Fossil Fuels
Show/Hide Sub-topics (Chemical Energetics | K324 / 6092)
Continue your study path
Natural gas and crude oil are named in the syllabus as non-renewable energy sources. This lesson also connects that fact to combustion, pollution, and the fractional distillation of crude oil.
The required energy-source statement is that natural gas (mainly methane) and crude oil are non-renewable. Fractional distillation and crude-oil fractions belong to the Organic Chemistry section, while combustion pollutants connect to Environmental Chemistry.
1. Definition
A. Fossil Fuels
Fossil fuels are non-renewable fuels formed from the remains of living organisms over millions of years.
Examples:
- Coal (mainly carbon, with impurities)
- Crude oil (petroleum) (mixture of hydrocarbons)
- Natural gas (mainly methane, CH₄)
B. Combustion (Burning)
Combustion is a reaction with oxygen that releases heat. It is usually exothermic (link: Exothermic Reactions).
2. Key Ideas
- Complete combustion (excess oxygen) produces CO₂ and H₂O.
- Incomplete combustion (limited oxygen) produces carbon monoxide, CO, and/or carbon (soot), C.
- Bunsen burner scan rule: air hole open → blue flame → more complete combustion; air hole closed → yellow smoky flame → more incomplete combustion → CO/soot.
- Burning fossil fuels releases energy because new bonds form in products (recall: Bond Breaking and Bond Forming).
- Crude oil is separated by fractional distillation because different fractions have different boiling points.
- Pollution from fossil fuels includes CO₂ (greenhouse gas), CO (toxic), SO₂ (acid rain), and nitrogen oxides (acid rain/smog).
CO is toxic. Incomplete combustion in enclosed spaces can be fatal.
3. Detailed Explanations
- Coal is mainly carbon; crude oil is a mixture of hydrocarbons; natural gas is mainly methane, CH₄.
- Complete combustion (excess oxygen) → CO₂ + H₂O.
- Incomplete combustion (limited oxygen) → CO and/or soot (C) + H₂O.
- CO is toxic, so incomplete combustion is dangerous.
- Fractional distillation separates crude oil by different boiling points (condense at different heights).
A. What Each Fossil Fuel Actually Is
| Fossil fuel | What it is (O-Level safe) | Typical use |
|---|---|---|
| Coal | mostly carbon, contains impurities (e.g., sulphur compounds) | power stations, industrial heating |
| Crude oil (petroleum) | mixture of hydrocarbons (alkanes, etc.) | fuels + chemical feedstock |
| Natural gas | mainly methane, CH₄ | cooking, heating, power generation |
B. Complete Combustion vs Incomplete Combustion
Complete combustion happens in excess oxygen:
Incomplete combustion happens when oxygen is limited.
Example (forms carbon monoxide):
Example (forms soot):
Limited oxygen → incomplete combustion → CO and/or C (soot).
C. Why Crude Oil Must Be Refined
Crude oil is not a single compound. It is a mixture of hydrocarbons with different chain lengths and therefore different boiling points.
This is why it can be separated by fractional distillation.
D. Fractional Distillation of Crude Oil
Crude oil is heated so it vaporises, and the vapours rise up a fractionating column.
Different fractions condense at different heights because they have different boiling points.
As you go down the fractionating column:
- boiling points increase
- molecules get larger
- liquids become more viscous (flow less easily)
- fractions become less flammable and burn more smokily
E. Common Fractions and Uses (O-Level)
| Fraction | Typical use |
|---|---|
| Refinery gas (petroleum gas) | bottled gas for cooking |
| Petrol (gasoline) | fuel for cars |
| Naphtha | starting material to make chemicals |
| Kerosene | jet fuel; heating/cooking |
| Diesel | fuel for diesel engines |
| Lubricating oil | lubricants for machines |
| Bitumen | road surfaces |
4. Common Mistakes
- Writing “fossil fuels = hydrocarbons”. Coal is mostly carbon; crude oil and natural gas contain hydrocarbons.
- Writing the definition of complete combustion without stating excess oxygen.
- Missing CO as a product of incomplete combustion.
- Forgetting that crude oil fractions are mixtures, so they boil over a range.
- Mixing up “condense” and “evaporate” in fractional distillation explanations.
5. Exam Tips
- State oxygen condition (excess/limited). 2) Write products (CO₂ + H₂O or CO/C + H₂O). 3) Mention pollution/hazard if asked.
- If the question mentions “pollution”, name the pollutant and the effect (e.g., CO₂ → greenhouse gas; SO₂ → acid rain; CO → toxic).
- For fractional distillation: use the words boiling point, condense, different heights.
6. Worked Examples
Example 1: Complete vs Incomplete Combustion (Logic)Core
Methane burns in a Bunsen burner with the air hole closed. State the type of combustion and give one product (other than water).
Show Answer
Air hole closed means limited oxygen, so it is incomplete combustion. One product (other than water) is carbon monoxide, CO, and/or carbon (soot), C.
Example 2: Write a Balanced Equation (Complete Combustion)Core
Write the balanced equation (with state symbols) for the complete combustion of methane.
Show Answer
Example 3: Fractionating Column TrendCore
State two properties that increase as you go down the fractionating column.
Show Answer
Boiling point increases, and viscosity increases (fractions flow less easily). (Also acceptable: molecules get larger / less flammable.)
Example 4: Match Fraction to UseCore
Which fraction is most suitable for road surfaces, and why?
Show Answer
Bitumen. It has very large molecules and is thick/viscous, so it forms a durable surface.
7. Mind Stretchers
Mind stretcher 1: “Clean” Fuel TrapExtension
A student says: “Natural gas is clean because it produces no pollutants.” Identify the mistake.
Show Answer
Natural gas can burn more cleanly than coal, but complete combustion still produces CO₂ (a greenhouse gas). Incomplete combustion can also produce toxic CO and soot if oxygen is limited.
Mind stretcher 2: Fractional Distillation MisconceptionExtension
A student claims: “Crude oil separates because lighter molecules rise to the top due to density.” Explain why this is wrong and give the correct reason.
Show Answer
Fractional distillation works mainly because fractions have different boiling points. Vapours rise and condense at different temperatures/heights in the column, not because of density.
8. Quiz
Test yourself on combustion products, pollution, and fractional distillation of crude oil.
Go to Quiz Page