Alkanes
Cambridge A-Level Chemistry (9701) · Unit 14: Hydrocarbons · 10 flashcards
Alkanes is topic 14.1 in the Cambridge A-Level Chemistry (9701) syllabus , positioned in Unit 14 — Hydrocarbons , alongside Alkenes. In one line: Complete combustion of alkanes occurs when there is a sufficient supply of oxygen. The alkane reacts with oxygen to produce carbon dioxide (CO₂) and water (H₂O).
Marked as A2 Level: examined at A Level in Paper 4 (A Level Structured Questions) and Paper 5 (Planning, Analysis and Evaluation). It is not tested on the AS-only papers (Papers 1, 2 and 3).
The deck below contains 10 flashcards — 5 definitions and 5 key concepts — covering the precise wording mark schemes reward. Use the 5 definition cards to lock down command-word answers (define, state), then move on to the concept and calculation cards to handle explain, describe, calculate and compare questions.
Describe the complete combustion of alkanes, including the products formed
Complete combustion of alkanes occurs when there is a sufficient supply of oxygen. The alkane reacts with oxygen to produce carbon dioxide (CO₂) and water (H₂O).
What the Cambridge 9701 syllabus says
Official 2025-2027 spec · A2 LevelThese are the exact learning outcomes Cambridge sets for this topic. The candidate is expected to be able to do each of these on the relevant paper.
- recall the reactions (reagents and conditions) by which alkanes can be produced: (a) addition of hydrogen to an alkene in a hydrogenation reaction, H2(g) and Pt/Ni catalyst and heat (b) cracking of a longer chain alkane, heat with Al 2O3
- describe: (a) the complete and incomplete combustion of alkanes (b) the free-radical substitution of alkanes by Cl 2 or Br2 in the presence of ultraviolet light, as exemplified by the reactions of ethane
- describe the mechanism of free-radical substitution with reference to the initiation, propagation and termination steps
- suggest how cracking can be used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions
- understand the general unreactivity of alkanes, including towards polar reagents in terms of the strength of the C–H bonds and their relative lack of polarity
- recognise the environmental consequences of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons arising from the combustion of alkanes in the internal combustion engine and of their catalytic removal
Cambridge syllabus keywords to use in your answers
These are the official Cambridge 9701 terms tagged to this section. Mark schemes credit responses that use the exact term — weave them into your answers verbatim rather than paraphrasing.
Tips to avoid common mistakes in Alkanes
- › Check if elimination products form a side chain or ring C=C bond, as this determines the optical isomerism of subsequent reaction products.
- › Treat the cyclohexene ring as non-planar and ensure straight-chain alkenes with the formula C6H10 have two C=C double bonds.
- › Identify every C=C double bond in the molecule and determine if cleavage results in a ketone, aldehyde, or carboxylic acid.
- › Only count hybridised orbitals belonging to carbon atoms; internal carbons in alkenes are sp2 hybridised, while terminal methyl carbons are sp3.
Describe the reagents and conditions required to produce an alkane from an alkene via hydrogenation.
Alkenes react with hydrogen gas (H₂(g)) in the presence of a platinum (Pt) or nickel (Ni) catalyst under heat. This is an addition reaction where hydrogen atoms add across the double bond, saturating the molecule to form an alkane.
Outline the process of cracking a longer chain alkane, including the necessary conditions.
Cracking involves breaking down large alkane molecules into smaller, more useful alkanes and alkenes. This is achieved by heating the alkane with aluminium oxide (Al₂O₃) as a catalyst.
Describe the complete combustion of alkanes, including the products formed.
Complete combustion of alkanes occurs when there is a sufficient supply of oxygen. The alkane reacts with oxygen to produce carbon dioxide (CO₂) and water (H₂O).
Describe the incomplete combustion of alkanes, including the products formed and its environmental impact.
Incomplete combustion occurs when there is a limited supply of oxygen. It produces carbon monoxide (CO), water (H₂O), and sometimes soot (carbon particles). Carbon monoxide is a toxic gas.
Describe the initiation step in the free-radical substitution of ethane by chlorine.
The initiation step involves the homolytic fission of a chlorine molecule (Cl₂) by ultraviolet (UV) light. This creates two chlorine free radicals (Cl•).
Describe the propagation steps in the free-radical substitution of ethane by chlorine.
Propagation involves two steps: 1) A chlorine free radical (Cl•) reacts with ethane (C₂H₆) to form an ethyl radical (C₂H₅•) and HCl. 2) The ethyl radical (C₂H₅•) reacts with another chlorine molecule (Cl₂) to form chloroethane (C₂H₅Cl) and another chlorine free radical (Cl•), which can then continue the chain reaction.
Describe the termination steps in the free-radical substitution of ethane by chlorine.
Termination steps involve the combination of two free radicals to form a stable molecule, removing free radicals from the reaction mixture and stopping the chain reaction.
Explain how cracking is used to obtain more useful alkanes and alkenes of lower Mr from heavier crude oil fractions.
Cracking breaks down long-chain hydrocarbons from crude oil into smaller, more valuable molecules. This increases the yield of gasoline and other useful chemicals, like alkenes for polymer production, which have a higher demand than the original long-chain alkanes.
Explain the general unreactivity of alkanes towards polar reagents.
Alkanes are generally unreactive due to the strong C–H bonds and their lack of polarity. Polar reagents are attracted to areas of high electron density or charge, which are absent in alkanes due to the similar electronegativity of carbon and hydrogen.
Outline the environmental consequences of carbon monoxide (CO), oxides of nitrogen (NOx), and unburnt hydrocarbons from internal combustion engines.
CO is a toxic gas that reduces oxygen transport in the blood. NOx contributes to acid rain and respiratory problems. Unburnt hydrocarbons contribute to smog and can be carcinogenic. Catalytic converters help to reduce these pollutants.
More Chemistry flashcards
Browse every 9701 flashcard topic by syllabus area.
All Chemistry FlashcardsMore topics in Unit 14 — Hydrocarbons
Alkanes sits alongside these A-Level Chemistry decks in the same syllabus unit. Each uses the same spaced-repetition system, so progress in one informs the next.
Key terms covered in this Alkanes deck
Every term below is defined in the flashcards above. Use the list as a quick recall test before your exam — if you can't define one of these in your own words, flip back to that card.
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