Organic chemistry — GCSE Chemistry Revision
Everything you need to revise organic chemistry for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Introduction to Organic Chemistry and Hydrocarbons
Organic chemistry is the study of carbon compounds. Carbon is unique because it can form four strong covalent bonds, including bonds with other carbon atoms, leading to long chains, rings, and branched structures. Hydrocarbons are the simplest organic compounds, made only of carbon and hydrogen atoms. They are found in crude oil, which is a finite resource formed over millions of years from dead marine organisms. Understanding hydrocarbons is fundamental to the study of organic chemistry.
Alkanes: Saturated Hydrocarbons
Alkanes are saturated hydrocarbons, meaning they only contain carbon-carbon single bonds. This makes them relatively unreactive. They follow the general formula CnH2n+2. Common alkanes include methane (CH4), ethane (C2H6), propane (C3H8), and butane (C4H10). As the number of carbon atoms increases, their boiling points increase due to stronger intermolecular forces. Alkanes are important fuels and raw materials, obtained by fractional distillation of crude oil.
Alkenes: Unsaturated Hydrocarbons
Alkenes are unsaturated hydrocarbons, meaning they contain at least one carbon-carbon double bond. This double bond makes them much more reactive than alkanes. They follow the general formula CnH2n. Common alkenes include ethene (C2H4), propene (C3H6), and butene (C4H8). Alkenes are crucial as starting materials for polymers and other organic compounds. Their unsaturation allows them to undergo addition reactions, for example, with bromine water, which is a test for unsaturation.
Fractional Distillation and Cracking
Crude oil is a mixture of hydrocarbons with different boiling points. Fractional distillation separates crude oil into fractions (groups of hydrocarbons) based on their boiling points. Hot crude oil enters a fractionating column, and vapours rise, cooling and condensing at different levels. Shorter chains with lower boiling points condense higher up, while longer chains condense lower down. Cracking is a process that breaks down long-chain hydrocarbons into smaller, more useful alkanes and alkenes, using heat and catalysts.
Key facts
- Carbon forms 4 covalent bonds.|Alkanes are saturated (C-C single bonds).|Alkenes are unsaturated (C=C double bond).|Crude oil is separated by fractional distillation.|Cracking breaks large hydrocarbons into smaller ones.|Bromine water tests for C=C double bonds.
Key terms
- Hydrocarbon::A compound containing only carbon and hydrogen atoms.|Saturated::An organic compound containing only carbon-carbon single bonds.|Unsaturated::An organic compound containing at least one carbon-carbon double or triple bond.|Crude oil::A finite, non-renewable mixture of hydrocarbons formed over millions of years.|Fractional distillation::A process to separate crude oil into fractions based on boiling points.|Cracking::Breaking down large, less useful hydrocarbons into smaller, more useful ones using heat and/or catalysts.
Common mistakes
- Confusing alkanes with alkenes, especially their reactivity and general formulae.|Mixing up the products of complete and incomplete combustion (e.g., CO2 vs CO/C).|Not explaining *why* alkanes have different boiling points (intermolecular forces).|Forgetting that cracking produces both alkanes and alkenes.
Exam tips
- Learn the general formulae for alkanes (CnH2n+2) and alkenes (CnH2n).|Practise drawing displayed formulae for the first four alkanes and alkenes.|Understand the process of fractional distillation and the properties of different fractions.|Be able to describe the test for unsaturation using bromine water and state the observation.
Quick quiz
1. Which of the following describes an alkane?
- Contains a C=C double bond.
- Is an unsaturated hydrocarbon.
- Has the general formula CnH2n.
- Contains only C-C single bonds.
Show answer
Contains only C-C single bonds. — Alkanes are saturated hydrocarbons, meaning they contain only carbon-carbon single bonds. Options A, B, and C describe alkenes.
2. Which process separates crude oil into fractions?
- Cracking
- Distillation
- Fractional distillation
- Polymerisation
Show answer
Fractional distillation — Crude oil is separated into fractions (groups of hydrocarbons with similar boiling points) using fractional distillation. Distillation is a general term, but 'fractional' is specific to crude oil separation.
3. What is the general formula for alkenes?
- CnH2n+2
- CnH2n
- CnH2n-2
- CnHn
Show answer
CnH2n — Alkenes are unsaturated hydrocarbons with one carbon-carbon double bond, and their general formula is CnH2n. CnH2n+2 is for alkanes.
4. Which of these is a product of cracking long-chain hydrocarbons?
- Only longer alkanes
- Only shorter alkenes
- Shorter alkanes and shorter alkenes
- Only carbon dioxide and water
Show answer
Shorter alkanes and shorter alkenes — Cracking breaks down large hydrocarbons into a mixture of smaller, more useful alkanes and alkenes.
5. What observation would you expect when testing ethene with bromine water?
- The bromine water remains orange-brown.
- The bromine water turns from orange-brown to colourless.
- The bromine water turns blue.
- The bromine water produces a white precipitate.
Show answer
The bromine water turns from orange-brown to colourless. — Ethene is an alkene, an unsaturated hydrocarbon. It reacts with bromine water in an addition reaction, causing the orange-brown colour of the bromine to disappear as it is consumed.
Exam-style questions
Explain why alkanes are described as saturated hydrocarbons and describe a chemical test to distinguish between an alkane and an alkene. [4 marks]
Show mark scheme
- Alkanes are saturated because they only contain carbon-carbon single bonds.
- Test: Add bromine water / aqueous bromine.
- Alkene: Bromine water turns from orange/brown to colourless.
- Alkane: Bromine water remains orange/brown.
Crude oil is a mixture of hydrocarbons. Describe how fractional distillation separates crude oil into different fractions. [3 marks]
Show mark scheme
- Crude oil is heated and vaporised (enters fractionating column).
- Vapours rise and cool.
- Hydrocarbons (fractions) condense at different temperatures/levels (due to different boiling points/chain lengths).
Give the general formula for an alkane and draw the displayed formula for propane. [3 marks]
Show mark scheme
- General formula: CnH2n+2
- Displayed formula for propane: 3 carbon atoms in a chain.
- Each carbon has 4 bonds, with hydrogen atoms correctly attached (C3H8).
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