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Fossil Fuels

Fossil fuels: what they are, complete vs incomplete combustion (CO2 vs CO/soot), pollution, and fractional distillation of crude oil.

  • SEC G3 Pure Chemistry 2027
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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.

Where this fits in the syllabus

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).
Carbon monoxide is not “just another gas”

CO is toxic. Incomplete combustion in enclosed spaces can be fatal.

3. Detailed Explanations

Quick Recall (combustion + refining)
  • 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 fuelWhat it is (O-Level safe)Typical use
Coalmostly carbon, contains impurities (e.g., sulphur compounds)power stations, industrial heating
Crude oil (petroleum)mixture of hydrocarbons (alkanes, etc.)fuels + chemical feedstock
Natural gasmainly methane, CH₄cooking, heating, power generation

B. Complete Combustion vs Incomplete Combustion

Complete combustion happens in excess oxygen:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

Incomplete combustion happens when oxygen is limited.

Example (forms carbon monoxide):

2CH₄(g) + 3O₂(g) → 2CO(g) + 4H₂O(l)

Example (forms soot):

CH₄(g) + O₂(g) → C(s) + 2H₂O(l)
Must-write line

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.

Fractional distillation of crude oilCrude oil is heated and its vapour enters the bottom of a fractionating column. The column is hot at the bottom and cooler at the top. Refinery gases leave at the top, followed downwards by petrol, naphtha, kerosene, diesel, fuel and lubricating oils, and bitumen. Boiling range and viscosity increase down the column, while ease of ignition decreases.Fractional distillation of crude oilcoolerhottervapours rise and coolrefinery gasesbottled gaspetrolnaphthakerosenejet fueldieselfuel and lubricating oilsfuel and lubricantsbitumencrude oilheated stronglydown the columnmolecules become largerboiling range increasesviscosity increasesease of ignition decreases
A refinery fractionating column is hot at the bottom and cooler at the top. Fractions with higher boiling ranges condense lower in the column; fractions with lower boiling ranges condense higher up.

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)

FractionTypical use
Refinery gas (petroleum gas)bottled gas for cooking
Petrol (gasoline)fuel for cars
Naphthastarting material to make chemicals
Kerosenejet fuel; heating/cooking
Dieselfuel for diesel engines
Lubricating oillubricants for machines
Bitumenroad 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

Combustion question checklist
  1. 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

Modelled example 1

Infer Combustion from the Air Supply

Core

Problem

Methane burns in a Bunsen burner with the air hole closed and gives a yellow smoky flame. State the combustion type, name one product other than water, and explain one associated hazard.
Study the worked solution

  1. Interpret the observation

    Method

    Use the closed air hole and smoky yellow flame as evidence of limited oxygen.

    Reason

    Limited oxygen prevents complete oxidation of the carbon in methane.

    Working

    Combustion type: incomplete combustion.

  2. Name a product

    Method

    State carbon monoxide, CO, and/or carbon soot, C.

    Reason

    These carbon-containing products can form when oxygen is insufficient for all carbon to become CO₂.

    Working

    One acceptable product other than water: CO.

  3. Connect product to hazard

    Method

    State that carbon monoxide is toxic, especially where ventilation is poor.

    Reason

    Identifying the pollutant is incomplete when the question also asks why it is dangerous.

    Working

    Limited oxygen → incomplete combustion → toxic CO and/or soot.

Guided practice 2

Balance Complete Combustion of Methane

About 6 min

Problem

Write the balanced equation, including state symbols, for complete combustion of methane.

Try this before viewing the solution

Products
Oxygen coefficient

Hints

Hint 1: products
Excess oxygen means the carbon product is CO₂.
Hint 2: balance order
Balance carbon, then hydrogen, then oxygen.
View solution step by step

  1. Write the products

    Method

    Use CO₂ and H₂O.

    Reason

    Complete combustion of a hydrocarbon in excess oxygen fully oxidises carbon to carbon dioxide.

    Working

    CH₄ + O₂ → CO₂ + H₂O.

  2. Balance hydrogen

    Method

    Place 2 before water.

    Reason

    One methane molecule contains four hydrogen atoms.

    Working

    CH₄ + O₂ → CO₂ + 2H₂O.

  3. Balance oxygen and add states

    Method

    Place 2 before oxygen and add the stated physical forms.

    Reason

    The products contain four oxygen atoms in total.

    Working

    CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).

Common misconception 3

A Crude-Oil Fraction Is Still a Mixture

Find and correct the mistake

Learner claim

“Each liquid collected from the fractionating column is one pure hydrocarbon with one exact boiling point.” Correct both parts of the claim.

Try this before viewing the solution

Composition of a fraction
Boiling behaviour

View solution step by step

  1. Correct the composition

    Method

    State that each fraction is a mixture of hydrocarbons with similar chain lengths and boiling points.

    Reason

    Fractional distillation groups substances by boiling behaviour; it does not normally isolate every individual compound.

    Working

    Fraction ≠ pure compound.

  2. Correct the boiling claim

    Method

    State that the mixture boils or condenses over a temperature range.

    Reason

    Its different components have different, though similar, boiling points.

    Working

    A fraction has a boiling range, not one sharp boiling point.

Examiner practice 4

Explain the Fractionating-Column Trend

4 marks

Examination question

State two properties that increase down a crude-oil fractionating column and explain why fractions are collected at different heights. [4 marks]

Try this before viewing the solution

View solution step by step

  1. State two trends

    2 marks

    Method

    State that boiling point and viscosity increase down the column.

    Reason

    Both are accepted properties for comparing the collected fractions.

    Working

    Down the column: higher boiling point; greater viscosity.

  2. Explain the separation

    2 marks

    Method

    State that hydrocarbons have different boiling points and condense where the column temperature falls below their boiling range.

    Reason

    The temperature gradient therefore causes different vapours to condense at different heights.

    Working

    High-boiling fractions condense lower; low-boiling fractions rise farther before condensing.

Challenge 5

Deduce the Road-Surface Fraction

Minimal support

Problem

An unnamed crude-oil fraction is collected near the bottom of the column. It has a very high boiling range, very high viscosity and low flammability. Identify the fraction most suitable for road surfaces and justify the use from the data.

Try this before viewing the solution

Fraction
Useful evidence

Hints

Hint 1: column position
Bottom fractions contain the largest, least volatile molecules.
Hint 2: use link
A road-surface material must be thick rather than easy to vaporise or ignite.
View solution step by step

  1. Identify the fraction

    Method

    Choose bitumen.

    Reason

    Bitumen is the heavy fraction or residue associated with the bottom of the column.

    Working

    Fraction: bitumen.

  2. Justify the use

    Method

    Use the high viscosity and low volatility implied by its high boiling range.

    Reason

    A thick material that does not vaporise readily can form a durable road surface.

    Working

    Very viscous bitumen → road surfacing.

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

Quiz Time!

Test yourself on combustion products, pollution, and fractional distillation of crude oil.

Go to Quiz Page