Evaluate why petrol is more suitable as a fuel for cars than diesel or bitumen, and explain why bitumen is used for road surfaces rather than as a fuel.
Answer: Petrol is more volatile and has a lower boiling point, meaning it vaporises easily to mix with air in an engine for efficient combustion. It also flows easily through fuel pipes. Bitumen is too viscous and has too high a boiling point to vaporise — it would not combust efficiently in an engine and would be difficult to pump. However, its sticky, viscous nature and high boiling point make it ideal as a waterproofing and binding agent for road surfaces.
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