Fuels and Petroleum
Summary: Petroleum (crude oil) is a mixture of hydrocarbons separated by fractional distillation into fractions (refinery gas, petrol, naphtha, kerosene, diesel, fuel oil, bitumen) based on boiling points. Burning fuels produces CO2, SO2, CO, and particulates. Catalytic converters reduce harmful emissions. Tags: igcse chemistry organic-chem Created: 2026-07-14 Last Updated: 2026-07-16
Learning Objectives
By the end of this topic, you should be able to:
- Describe petroleum as a mixture of hydrocarbons (not a compound)
- Explain how fractional distillation separates crude oil into useful fractions
- Name the main fractions, their boiling point ranges, and their uses
- Explain why fractional distillation works (different chain lengths, different boiling points, temperature gradient in column)
- Describe the environmental issues from burning fuels (CO2, SO2, CO, particulates)
- Explain how a catalytic converter works (2CO + 2NO → 2CO2 + N2)
Content
1. What is Petroleum?
Petroleum (crude oil) is a mixture of many different hydrocarbon compounds — mostly alkanes with different chain lengths.
Key distinction: Petroleum is a mixture, NOT a compound. As a mixture, it has no fixed boiling point — it boils over a range of temperatures. The different hydrocarbons in the mixture retain their individual properties.
Hydrocarbons in crude oil range from:
- Short chains: 1-4 carbons (gases at room temperature)
- Medium chains: 5-15 carbons (liquids)
- Long chains: 16+ carbons (thick liquids / solids)
2. Fractional Distillation — How It Works
Fractional distillation separates crude oil into useful fractions based on differences in boiling points.
The Fractionating Column
| Part of Column | Temperature | What Happens |
|---|---|---|
| Bottom (near furnace) | Hottest (~400°C) | Crude oil vaporised; longest-chain hydrocarbons condense here |
| Middle | Moderate (~150-300°C) | Medium-chain hydrocarbons condense |
| Top | Coolest (~25-40°C) | Shortest-chain hydrocarbons condense here; gases may not condense at all |
Step-by-Step Process
- Crude oil is heated in a furnace until it vaporises (turns to gas)
- The vapour enters at the bottom of the fractionating column
- The column has a temperature gradient — hottest at the bottom, coolest at the top
- Vapour rises through the column
- When a hydrocarbon vapour reaches a height where the temperature is below its boiling point, it condenses to liquid
- The liquid fraction is collected on trays/bubble caps at that level
- The shortest-chain hydrocarbons (lowest boiling points) rise to the top; the longest-chain hydrocarbons (highest boiling points) condense near the bottom
Why Different Fractions Have Different Boiling Points
- Longer-chain hydrocarbons have stronger intermolecular forces (van der Waals forces / London dispersion forces) because there are more points of contact between molecules
- More energy is needed to overcome these forces → higher boiling point
- Shorter-chain hydrocarbons have weaker intermolecular forces → lower boiling point
3. The Fractions — Table
| Fraction | Approx. Carbon Chain Length | Boiling Point Range (°C) | State at Room Temp | Uses |
|---|---|---|---|---|
| Refinery Gas | C1–C4 | Below 40 | Gas | Bottled gas (LPG), cooking, heating |
| Petrol (Gasoline) | C5–C10 | 40–100 | Liquid | Fuel for cars (petrol engines) |
| Naphtha | C7–C11 | 70–180 | Liquid | Feedstock for making chemicals (via cracking/reforming) |
| Kerosene (Paraffin) | C10–C16 | 150–250 | Liquid | Jet fuel (aviation), heating oil |
| Diesel (Gas Oil) | C14–C20 | 220–350 | Liquid | Fuel for diesel engines (lorries, buses, trains) |
| Fuel Oil | C20–C35 | 350–500 | Thick liquid | Ships’ engines, power stations, heating |
| Bitumen (Residue) | C35+ | Above 500 | Solid / semi-solid | Road surfacing, roofing, waterproofing |
Exam tip: The exact boiling point ranges and carbon numbers are approximate and may vary slightly between syllabi. Focus on the ORDER (top = shortest chains, lowest bp; bottom = longest chains, highest bp) and the USES.
4. Fractional Distillation vs Simple Distillation
| Feature | Fractional Distillation | Simple Distillation |
|---|---|---|
| What it separates | Mixture of liquids with different boiling points | A liquid from a dissolved solid, or two liquids with very different boiling points |
| Apparatus | Fractionating column with trays/bubble caps | Simple flask, condenser, receiving flask |
| Example | Separating crude oil fractions | Obtaining pure water from salt water |
5. Fuels from Petroleum
| Fuel | Source | Uses |
|---|---|---|
| Petrol | Fractional distillation; cracking | Cars (petrol/spark-ignition engines) |
| Diesel | Fractional distillation | Lorries, buses, trains, some cars |
| Natural Gas | Often found with petroleum deposits | Cooking, heating, power stations — mainly methane (CH4) |
| Kerosene | Fractional distillation | Aircraft jet engines |
| Fuel Oil | Fractional distillation (residue) | Ships, power stations |
6. Environmental Issues from Burning Fuels
When hydrocarbon fuels burn (combustion), the products depend on the conditions:
Complete Combustion
Hydrocarbon + O2 → CO2 + H2O
Example: CH4 + 2O2 → CO2 + 2H2O
Carbon dioxide (CO2) is a greenhouse gas → contributes to global warming / climate change.
Incomplete Combustion (Limited O2)
Hydrocarbon + O2 → CO + H2O (or C + H2O)
Carbon monoxide (CO) is a toxic gas — it binds to haemoglobin in red blood cells more strongly than oxygen, preventing oxygen transport → asphyxiation.
Carbon particulates (soot) — black smoke, causes respiratory problems, deposits on buildings (blackening).
Sulfur Impurities
Crude oil contains sulfur compounds as impurities. When burned:
S + O2 → SO2
Sulfur + oxygen → sulfur dioxide
Sulfur dioxide (SO2) dissolves in atmospheric water → acid rain (H2SO3 / sulfurous acid, which oxidises to H2SO4 / sulfuric acid).
Effects of acid rain:
- Kills trees and vegetation
- Acidifies lakes and rivers → kills fish
- Corrodes limestone/marble buildings and statues
- Damages metal structures
Nitrogen Oxides (NOx)
At high temperatures in car engines, N2 and O2 from the air react:
N2 + O2 → 2NO (nitrogen monoxide) N2 + 2O2 → 2NO2 (nitrogen dioxide)
NO2 also contributes to acid rain and respiratory problems (asthma, bronchitis).
7. Catalytic Converters
Catalytic converters are fitted to car exhausts to reduce harmful emissions.
Key Equation: 2CO + 2NO → 2CO2 + N2
| Pollutant In | Pollutant Out | Change |
|---|---|---|
| CO (toxic) | CO2 (less toxic) | CO is oxidised |
| NO (toxic, forms acid rain) | N2 (harmless, already in air) | NO is reduced |
| Unburnt hydrocarbons | CO2 + H2O | Hydrocarbons are oxidised |
Catalyst: Platinum (Pt), palladium (Pd), and rhodium (Rh) on a honeycomb ceramic structure (to maximise surface area).
Conditions: The catalyst works best when the engine is hot — catalytic converters are less effective during short journeys (cold starts).
Worked Examples (Exam-Style)
Example 1: Fractional Distillation
Question: Explain why fractional distillation can be used to separate crude oil into different fractions. [3 marks]
Solution (model answer):
- Crude oil is a mixture of hydrocarbons with different chain lengths / boiling points ✓
- The fractionating column has a temperature gradient — hottest at the bottom, coolest at the top ✓
- Hydrocarbons condense at different heights when the temperature falls below their boiling point ✓
Example 2: Environmental Issues
Question: Explain how burning fuels containing sulfur impurities can damage the environment. [3 marks]
Solution (model answer):
- Sulfur burns to form sulfur dioxide: S + O2 → SO2 ✓
- SO2 dissolves in atmospheric water to form acid rain ✓
- Acid rain kills trees, acidifies lakes (kills fish), and corrodes limestone buildings ✓
Example 3: Catalytic Converter
Question: Write a balanced equation for a reaction that occurs in a catalytic converter. [1 mark]
Solution: 2CO + 2NO → 2CO2 + N2 ✓
Practice Questions
-
Explain why petroleum is described as a mixture and not a compound. [2 marks]
-
State the name and one use of: (a) the fraction with the lowest boiling point range; (b) the fraction used as jet fuel. [4 marks]
-
Explain why hydrocarbons with longer carbon chains have higher boiling points. [2 marks]
-
Carbon monoxide is produced during the incomplete combustion of petrol. State why carbon monoxide is dangerous. [2 marks]
-
Write the equation for a reaction that occurs inside a catalytic converter. [1 mark]
-
Describe two effects of acid rain on the environment. [2 marks]
Key Facts to Memorise
- Petroleum is a mixture of hydrocarbons; NOT a compound
- Separated by fractional distillation — temperature gradient in the column
- Fractions (top → bottom): Refinery gas → Petrol → Naphtha → Kerosene → Diesel → Fuel oil → Bitumen
- Longer-chain = higher bp (stronger intermolecular forces); shorter-chain = lower bp
- Complete combustion: hydrocarbon + O2 → CO2 + H2O
- Incomplete combustion: produces CO (toxic) and C particulates (soot)
- Sulfur impurities → SO2 → acid rain
- Catalytic converter equation: 2CO + 2NO → 2CO2 + N2
- Catalytic converter catalyst: platinum, palladium, rhodium
Common Misconceptions
| Students often think… | But the correct understanding is… |
|---|---|
| ”Petroleum is a compound because it has a formula” | Petroleum is a MIXTURE — it has no fixed formula, no fixed boiling point, and its composition varies depending on its source |
| ”Fractions are pure substances” | Each fraction is still a MIXTURE of hydrocarbons within a boiling point range, not a pure compound |
| ”Longer chain hydrocarbons have stronger covalent bonds so higher boiling points” | Boiling point depends on intermolecular forces, NOT covalent bond strength. Covalent C-C and C-H bonds are not broken during boiling — only intermolecular forces are overcome |
| ”Catalytic converters remove CO2 from exhaust” | Catalytic converters actually PRODUCE CO2 (from CO). The problem of CO2 as a greenhouse gas remains |
| ”Burning fuels always produces CO2 and H2O” | Only if combustion is COMPLETE (excess O2). Incomplete combustion (limited O2) produces CO and/or C instead |
Key Concepts from Past Papers
Definitions You MUST Know (Exact Mark Scheme Wording)
- Petroleum (crude oil): a mixture of hydrocarbons
- Fraction: a mixture of hydrocarbons with similar boiling points
- Fractional distillation: a process that separates a mixture of liquids with different boiling points using a fractionating column
- Catalytic converter: a device in car exhausts that uses catalysts (Pt, Pd, Rh) to convert harmful gases (CO, NO) into less harmful gases (CO2, N2)
Recurring Mark Scheme Answers
- “Crude oil is heated/vaporised before entering the fractionating column”
- “The column is hotter at the bottom and cooler at the top (temperature gradient)”
- “Hydrocarbons condense when the temperature falls below their boiling point”
- “Longer-chain hydrocarbons have higher boiling points because intermolecular forces are stronger”
- “Sulfur in fuel + oxygen → sulfur dioxide → dissolves in water → acid rain”
- (property on which fractional distillation) depends is the boiling point AND 1 mark each for any two of:
- idea of (fractional distillation) column
- petroleum heated / petroleum vaporised
- petroleum vapour enters at bottom of fractionating column
- temperatures high at bottom of column and low at the top
- different fractions / compounds move different distances
- vapour turns to liquid in condenser / vapour turns to liquid when it reaches a certain height
- with methane (aqueous) bromine remains orange with ethene (aqueous) bromine decolourised
Common Question Types
- Short recall (state/give/name): “State the use of the kerosene fraction”
- Extended writing (describe/explain/suggest): “Describe how crude oil is separated by fractional distillation”
- Environmental explanation: “Explain how acid rain is formed from the burning of fossil fuels”
- Equation writing: “Write the equation for the reaction in a catalytic converter”
- Comparison: “Compare the boiling points and uses of petrol and diesel fractions”
Exam Tips
- This topic appears in 29 papers in the database
- Total Q+A entries: 68
- Average marks per question: 1.1
- The fractional distillation diagram (empty column to label) is a very common exam task — practise drawing and labelling it
- For environmental questions, you need the FULL sequence: fuel burned → gas produced → environmental effect. Don’t jump straight to the effect
- The catalytic converter equation (2CO + 2NO → 2CO2 + N2) is one of the most tested equations in the entire syllabus — learn it exactly
- When explaining boiling points, always mention “intermolecular forces” (not “covalent bonds”)
Keywords from Past Papers
methane, ethene, fractions, point, make, bonds, carbon, demand, smaller, boiling, idea, column, petroleum, vapour, alkenes
Related Notes
- Organic Chemistry Fundamentals — Homologous series, alkanes, alkenes
- Alkanes — Saturated hydrocarbons (main components of petroleum)
- Alkenes — Products of cracking
- Polymers — Addition polymers from alkene monomers (from cracking)
- Climate Change and Greenhouse Gases — CO2 from burning fuels
- Air and Combustion — Complete vs incomplete combustion
- Atoms, Elements and Compounds — Difference between mixtures and compounds
- IGCSE-Chem-Index — Full IGCSE Chemistry index
Sources
- OpenStax Chemistry 2e — [Chapter 20: Organic Chemistry (Hydrocarbons and Fuels)], Rice University (free, CC BY 4.0)
- BBC Bitesize GCSE Chemistry — [Fuels and Fractional Distillation], BBC (free educational resource)
- Cambridge IGCSE Chemistry 0620 — Syllabus Section 11: Organic Chemistry (Fuels and Petroleum), Cambridge Assessment International Education
- CK-12 Chemistry for High School — [Chapter 25: Fossil Fuels], CK-12 Foundation (free, CC BY-NC 3.0)
Past Paper Sources
- 0620/32 Feb/March 2017: Q33(b)(iv) (1m), Q33(c)(iii) (1m)
- 0620/32 Feb/March 2018: Q66(a)(i) (1m)
- 0620/32 Feb/March 2019: Q55(d)(iii) (1m)
- 0620/32 Feb/March 2020: Q11(a)(ii) (1m), Q11(a)(v) (1m)
- 0620/32 Feb/March 2021: Q66(c)(i) (1m)
- 0620/32 Feb/March 2022: Q22(a)(ii) (2m), Q77(b)(i) (1m)
- 0620/32 Feb/March 2023: Q66(a)(iv) (2m)
- 0620/32 May/June 2018: Q11(a)(v) (1m), Q33(a)(iv) (1m), Q33(c)(ii) (0m)
- 0620/33 May/June 2016: Q44(c)(iv) (1m), Q66(e)(i) (1m)
- 0620/33 May/June 2017: Q44(a)(ii) (1m)
- 0620/33 May/June 2019: Q11(a)(i) (2m), Q11(a)(iv) (1m), Q11(a)(vi) (1m) (+3 more)
- 0620/33 May/June 2021: Q44(a)(iii) (1m), Q44(d)(i) (1m), Q44(d)(ii) (1m) (+1 more)