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This lesson provides comprehensive exam practice for the entire AQA GCSE Chemistry Organic Chemistry topic (5.8). It brings together all the key concepts from across the topic and helps you prepare for the types of questions you will face in the exam. Use this lesson to consolidate your understanding, practise key skills, and identify any areas where you need further revision.
The organic chemistry topic covers the following areas:
| Subtopic | Key Content | Specification Reference |
|---|---|---|
| Crude oil, hydrocarbons and alkanes | Crude oil as a mixture, fractional distillation, properties of hydrocarbons, combustion | 5.8.1 |
| Cracking and alkenes | Cracking methods, alkene properties, bromine water test | 5.8.1 |
| Reactions of alkenes [H] | Addition reactions — hydrogenation, hydration, halogenation | 5.8.2 |
| Alcohols, carboxylic acids and esters [H] | Functional groups, properties, reactions, esterification | 5.8.2 |
| Polymers | Addition polymerisation, condensation polymerisation [H], natural polymers [H] | 5.8.2 |
Exam Tip: Before attempting any question, read it carefully and identify which part of the specification it relates to. This will help you focus your answer on the relevant content.
You must be able to define the following terms precisely:
| Term | Definition |
|---|---|
| Hydrocarbon | A compound containing only carbon and hydrogen atoms |
| Alkane | A saturated hydrocarbon with the general formula CnH2n+2 |
| Alkene | An unsaturated hydrocarbon with the general formula CnH2n, containing at least one C=C double bond |
| Saturated | A molecule containing only single covalent bonds between carbon atoms |
| Unsaturated | A molecule containing at least one C=C double bond between carbon atoms |
| Fraction | A group of hydrocarbons with similar numbers of carbon atoms and similar boiling points, collected at the same level in a fractionating column |
| Fractional distillation | The separation of crude oil into fractions based on differences in boiling point |
| Cracking | The thermal decomposition of long-chain hydrocarbons into shorter, more useful hydrocarbons |
| Polymer | A large molecule made by joining many small monomer molecules together |
| Monomer | A small molecule that joins with many others to form a polymer |
| Addition polymerisation | Polymerisation of alkene monomers where the C=C bonds open and monomers join; no by-product |
| Condensation polymerisation [H] | Polymerisation where monomers with two functional groups join, releasing a small molecule (usually water) |
| Functional group | An atom or group of atoms that determines the chemical properties of a compound |
| Homologous series | A family of compounds with the same general formula and similar chemical properties, with a gradual trend in physical properties |
Questions may ask you to describe or explain trends in the properties of hydrocarbons.
Example Question: Explain why the boiling points of alkanes increase as the chain length increases.
Model Answer:
You must be able to write and balance equations for combustion and cracking reactions.
Example Question: Write a balanced symbol equation for the complete combustion of propane (C3H8).
Model Answer: C3H8 + 5O2 → 3CO2 + 4H2O
Example Question: Decane (C10H22) is cracked. One of the products is ethene (C2H4). Determine the formula of the other product.
Model Answer:
graph TD
A["Exam Question Types"] --> B["Property Trends"]
A --> C["Balancing Equations"]
A --> D["Identifying Products"]
A --> E["Drawing Structures"]
A --> F["Compare and Contrast"]
A --> G["Extended Response (6-mark)"]
B --> B1["Explain using intermolecular forces"]
C --> C1["Combustion, cracking equations"]
D --> D1["Use atom counting"]
E --> E1["Displayed formulae, repeat units"]
F --> F1["Complete vs incomplete combustion<br/>Addition vs condensation polymerisation"]
G --> G1["Plan, use key terms, link ideas"]
style A fill:#8e44ad,color:#fff
style G fill:#e74c3c,color:#fff
Exam Tip: For 6-mark extended response questions, always plan your answer before writing. Use scientific terminology, give specific examples, and make sure your answer has a logical structure. Quality of written communication (QWC) marks are often awarded for these questions.
| Reaction | Equation |
|---|---|
| Complete combustion of methane | CH4 + 2O2 → CO2 + 2H2O |
| Complete combustion of propane | C3H8 + 5O2 → 3CO2 + 4H2O |
| Incomplete combustion (CO) | 2CH4 + 3O2 → 2CO + 4H2O |
| Incomplete combustion (C) | CH4 + O2 → C + 2H2O |
| Reaction | Equation |
|---|---|
| Cracking of decane (example 1) | C10H22 → C8H18 + C2H4 |
| Cracking of decane (example 2) | C10H22 → C5H12 + C5H10 |
| Cracking of hexane | C6H14 → C4H10 + C2H4 |
| Reaction | Equation |
|---|---|
| Hydrogenation of ethene | C2H4 + H2 → C2H6 |
| Hydration of ethene | C2H4 + H2O → C2H5OH |
| Bromination of ethene | C2H4 + Br2 → C2H4Br2 |
| Reaction | Equation |
|---|---|
| Combustion of ethanol | C2H5OH + 3O2 → 2CO2 + 3H2O |
| Ethanol + sodium | 2C2H5OH + 2Na → 2C2H5ONa + H2 |
| Esterification | C2H5OH + CH3COOH → CH3COOC2H5 + H2O |
| Ethanoic acid + NaOH | CH3COOH + NaOH → CH3COONa + H2O |
| Homologous Series | Functional Group | General Formula | Example |
|---|---|---|---|
| Alkanes | None (C–C and C–H only) | CnH2n+2 | Methane CH4 |
| Alkenes | C=C double bond | CnH2n | Ethene C2H4 |
| Alcohols | –OH (hydroxyl) | CnH2n+1OH | Ethanol C2H5OH |
| Carboxylic acids | –COOH (carboxyl) | CnH2n+1COOH | Ethanoic acid CH3COOH |
| Esters | –COO– (ester link) | — | Ethyl ethanoate CH3COOC2H5 |
| Mistake | Correction |
|---|---|
| Saying crude oil is a compound | Crude oil is a mixture of hydrocarbons |
| Saying bonds are broken during boiling | Intermolecular forces are overcome during boiling, not covalent bonds |
| Saying nitrogen oxides come from nitrogen in the fuel | They form from nitrogen and oxygen in the air at high temperatures |
| Saying bromine water turns "clear" | It turns colourless (all liquids are clear if you can see through them) |
| Confusing weak and dilute | Weak = partially ionises; dilute = low concentration |
| Saying energy is "made" in combustion | Energy is transferred or released, not created |
| Missing the by-product in condensation polymerisation | Condensation polymerisation releases a small molecule (usually water) |
| Drawing repeat units with double bonds | In addition polymers, the double bond opens — the repeat unit has single bonds |
Exam Tip: Read through the "Common Mistakes" table before your exam. Many students lose marks on these points every year. Being aware of them will help you avoid them.
The organic chemistry topic is highly interconnected. You should be able to trace the connections between different concepts:
| Starting Material | Process | Product | Everyday Use |
|---|---|---|---|
| Crude oil | Fractional distillation | Petrol | Car fuel |
| Long-chain fraction | Cracking | Ethene | Feedstock |
| Ethene | Addition polymerisation | Poly(ethene) | Plastic bags |
| Ethene | Hydration [H] | Ethanol | Solvent, fuel |
| Ethanol | Oxidation [H] | Ethanoic acid | Vinegar |
| Ethanol + ethanoic acid | Esterification [H] | Ethyl ethanoate | Flavouring, solvent |
"Compare complete and incomplete combustion of hydrocarbons. In your answer, discuss the conditions, products, and environmental impacts of each."
Model Answer Plan:
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