C19: Hydrocarbons and Crude Oil
Crude oil, fractional distillation, alkanes, alkenes and cracking
📋 Key Concepts
Crude oil is a mixture of hydrocarbons that formed from the remains of ancient biomass (mostly plankton) over millions of years under high pressure and temperature. It is a finite resource — it cannot be replaced once used.
- Crude oil is found in porous rocks beneath the Earth's surface.
- It is a mixture, not a pure substance — it contains many different hydrocarbons with different chain lengths.
- Because it is finite, we must use it carefully and find alternative resources.
📝 Fractional Distillation
Fractional distillation separates crude oil into fractions (groups of hydrocarbons with similar boiling points). The crude oil is heated and vapours rise up a fractionating column. Fractions condense at different heights depending on their boiling points.
- The column is hottest at the bottom and coolest at the top.
- Short-chain hydrocarbons have low boiling points — they condense near the top.
- Long-chain hydrocarbons have high boiling points — they condense near the bottom.
| Fraction | Chain Length | Boiling Point Range (°C) | Uses |
|---|---|---|---|
| Petroleum gases | C₁–C₄ | Below 40 | Heating, cooking (LPG) |
| Petrol (gasoline) | C₅–C₁₀ | 40–150 | Fuel for cars |
| Kerosene | C₁₀–C₁₆ | 150–250 | Aircraft fuel |
| Diesel oil | C₁₄–C₂₀ | 250–350 | Fuel for lorries, trains |
| Fuel oil | C₂₀–C₃₀ | 350–500 | Ship engines, power stations |
| Bitumen | C₃₀+ | Over 500 | Road surfacing, roofing |
Example 1
Explain how fractional distillation separates the fractions in crude oil.
Solution: Crude oil is heated and vaporised. The vapours enter a fractionating column which is hot at the bottom and cool at the top. As the vapours rise, they cool down. Each fraction condenses at a different height because hydrocarbons have different boiling points. Long-chain hydrocarbons have high boiling points so they condense near the bottom (e.g. bitumen). Short-chain hydrocarbons have low boiling points so they condense near the top (e.g. petroleum gases).
📝 Hydrocarbons: Alkanes and Alkenes
A hydrocarbon is a compound of carbon and hydrogen only. No other elements are present.
Alkanes are saturated hydrocarbons — they contain only single carbon–carbon bonds. The general formula for alkanes is CₙH₂ₙ₊₂.
Alkane general formula: CₙH₂ₙ₊₂
| Alkane | Molecular Formula | n Value | Check |
|---|---|---|---|
| Methane | CH₄ | n = 1 | 2(1) + 2 = 4 H |
| Ethane | C₂H₆ | n = 2 | 2(2) + 2 = 6 H |
| Propane | C₃H₈ | n = 3 | 2(3) + 2 = 8 H |
| Butane | C₄H₁₀ | n = 4 | 2(4) + 2 = 10 H |
Alkenes are unsaturated hydrocarbons — they contain a C=C double bond. The general formula for alkenes is CₙH₂ₙ. Alkenes are more reactive than alkanes because of the double bond.
Alkene general formula: CₙH₂ₙ
| Property | Alkanes | Alkenes |
|---|---|---|
| Bond type | C—C single bonds only (saturated) | Contains C=C double bond (unsaturated) |
| General formula | CₙH₂ₙ₊₂ | CₙH₂ₙ |
| Reactivity | Less reactive | More reactive |
| Bromine water test | No change (stays orange) | Decolourises (orange → colourless) |
Bromine water test for unsaturation: Add bromine water (orange) to the hydrocarbon. If it decolourises (goes colourless), the substance is an alkene (unsaturated). If it stays orange, it is an alkane (saturated).
Example 2
A hydrocarbon has the formula C₃H₆. Is it an alkane or an alkene? How would bromine water confirm this?
Solution: For an alkane with n = 3, the formula would be C₃H₈ (CₙH₂ₙ₊₂). C₃H₆ matches CₙH₂ₙ (with n = 3), so it is an alkene (propene). Bromine water would be decolourised from orange to colourless because the C=C double bond opens and reacts with bromine.
📝 Properties of Hydrocarbon Fractions
The properties of a hydrocarbon fraction depend on the length of the carbon chain.
| Property | Shorter Chains | Longer Chains |
|---|---|---|
| Boiling point | Lower | Higher |
| Viscosity (how runny) | More runny (less viscous) | Less runny (more viscous) |
| Flammability | More flammable | Less flammable |
| Volatility (how easily evaporates) | More volatile | Less volatile |
Example 3
Petrol is a shorter-chain fraction than diesel. Compare their viscosity, flammability and boiling point.
Solution: Petrol has shorter chains so it is less viscous (more runny), more flammable, and has a lower boiling point than diesel. Diesel has longer chains so it is more viscous (thicker), less flammable, and has a higher boiling point.
📝 Cracking
Cracking breaks long-chain hydrocarbons into shorter, more useful molecules. It always produces at least one alkene (which has a C=C double bond) and an alkane.
- Thermal cracking — high temperature (up to 750°C) and high pressure.
- Catalytic cracking — moderate temperature (~500°C) with a catalyst (zeolite).
- Cracking is needed because there is high demand for short-chain fractions (e.g. petrol) but supply from fractional distillation is limited.
Example 4
Write a balanced equation for the cracking of C₁₄H₃₀ to produce ethene and one other product.
Solution: C₁₄H₃₀ → C₂H₄ + C₁₂H₂₆. Check: C — 14 = 2 + 12 ✓, H — 30 = 4 + 26 ✓. Ethene (C₂H₄) is an alkene and C₁₂H₂₆ is an alkane (satisfies CₙH₂ₙ₊₂ with n = 12).
❓ Practice Questions
Q1: What is crude oil a mixture of? Explain why crude oil is described as a finite resource.
Q2: Describe how fractional distillation separates crude oil into fractions. Explain why fractions condense at different heights in the column.
Q3: An alkane has 5 carbon atoms. Use the general formula to work out its molecular formula and name it.
Q4: Compare the properties of a short-chain hydrocarbon fraction (e.g. petrol) with a long-chain fraction (e.g. bitumen) in terms of boiling point, viscosity, flammability and volatility.
Q5: Higher Write a balanced symbol equation for the cracking of C₁₀H₂₂ to produce ethene and one other product. State whether each product is an alkane or an alkene.
✅ Answers
- Crude oil is a mixture of hydrocarbons (compounds of carbon and hydrogen only). It is finite because it was formed from ancient biomass over millions of years and cannot be replaced within a human lifetime — once used, supplies are gone.
- Crude oil is heated and vaporised, then the vapours enter a fractionating column. The column is hot at the bottom and cool at the top. As vapours rise they cool. Each fraction condenses at a different height because hydrocarbons have different boiling points — long-chain hydrocarbons have high boiling points and condense near the bottom, while short-chain hydrocarbons have low boiling points and condense near the top.
- Using CₙH₂ₙ₊₂ with n = 5: H = 2(5) + 2 = 12. Molecular formula = C₅H₁₂. The name is pentane.
- Petrol (short-chain) has a lower boiling point, is less viscous (more runny), is more flammable, and is more volatile than bitumen. Bitumen (long-chain) has a higher boiling point, is more viscous (thicker/sticky), is less flammable, and is less volatile.
- C₁₀H₂₂ → C₂H₄ + C₈H₁₈. C₂H₄ (ethene) is an alkene — it has a C=C double bond and fits CₙH₂ₙ. C₈H₁₈ is an alkane — it has only single bonds and fits CₙH₂ₙ₊₂ (n = 8, 2×8+2 = 18).
🎯 Exam Tips
- When asked about fractional distillation, always mention the temperature gradient (hot at bottom, cool at top) and that separation happens because different fractions have different boiling points
- When writing cracking equations, always check your carbon and hydrogen atoms balance on both sides
- Cracking always produces at least one alkene — this is a common exam point
- Bromine water test: decolourises = alkene (unsaturated); stays orange = alkane (saturated)
- Properties trend: longer chains → higher boiling point, more viscous, less flammable, less volatile
🔢 Maths Skills
Mathematical Skills
You need to use the general formulae CₙH₂ₙ₊₂ (alkanes) and CₙH₂ₙ (alkenes) to work out molecular formulae, and balance cracking equations by ensuring the same number of C and H atoms on both sides.
Maths Example
An alkane has 10 carbon atoms. Calculate its molecular formula.
Using CₙH₂ₙ₊₂ with n = 10: H = 2(10) + 2 = 22. Formula = C₁₀H₂₂ (decane).
Maths Example 2
<C₁₆H₃₄ is cracked to produce ethene (C₂H₄) and one alkane product. Calculate the formula of the alkane.
Carbon atoms in product: 16 − 2 = 14. Hydrogen atoms in product: 34 − 4 = 30. Alkane formula = C₁₄H₃₀. Check: 2(14) + 2 = 30 ✓
⚠️ Common Misconceptions
Watch Out!
1. Wrong: Crude oil is a pure substance Correct: Crude oil is a MIXTURE of many different hydrocarbons with different chain lengths — it is not a single compound
2. Wrong: Shorter chains have higher boiling points Correct: Shorter chains have LOWER boiling points — they need less energy to overcome the weaker intermolecular forces
3. Wrong: Cracking always produces two alkenes Correct: Cracking produces at least one alkene and one alkane — the alkene has the C=C double bond, but the other product can be a saturated alkane
4. Wrong: Alkenes are saturated because they have more hydrogens Correct: Alkenes are UNSATURATED because they have a C=C double bond — they actually have FEWER hydrogen atoms than the corresponding alkane (CₙH₂ₙ vs CₙH₂ₙ₊₂)
✍️ 6-Mark Question
Extended Answer
6 marks: Describe how fractional distillation separates crude oil, and explain why cracking is necessary even after fractional distillation.
Fractional distillation separates crude oil by heating it to vaporise the hydrocarbons. The vapours enter a fractionating column which has a temperature gradient — hot at the bottom and cool at the top. As the vapours rise, they cool. Each fraction condenses at a different height because hydrocarbons have different boiling points. Long-chain hydrocarbons have high boiling points and strong intermolecular forces, so they condense near the bottom at higher temperatures (e.g. bitumen). Short-chain hydrocarbons have low boiling points and weaker intermolecular forces, so they rise higher and condense near the top at lower temperatures (e.g. petroleum gases, petrol). However, fractional distillation produces more long-chain fractions than are needed and not enough short-chain fractions like petrol, which are in high demand. Cracking solves this problem by breaking long-chain hydrocarbons (which are in surplus) into shorter, more useful molecules. Cracking always produces at least one alkene (useful for making polymers) and shorter alkanes (useful as fuels).
Mark scheme: 1 mark for heating/vaporising crude oil; 1 mark for temperature gradient in column; 1 mark for different boiling points causing condensation at different heights; 1 mark for explaining supply/demand mismatch; 1 mark for cracking breaking long chains into shorter ones; 1 mark for mentioning alkene production from cracking
📊 AO3: Analyse & Evaluate
Analysis and Evaluation
A country produces crude oil and uses fractional distillation. The demand for different fractions and the supply from distillation are:
- Petrol: demand 40%, supply 15%
- Diesel: demand 30%, supply 25%
- Kerosene: demand 15%, supply 20%
- Fuel oil: demand 5%, supply 25%
- Bitumen: demand 10%, supply 15%
(a) Which fraction has the greatest surplus, and which has the greatest shortage?
(b) Explain how cracking could help balance supply and demand.
(c) A student suggests that cracking fuel oil produces only petrol. Evaluate this claim.
Answers: (a) Fuel oil has the greatest surplus (25% supply vs 5% demand = 20% surplus). Petrol has the greatest shortage (40% demand vs 15% supply = 25% shortage). (b) Cracking could break down surplus long-chain fractions (like fuel oil and bitumen) into shorter-chain fractions (like petrol) that are in high demand. This would reduce the surplus of fuel oil and help meet the demand for petrol. (c) The student is incorrect. Cracking does not produce only petrol — it breaks a long-chain hydrocarbon into a shorter alkane AND an alkene. The products depend on the specific cracking reaction and could include various chain lengths, not just petrol-range molecules. The alkene by-product is also useful for polymer production, not just as fuel.
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