FuelsEdexcel GCSE Chemistry: Revision notes

Hydrocarbons are compounds that contain carbon and hydrogen only.

Crude oil is a complex mixture of hydrocarbons, mostly alkanes, with molecules containing carbon atoms in chains or rings. It is:

• An important source of fuels and of feedstock for the petrochemical industry
• A finite resource (non-renewable) — it cannot be replaced once used up

Crude oil is separated into simpler, more useful mixtures (fractions) by fractional distillation:

1. Crude oil is heated until most of it turns into vapour
2. The vapour enters a fractionating column, which is hot at the bottom and cools towards the top
3. Hydrocarbons with higher boiling points condense and are collected lower down the column
4. Hydrocarbons with lower boiling points rise further before condensing and are collected near the top

This works because different hydrocarbon chain lengths have different boiling points.

Moving from the top to the bottom of the fractionating column, hydrocarbon molecules have:

• More carbon and hydrogen atoms per molecule
• Higher boiling points
• Are harder to ignite
• Are more viscous (thicker, flow less easily)

Most of the hydrocarbons in crude oil belong to the alkane homologous series: a family of compounds that

• Share the same general formula
• Differ by CH2 in molecular formula between neighbouring members
• Show a gradual variation in physical properties (e.g. boiling point increases steadily with chain length)
• Have similar chemical properties

Complete combustion (plenty of oxygen) produces carbon dioxide and water, releasing energy:

hydrocarbon + oxygen → carbon dioxide + water (+ energy)

Incomplete combustion (limited oxygen) can produce carbon monoxide and carbon (soot) instead of, or as well as, carbon dioxide. Carbon monoxide is a toxic gas — it is colourless and odourless, and binds to haemoglobin in the blood, preventing it from carrying oxygen, which can be fatal. Soot can also block appliance vents, making the incomplete combustion problem worse.

Other pollution problems from burning fuels:

• Sulfur dioxide: impurities (sulfur) in some hydrocarbon fuels burn to form sulfur dioxide, which dissolves in rainwater to cause acid rain
• Oxides of nitrogen: at the high temperatures inside an engine, oxygen and nitrogen from the air can react together to form nitrogen oxides, which are also pollutants

Cracking breaks down larger, saturated hydrocarbon molecules (alkanes) into smaller, more useful molecules, some of which are unsaturated (alkenes). Cracking is necessary because:

• Fractional distillation produces more long-chain fractions than there is demand for
• It produces more of the shorter-chain, highly useful fuels such as petrol
• Alkenes made are useful as feedstock for making polymers

Hydrogen as a fuel for cars — compared with petrol:

• Advantages: only produces water when it combusts (no CO2, CO or soot); can be made from renewable sources
• Disadvantages: hydrogen is hard to store and transport safely (flammable, low density gas), there is currently little refuelling infrastructure, and producing hydrogen itself often uses energy from fossil fuels

Non-renewable fossil fuels: petrol, kerosene and diesel oil (from crude oil) and methane (from natural gas) are all non-renewable — they cannot be replaced once used.