Fuels and Fractional Distillation
ChemistryΒ· 11.3Β· 12 min read
1. Crude Oil: Composition and Global Importanceβ βββββ± 3 min
Crude oil (or petroleum) is a finite, non-renewable fossil fuel formed over millions of years from the remains of tiny marine organisms buried under seabed sediment. It is a complex mixture of hydrocarbons: compounds made only of hydrogen and carbon atoms, mostly from the alkane homologous series.
Crude Oil
A dark, viscous mixture of hydrocarbon compounds extracted from underground rock formations, used as a source of fuels and petrochemical feedstock.
State two reasons crude oil is considered a critical global resource.
- 1
Step 1: First use case: It is processed to make transport fuels (petrol, diesel, kerosene) for cars, aircraft and ships.
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Step 2: Second use case: It provides raw material (feedstock) for the petrochemical industry to make plastics, detergents, solvents and synthetic fibres.
2. The Fractional Distillation Processβ β ββββ± 4 min
Fractional distillation is the physical process that separates crude oil into useful fractions (groups of hydrocarbons with similar boiling points). It takes place in a tall, steel fractionating column that is kept hot at the bottom (~400Β°C) and much cooler at the top.
Fractional Distillation
Physical separation technique that splits crude oil into fractions based on differences in the boiling points of its hydrocarbon components.
First, crude oil is heated to ~400Β°C to vaporise almost all of its components. The hot vapour rises up the column. As it rises, it cools: hydrocarbons with higher boiling points (larger molecules) condense first near the hot bottom of the column, while smaller, lower-boiling point hydrocarbons rise further up before condensing near the cool top. Liquid fractions are tapped off at different heights in the column.
Explain why larger hydrocarbon fractions are collected near the bottom of the fractionating column.
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Step 1: Link molecular size to boiling point: Larger hydrocarbon molecules have stronger intermolecular forces, so they have higher boiling points.
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Step 2: Link boiling point to column position: The bottom of the column is the hottest region, so only high-boiling point (large) molecules condense here, rather than rising further up the column.
3. Fraction Properties and Common Usesβ β ββββ± 3 min
All fractions follow consistent property trends as you move from the bottom to the top of the distillation column: boiling point decreases, viscosity (thickness) decreases, and flammability increases, as molecular size gets smaller.
Fraction | Boiling Point Range (Β°C) | Main Use |
|---|---|---|
Refinery gas | <40 | Bottled fuel for cooking and heating |
Gasoline (petrol) | 40-170 | Fuel for passenger cars |
Kerosene (paraffin) | 170-250 | Aircraft fuel, domestic heating |
Diesel oil | 250-350 | Fuel for lorries, buses and trains |
Fuel oil | 350-600 | Fuel for ships and power stations |
Bitumen |
| Road surfacing, roof waterproofing |
A fraction collected from the upper part of the column flows easily and ignites readily when exposed to a flame. Name this fraction and state its main use.
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Step 1: Identify the fraction: Low viscosity, high flammability and collection high up the column matches gasoline (petrol) or refinery gas.
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Step 2: State its use: Gasoline is used as a fuel for passenger cars.
4. Combustion of Hydrocarbon Fuelsβ β β βββ± 2 min
Hydrocarbon fuels release useful energy when burned (combusted) in oxygen. Two types of combustion can occur, depending on the volume of oxygen available.
Complete Combustion
Combustion that occurs when there is a plentiful supply of oxygen, producing only carbon dioxide and water as waste products, and releasing the maximum possible energy from the fuel.
Incomplete combustion occurs when oxygen supply is limited. It releases far less energy than complete combustion, and produces harmful waste products including toxic carbon monoxide gas, solid carbon (soot) and water.
A faulty gas heater produces yellow flames and leaves black soot deposits on nearby walls. Name the type of combustion occurring and state one associated health risk.
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Step 1: Identify combustion type: Yellow flames and soot are signs of incomplete combustion, caused by limited oxygen supply to the heater.
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Step 2: State health risk: Incomplete combustion produces toxic carbon monoxide gas, which causes fatal poisoning if inhaled in unventilated spaces.
5. Common Pitfalls
Wrong move:
Stating fractional distillation involves chemical reactions to break down hydrocarbons
Why:
Fractional distillation is a physical separation process, no chemical bonds are broken, only intermolecular forces are overcome during boiling
Correct move:
Always refer to fractional distillation as a physical separation based on boiling point differences
Wrong move:
Claiming high boiling point fractions like bitumen are collected at the top of the column
Why:
Bitumen has very large molecules with very high boiling points, so it condenses first at the hot bottom of the column
Correct move:
Remember the trend: smaller, lower boiling point fractions are collected higher up the column, larger fractions at the bottom
Wrong move:
Stating incomplete combustion only produces carbon dioxide and water
Why:
Incomplete combustion occurs with limited oxygen, so it produces carbon monoxide, soot and water, not just carbon dioxide
Correct move:
Associate complete combustion with COβ + HβO, incomplete combustion with CO/soot + HβO
Wrong move:
Describing crude oil as a pure substance made of one type of hydrocarbon
Why:
Crude oil is a mixture of hundreds of different hydrocarbon compounds, which is why separation is required
Correct move:
Always describe crude oil as a complex mixture of hydrocarbons
Wrong move:
Confusing property trends, e.g. claiming larger hydrocarbon molecules are more flammable
Why:
Larger hydrocarbon molecules are less flammable, more viscous, and have higher boiling points than smaller molecules
Correct move:
Use the trend: as molecular size increases, boiling point β, viscosity β, flammability β
6. Quick Reference Cheatsheet
Concept | Key Exam Fact |
|---|---|
Crude oil | Mixture of hydrocarbons, non-renewable fossil fuel |
Fractional distillation | Physical separation, based on boiling point differences |
Column temperature | Hot at bottom, cool at top |
Top column fractions | Low boiling point, low viscosity, highly flammable (e.g. petrol) |
Bottom column fractions | High boiling point, high viscosity, low flammability (e.g. bitumen) |
Complete combustion | Excess Oβ β COβ + HβO, maximum energy released |
Incomplete combustion | Limited Oβ β CO/soot + HβO, toxic products |
Going deeper
What's Next
Now that you have mastered core content for fuels and fractional distillation for CIE IGCSE Chemistry 0620, you are ready to build on this knowledge to explore related organic chemistry and atmospheric chemistry topics. First, you will learn about cracking, a chemical reaction that breaks down large, low-demand fractions (like fuel oil) into smaller, high-demand fractions (like petrol) and alkenes, the feedstock for plastic production. Next, you will study the environmental impacts of fuel combustion, including greenhouse gas emissions and atmospheric pollution from sulfur dioxide and nitrogen oxides. You will also explore alternative fuels that reduce reliance on finite crude oil supplies. Make sure to practice writing structured 2-3 mark answers describing the fractional distillation process, as these are very common in Core exam papers.