Fossil fuels are natural energy sources formed over millions of years from the remains of ancient organisms. 

The three main fossil fuels are:

1. Coal – A solid fuel mainly composed of carbon.

2. Natural Gas – A gaseous fuel primarily composed of the hydrocarbon methane (CH₄).

3. Petroleum (Crude Oil) – A complex liquid mixture of hydrocarbons that can be separated into useful fractions through fractional distillation.

Hydrocarbons

• Hydrocarbons are compounds made up of only carbon and hydrogen atoms.

• Examples:

  • Methane (CH₄) – The simplest hydrocarbon.

  • Ethane (C₂H₆) and Propane (C₃H₈) – Also found in natural gas.

  • Octane (C₈H₁₈) – A component of petrol.

Fractional distillation of petroleum

Fractional distillation is a process that separates crude oil into useful fractions based on boiling points.

Process:

1. Crude oil is heated to 450°C and enters a fractionating column.

2. The column has a temperature gradient (hot at the bottom, cool at the top).

3. Different hydrocarbon fractions condense at different heights based on their boiling points.

4. Small molecules (lower boiling points) rise higher, while large molecules (higher boiling points) stay lower.

5. Each fraction is collected and used for different purposes.

Properties of the fractions

From bottom (large molecules) to top (small molecules), fractions change as follows:

Bottom of Column (e.g., Bitumen) - to - Top of Column (e.g., Refinery Gas)

Chain Length - Longer hydrocarbon chains - to - Shorter hydrocarbon chains

Volatility - Low (does not evaporate easily) - to - High (evaporates easily)

Boiling Point - High (above 350°C) - to - Low (below 25°C)

Viscosity - Thick & Sticky - to - Thin & Runny

Uses of petroleum fractions

The molecular formula of a compound tells us:

1. The exact number of atoms of each element present in a single molecule.

2. The type of atoms in the molecule (i.e., which elements it contains).

Examples of Molecular Formulae:

Water = H₂O (2 hydrogen atoms, 1 oxygen atom)

Methane = CH₄ (1 carbon atom, 4 hydrogen atoms)

Ethanol = C₂H₆O (2 carbon, 6 hydrogen, 1 oxygen atom)

Glucose = C₆H₁₂O₆ (6 carbon, 12 hydrogen, 6 oxygen atoms)

Displayed formulae

Alkanes are saturated hydrocarbons. They are less reactive than unsaturated compounds.

• General formula: CₙH₂ₙ₊₂

• Functional group: None, alkanes have single covalent bonds between carbon atoms (C–C).

• Have the suffix -ane.

• Cracking is the process of breaking down large alkane molecules from crude oil into smaller, alkenes and hydrogen.

• Reasons for cracking:

  • To produce smaller, more useful hydrocarbons (e.g., gasoline, diesel).

  • To make more valuable alkenes, which are used to manufacture plastics and chemicals.

  • To produce hydrogen gas (H₂), which is used in fuel and ammonia production.

• This process requires:

  • High temperature (~600-700°C)

  • A catalyst (e.g., alumina or silica)

Example Reaction:

C10H22 → C5H10 + C3H6 + H2 

Test for alkenes

(a) As a Solvent

• Dissolves substances that water cannot (e.g., paints, perfumes, medicines, and cosmetics).

• Used in industrial processes and chemical extractions.

(b) As a Fuel

• Used as bioethanol (a renewable fuel).

• Mixed with petrol to reduce carbon emissions (e.g., E10 fuel: 10% ethanol, 90% petrol).

• Used in spirit burners and alcohol stoves.

Combustion of ethanol

• Polymers are large molecules formed by linking many small molecules called monomers.

• The process of forming polymers is known as polymerisation.

Examples of Polymers:

• Natural Polymers: Cellulose, proteins, DNA.

• Synthetic Polymers: Poly(ethene), nylon, polyester.

Formation

• Plastics are synthetic materials made from polymers.

• They are lightweight, durable, and moldable, which makes them suitable for various applications like packaging, electronics, and textiles.

The properties that make plastics useful also create disposal challenges:

• Durability: Plastics take hundreds of years to decompose, leading to long-term environmental impacts.

• Resistance to Chemicals: Limits the breakdown of plastics in natural environments.

• Lightweight: Easily transported by wind and water, contributing to widespread pollution.

Environmental challenges of plastics