Fuels

Fossil fuels are natural fuels formed from the remains of long-dead plants and animals, compressed and heated underground over millions of years.

Three big ones.

• Coal: from ancient plants. Mostly carbon. Burned for electricity, steel-making.
• Crude oil (petroleum): from ancient marine organisms. Mixture of hydrocarbons. Source of fuels and petrochemicals.
• Natural gas: mostly methane (CH₄). Often found alongside oil.

Why "fossil"? Fossils = preserved remains of ancient life. Coal seams sometimes have fossilised plant imprints; oil drilling often turns up shells, plankton fragments.

Why non-renewable? Formed at GEOLOGICAL timescales (millions of years). We're using them at MUCH faster rates → reserves are finite.

Pollution. Combustion produces CO₂ (greenhouse gas) and may produce SO₂, NOₓ, CO, particulates. (See Air study note.)

Process.

1. Crude oil heated in a furnace ($\sim 350\degree\text{C}$) → vaporises.
2. Vapour enters a fractionating column with temperature gradient (HOT at bottom, COOL at top).
3. As vapour rises, different molecular sizes condense at different heights:
   • SHORT-chain molecules (low b.p.) rise highest before condensing.
   • LONG-chain molecules (high b.p.) condense low.
4. Each fraction collected at a different level.

Fractions, top to bottom (small to large molecule, low to high b.p.).

Fraction	Carbons	Use
Refinery gas	C₁-C₄	Bottled gas (LPG), home cooking
Gasoline (petrol)	C₅-C₁₀	Car fuel
Naphtha	C₆-C₁₀	Petrochemical feedstock (plastics, etc.)
Kerosene (paraffin)	C₁₀-C₁₆	Aircraft fuel, heating
Diesel	C₁₄-C₂₀	Road vehicles, trains
Fuel oil	C₂₀-C₃₀	Power stations, ships
Bitumen	C₃₀+	Roads, roofing

Properties going DOWN the column.

• Boiling point INCREASES.
• Viscosity (thickness) INCREASES.
• Volatility (ease of evaporation) DECREASES.
• Flammability DECREASES.
• Colour darkens.

Why? Larger molecules = stronger intermolecular forces (more contact area) = harder to vaporise = higher b.p.

Cracking. Long-chain hydrocarbons are LESS USEFUL than short-chain. Cracking breaks long molecules into short, more useful ones (esp. alkenes for plastics): $\text{C}_{12}\text{H}_{26} \to \text{C}_{8}\text{H}_{18} + \text{C}_{4}\text{H}_{8} \text{ (an alkene)}.$

Done at high T with a catalyst (e.g. Al₂O₃).

Complete combustion (plenty of oxygen). $\text{hydrocarbon} + \text{O}_{2} \to \text{CO}_{2} + \text{H}_{2}\text{O}.$

Worked. Methane: $\text{CH}_{4} + 2\text{O}_{2} \to \text{CO}_{2} + 2\text{H}_{2}\text{O}$. Worked. Octane: $2\text{C}_{8}\text{H}_{18} + 25\text{O}_{2} \to 16\text{CO}_{2} + 18\text{H}_{2}\text{O}$.

Strategy for balancing: balance C, then H, then O.

Incomplete combustion (insufficient oxygen).

• Produces CO (carbon monoxide) and/or C (soot/carbon).
• $2\text{CH}_{4} + 3\text{O}_{2} \to 2\text{CO} + 4\text{H}_{2}\text{O}$.
• $\text{CH}_{4} + \text{O}_{2} \to \text{C} + 2\text{H}_{2}\text{O}$.

Why bad?

• CO: poisonous (binds haemoglobin).
• Soot: particulates → respiratory issues, blackens surfaces.
• Less heat released per mole of fuel (incomplete reaction).

Cambridge tip. When given a combustion question, ALWAYS write the balanced equation. Check C, H, O atom counts on both sides.

Worked qualitative. A faulty gas heater produces CO. What's the cause? Insufficient air supply, poor ventilation, or a blocked vent. Fix: ensure good ventilation; service the heater regularly. CO detectors save lives in homes with gas appliances.

Hydrogen.

• Combustion: $2\text{H}_{2} + \text{O}_{2} \to 2\text{H}_{2}\text{O}$. Only product: water.
• Excellent fuel by mass: huge energy density per kg.
• Used in fuel cells (see study note) and rockets.
• Drawbacks: storage (H₂ leaks easily, must be high-pressure or liquid); production (most H₂ today comes from natural gas, releasing CO₂; "green" H₂ from electrolysis with renewable electricity).

Biofuels.

• Bioethanol: from fermentation of sugar/starch crops (sugar cane, corn, wheat).
  • $\text{C}_{6}\text{H}_{12}\text{O}_{6} \to 2\text{C}_{2}\text{H}_{5}\text{OH} + 2\text{CO}_{2}$ (yeast fermentation).
• Biodiesel: from vegetable oils.
• Biogas (methane): from anaerobic decomposition of waste.
• Renewable: crops can be regrown.
• BUT: needs large land area (food vs fuel debate); can release CO₂ in production / processing; not zero-impact.

Carbon-neutral? In theory, biofuels are CARBON NEUTRAL: the CO₂ released by burning equals the CO₂ absorbed by the crops as they grew. In practice, fertiliser production, farm machinery and transport contribute CO₂ → not quite zero-net.

Worked qualitative. Why is hydrogen often called a "clean fuel"? Combustion produces only water — no CO₂, CO, SO₂, NOₓ, or particulates. The catch: H₂ production may not be clean unless renewable electricity is used.