Organic chemistry

1. Crude oil: a useful, limited mixture

Crude oil is a dark liquid found in rocks underground. It formed over millions of years from ancient biomass—the remains of living things, mainly tiny plankton buried in mud. Heat and pressure changed these remains.

Crude oil is finite: there is a limited supply, and we use it far faster than it forms. It is a mixture of many compounds, not one pure chemical. Most are hydrocarbons, molecules containing carbon and hydrogen atoms only.

Carbon atoms can bond to other carbon atoms, forming chains and many families of compounds. This helps explain the huge range of carbon-based materials around us.

2. Alkanes: a family with a pattern

Most hydrocarbons in crude oil are alkanes. They have only single bonds between carbon atoms. They are saturated: they have the maximum number of hydrogen atoms for these open carbon chains.

A homologous series is a family of compounds with the same general formula and similar chemical reactions. Alkanes follow CₙH₂ₙ₊₂. Here, n is the number of carbon atoms.

A molecular formula tells you the number of each atom. A displayed formula also shows how the atoms are connected. Do not change the number of atoms when switching between them.

3. Separate the mixture using boiling points

Different hydrocarbons boil at different temperatures. In fractional distillation, crude oil is heated so that much of it vaporises. The vapour enters a column that is hot at the bottom and cooler at the top.

As vapours rise, they cool. A hydrocarbon condenses when it reaches a region below its boiling point. Large molecules with higher boiling points condense lower down. Smaller molecules with lower boiling points travel further up before condensing. The smallest remain gases and leave at the top. Heavy residue remains at the bottom.

Fractional distillation is a physical separation: molecules are separated, not split apart. Each fraction contains molecules with similar numbers of carbon atoms and similar boiling points.

4. Fuels and feedstock: two different jobs

A fuel is burned to release energy. A feedstock is a starting material used to make other chemicals. The same oil industry supplies both.

Modern transport, packaging, cleaning and manufacturing use these products. A polymer can be made into a useful material instead of being burned as a fuel. This gives a reason to use finite oil resources carefully.

5. Size changes the properties

Viscosity means resistance to flowing. Honey is more viscous than water. Flammability describes how easily a substance catches fire.

These trends help explain uses. Small-molecule fuels vaporise and ignite easily. Larger molecules can be useful as lubricants because they are thicker and less easily evaporated. When a liquid boils, attractions between molecules are overcome; the covalent bonds inside each molecule are not broken.

6. Complete combustion: enough oxygen

When a hydrocarbon burns in plenty of oxygen, its carbon and hydrogen are oxidised. Complete combustion produces carbon dioxide and water and releases energy to the surroundings.

Hydrocarbon + oxygen → carbon dioxide + water

Water may initially be a vapour because the flame is hot. You cannot identify water only by looking for visible liquid.

For propane: C₃H₈ + 5O₂ → 3CO₂ + 4H₂O. There are 3 carbons and 8 hydrogens, so make 3CO₂ and 4H₂O. These need 10 oxygen atoms: 5O₂.

With too little oxygen, combustion can be incomplete and produce toxic carbon monoxide or soot. This links to Chemistry of the atmosphere. Use proper ventilation and never test fuels casually at home.

7. Cracking: turn surplus into useful products

Refineries may have more long-chain hydrocarbons than customers need, while demand for smaller fuel molecules is high. Cracking breaks large hydrocarbon molecules into smaller ones. It is a chemical reaction, unlike fractional distillation.

In catalytic cracking, the hydrocarbon is vaporised and passed over a hot catalyst. In steam cracking, hydrocarbon vapour is mixed with steam and heated to a very high temperature. You need these general conditions, not an exact industrial temperature.

Products include smaller alkanes, useful as fuels, and alkenes, useful for making polymers and many other chemicals. Cracking increases the supply of products in high demand.

8. Use bromine water to recognise an alkene

Alkenes contain a carbon–carbon double bond. They are unsaturated and more reactive than alkanes. In a teacher-supervised test, an alkene reacts with bromine water, changing it from orange to colourless. An alkane does not give this rapid colour change under the normal test conditions.

Use small samples and eye protection. Bromine water is harmful, so follow your teacher’s instructions. “Colourless” is the observation: “clear” is not precise because a coloured solution can also be clear.

9. Read evidence and explain a process

Always use the values and units given in a data question. For a percentage, divide the part by the whole and multiply by 100.

For a six-mark explanation, connect stages using “because”, “so” and “therefore”. For distillation, link heating, vapour, the temperature gradient, boiling points and condensation. For cracking, explain both how it works and why the products are useful.

There is no separate required practical in topic 5.7. Demonstrations of cracking or fuel properties must follow school safety procedures. All questions here are original exam-style practice. This topic has no Higher-only specification content; the optional challenges ask you to apply the same content with less support.

Use a refinery’s evidence

A refinery needs more small-molecule fuels and polymer feedstocks. Explain how fractional distillation and cracking work together. Refer to the diagrams and give two differences between the processes.

This is a diagram and application task. AQA does not specify a separate required practical for Organic chemistry.

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One final thought

Use a scientific idea and a specific example in your reflection.