11.3 Fuels Revision Notes

1. Overview

Fuels are substances that release energy, usually in the form of heat, when they undergo combustion. This topic focuses on fossil fuels—specifically petroleum and natural gas—which are the primary energy sources for modern transport, heating, and the chemical industry.

Key Definitions

• Fossil Fuel: A natural fuel such as coal or gas, formed in the geological past from the remains of living organisms.
• Hydrocarbon: A compound that contains only hydrogen and carbon atoms.
• Petroleum (Crude Oil): A complex mixture of hydrocarbons.
• Volatility: The tendency of a substance to vaporize (turn into a gas).
• Viscosity: A measure of a fluid's resistance to flow (thickness).
• Fractional Distillation: A process used to separate a liquid mixture into "fractions" with different boiling points.

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Core Content

The Fossil Fuels

There are three main fossil fuels:

1. Coal (solid)
2. Natural Gas (gas)
3. Petroleum (liquid)

Methane ($CH_4$) is the main constituent of natural gas.

Petroleum and Hydrocarbons

Petroleum is a thick, black liquid that is a mixture of many different hydrocarbons. Because these molecules have different sizes and properties, they must be separated before they can be used.

Fractional Distillation

Petroleum is separated into useful fractions in a fractionating column.

1. The petroleum is heated and vaporized before entering the bottom of the column.
2. The column is hot at the bottom and cooler at the top.
3. Vapors rise up the column.
4. Hydrocarbons condense into liquids when they reach a level that is cooler than their boiling point.
5. Small molecules rise to the top; large molecules stay at the bottom.

Property Trends (From Bottom to Top)

As you move from the bottom (large molecules) to the top (small molecules) of the fractionating column:

• Chain length decreases: Molecules get smaller.
• Volatility increases: They turn into gas more easily.
• Boiling point decreases: They condense at lower temperatures.
• Viscosity decreases: The liquids become "runnier" and less thick.

Uses of Petroleum Fractions

You must memorize these specific uses for the exam:

Fraction	Use
Refinery gas	Heating and cooking
Gasoline (Petrol)	Fuel for cars
Naphtha	Chemical feedstock (making other chemicals)
Kerosene (Paraffin)	Jet fuel
Diesel oil (Gas oil)	Fuel for diesel engines (trucks/trains)
Fuel oil	Fuel for ships and home heating systems
Lubricating oil	Lubricants, waxes, and polishes
Bitumen	Making roads and roofing

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Extended Content (Extended Only)

Note: There is no specific "Extended" content for topic 11.3; all objectives listed are Core requirements for all students.

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Key Equations

While this topic focuses on separation, you should be able to write the combustion equation for methane (the main component of natural gas).

Complete Combustion of Methane:

• Word Equation: methane + oxygen → carbon dioxide + water
• Symbol Equation: $CH_4(g) + 2O_2(g) \rightarrow CO_2(g) + 2H_2O(l)$

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Common Mistakes to Avoid

• ❌ Wrong: Defining a hydrocarbon as "a molecule containing hydrogen and carbon."
• ✅ Right: Defining a hydrocarbon as "a compound containing hydrogen and carbon only." (Omitting "only" loses the mark).
• ❌ Wrong: Thinking that fractional distillation involves a chemical reaction.
• ✅ Right: Understanding it is a physical separation based on different boiling points.
• ❌ Wrong: Stating that the bottom of the column is the coolest.
• ✅ Right: The bottom of the fractionating column is the hottest; large molecules with high boiling points condense there.
• ❌ Wrong: Confusing the use of Naphtha.
• ✅ Right: Naphtha is used as a chemical feedstock, not as a fuel for transport.

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Exam Tips

• Command Words: If asked to "Describe the trends," you must mention at least two properties (e.g., "As you go up the column, the boiling point decreases and the viscosity decreases").
• Order of Fractions: A common mnemonic to remember the order from top to bottom is: Really Good Notes Keep Dads From Losing Brains (Refinery gas, Gasoline, Naphtha, Kerosene, Diesel, Fuel oil, Lubricating oil, Bitumen).
• Context: Questions often ask why Bitumen is found at the bottom. Your answer should focus on it having the largest molecules and the highest boiling point.
• Typical Values: You do not need to memorize the exact boiling points in degrees Celsius, just the relative trends.