Alkanes, Alkenes and Crude Oil
Alkanes, alkenes and crude oil is the separate GCSE chemistry topic that builds on the Combined Science content on crude oil and hydrocarbons, covering the idea of a homologous series, naming and writing formulae for alkanes and alkenes, using bromine water to distinguish saturated from unsaturated hydrocarbons, and the addition reactions alkenes undergo with hydrogen, steam and halogens.
Before you start
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Method
- To decide whether a hydrocarbon is an alkane or an alkene from its molecular formula, compare it with the general formulae: CnH2n+2 for a saturated alkane, or CnH2n for an unsaturated alkene with one carbon-to-carbon double bond.
- To name a straight-chain alkane or alkene, count the carbon atoms to choose the correct prefix (meth- for 1, eth- for 2, prop- for 3, but- for 4), then add the ending '-ane' for an alkane or '-ene' for an alkene.
- When asked to test for an alkene, describe the bromine water test precisely: shake the sample with orange bromine water; a colour change to colourless confirms an alkene, while an alkane leaves the bromine water unchanged (orange).
- For an addition reaction question, identify what is adding across the C=C double bond (hydrogen, steam, or a halogen), name the correct catalyst if one is needed, and write the single organic product with no other by-product.
- When drawing or describing a displayed formula, show every atom and every bond, including the C=C double bond of an alkene (drawn as two parallel lines between the two carbon atoms).
- Check any proposed structure carefully: every carbon atom in an alkane must have exactly four single bonds, and every carbon atom in an alkene, other than the two joined by the double bond, must still have four single bonds.
Worked example
A student bubbles a gas through orange bromine water in two separate test tubes: gas A leaves the bromine water orange and unchanged, while gas B turns the bromine water colourless. Identify which gas is an alkene and which is an alkane, and explain your reasoning.
- Recall the rule for the bromine water test: an alkene decolourises bromine water because it undergoes an addition reaction with the bromine across its C=C double bond.
- Recall that an alkane does not react with bromine water because it is saturated and has no C=C double bond to react, so the bromine water stays orange.
- Apply this to gas A: the bromine water stayed orange and unchanged, so gas A did not react.
- Apply this to gas B: the bromine water turned colourless, so gas B reacted with the bromine.
- Final answer: gas A is an alkane (no reaction); gas B is an alkene (decolourised the bromine water).
Practice questions
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Q1Define the term 'homologous series'.Show answer
Answer: A family of compounds with the same general formula and similar chemical properties, showing a gradual change in physical properties (such as boiling point) as chain length increases, where each member differs from the next by a CH2 unit.
Q2State the general formula of the alkenes.Show answer
Answer: CnH2n
Q3Name the alkene with the molecular formula C4H8.Show answer
Answer: Butene.
Q4Describe the colour change seen when an alkene is shaken with bromine water.Show answer
Answer: The bromine water changes from orange to colourless.
Q5Name the catalyst used when an alkene reacts with hydrogen to form an alkane.Show answer
Answer: Nickel.
Q6State the type of reaction that occurs when bromine adds across the double bond of an alkene.Show answer
Answer: An addition reaction.
Q7State the number of covalent bonds a carbon atom forms in a saturated hydrocarbon.Show answer
Answer: Four (four single covalent bonds).
Q8Propene (C3H6) reacts with steam in the presence of a catalyst. Name the catalyst, and name the type of compound formed.Show answer
Answer: Phosphoric acid catalyst; an alcohol (propanol) is formed.
Exam-style questions
Written in the style of a GCSE Science exam paper, with a full mark scheme.
A student is given two unlabelled gas jars, one containing ethane and one containing ethene. Describe a chemical test the student could use to identify which gas jar contains ethene, including the result that would confirm it.
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(a) Give the molecular formula of pentene, an alkene with 5 carbon atoms. (b) Pentene reacts with hydrogen gas in the presence of a nickel catalyst. Name the type of reaction that occurs, and give the molecular formula of the product formed.
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A technician has an unlabelled sample of a gaseous hydrocarbon and needs to determine whether it is an alkane or an alkene. Describe a test the technician could carry out, and explain, in terms of bonding, why the two types of hydrocarbon behave differently in this test.
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Free printable worksheet
Want more practice on paper? Download the alkanes, alkenes and crude oil worksheet pack - 14 pages of exam-style questions with a full mark scheme. One email opens every download in this browser for 14 days - no account, no card. Print it for personal and classroom use.
This topic is chapter 24 of GCSE Chemistry Workbook, the whole course as one free printable PDF.
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