AQA GCSE Chemistry 4.7 Organic chemistry: a complete overview

What topic 4.7 actually demands

Organic chemistry is the chemistry of carbon compounds, and at GCSE it focuses on crude oil and the fuels and plastics that come from it. Topic 4.7 rewards clear recall of definitions, general formulae and tests, and an understanding of why crude oil is processed the way it is. It links to using resources (finite resources, recycling) and the atmosphere (combustion and pollutants).

This guide walks through all three dot points of the topic in specification order, then sets out the exam patterns AQA repeats. Each dot point has a matching page with practice questions; this overview ties them together.

Crude oil and hydrocarbons

Crude oil is a finite resource formed over millions of years from ancient biomass. It is a mixture of hydrocarbons, compounds of hydrogen and carbon only, most of which are alkanes with the general formula $C_nH_{2n+2}$. As the chain gets longer, boiling point and viscosity increase and flammability decreases, which is why short hydrocarbons make better fuels. Complete combustion gives carbon dioxide and water.

Fractional distillation and cracking

Fractional distillation separates crude oil by boiling point in a column that is hot at the bottom and cool at the top, giving fractions such as petrol, kerosene, diesel and bitumen, used as fuels and feedstocks. Cracking breaks long hydrocarbons into shorter, more useful ones plus alkenes, using a catalyst or steam, to meet demand for short-chain fuels and to make alkenes for polymers.

Alkenes and polymers

Alkenes are unsaturated hydrocarbons with a $C=C$ double bond and general formula $C_nH_{2n}$, making them more reactive than alkanes. They turn bromine water from orange to colourless. In addition polymerisation, many alkene monomers join, with the double bonds opening, to form a long polymer chain with no other product, making plastics such as poly(ethene).

How topic 4.7 is examined

A typical AQA profile for this topic:

• Recall. General formulae, the first members of each series, and the fractions and their uses.
• Tests. The bromine water test to distinguish alkenes from alkanes.
• Explanation. How fractional distillation works and why cracking is done.
• Polymers. Drawing or interpreting addition polymerisation and monomer-polymer relationships.

Check your knowledge

A mix of recall and explanation questions covering topic 4.7. Attempt them under timed conditions, then check against the solutions.

1. Define a hydrocarbon. (1 mark)
2. Give the general formula of an alkane and an alkene. (2 marks)
3. Describe how the viscosity of hydrocarbons changes with chain length. (1 mark)
4. Explain how fractional distillation separates crude oil. (2 marks)
5. Give two reasons why cracking is carried out. (2 marks)
6. Describe the bromine water test and its results for an alkane and an alkene. (3 marks)
7. State the products of the complete combustion of a hydrocarbon. (2 marks)
8. Describe what happens in addition polymerisation. (2 marks)