Hydrocarbons and fossil fuelsIB MYP Sciences: Revision notes

Fossil fuels are fuels formed from the remains of living things that died millions of years ago: coal, oil and natural gas.

• Coal formed from dead plants in swamps, which were buried by sediment. Heat and pressure over millions of years turned them into coal.
• Oil and natural gas formed from the remains of tiny sea organisms (plankton) that sank to the sea bed and were buried under mud. Heat and pressure over millions of years turned them into oil and gas, which are trapped in rocks.

Fossil fuels are non-renewable because they take millions of years to form, so we are using them much faster than they are made.

Crude oil is a mixture of many different hydrocarbons. A hydrocarbon is a compound made of hydrogen and carbon atoms only.

Crude oil is not useful until it has been separated into groups of hydrocarbons with similar boiling points, called fractions. This is done by fractional distillation.

In an oil refinery, crude oil is heated and the vapour enters a fractionating column, which is hot at the bottom and cooler at the top.

• hydrocarbons with long molecules have high boiling points and condense near the bottom
• hydrocarbons with short molecules have low boiling points and rise to the top before they condense

So the fractions separate by boiling point. From the top to the bottom the properties change:

• boiling point increases
• the liquids become thicker (more viscous)
• they become harder to ignite (less flammable)

Each fraction has its own uses:

• refinery gases: heating and cooking (LPG)
• petrol: fuel for cars
• naphtha: raw material for making chemicals and plastics
• kerosene: jet fuel
• diesel: fuel for lorries, buses and some cars
• fuel oil: fuel for ships and power stations
• bitumen: surfacing roads and roofs

The top fractions are the most useful as fuels because they are runny and ignite easily.

Alkanes are the main hydrocarbons in crude oil. They have only single bonds between the atoms. The first three are:

• methane, CH₄ (1 carbon atom)
• ethane, C₂H₆ (2 carbon atoms)
• propane, C₃H₈ (3 carbon atoms)

An alkane with n carbon atoms has 2n + 2 hydrogen atoms, so the formula is CₙH₂ₙ₊₂. Methane is the main part of natural gas.

Burning fossil fuels causes pollution:

• carbon dioxide (CO₂): made when any carbon fuel burns. It is a greenhouse gas that contributes to global warming.
• sulfur dioxide (SO₂): made when sulfur impurities in coal and oil burn. It dissolves in rain water to form acid rain, which damages buildings and kills trees and fish.
• nitrogen oxides: formed in hot engines when nitrogen and oxygen from the air react. They cause acid rain and breathing problems.
• particulates: tiny particles of soot (carbon) made by incomplete combustion, especially in diesel engines. They cause haze and lung problems.