Alkanes
Alkanes are the main constituent of crude oil, which is an important raw material for the chemical industry.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/a-level/chemistry/organic-chemistry/alkanes-a-level.
Topic preview: Alkanes
Sample stems from the StudyVector question bank (AQA · Edexcel · OCR) — not generic filler text.
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Coverage and provenance
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Topic explanation
Alkanes are saturated hydrocarbons that are the main components of crude oil. At A-Level, the focus is on their fractional distillation, cracking (to produce more useful shorter-chain alkanes and alkenes), and reformation. A key reaction is their free-radical substitution with halogens in the presence of UV light, which proceeds via a three-step mechanism: initiation, propagation, and termination.
Alkanes is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In A-Level Chemistry, the goal is to recognise how the topic appears in a question, identify the command word, and decide what evidence, method, or vocabulary earns marks. StudyVector keeps this page tied to AQA · Edexcel · OCR language where coverage is available, then routes practice towards the same topic so revision moves from explanation into retrieval.
A strong revision session starts with a short recall check. Write down the rule, definition, process, or method linked to Alkanes before looking at any notes. Then answer one exam-style prompt and compare your answer with the mark-scheme logic: did you make a clear point, support it with the right step, and avoid drifting into a nearby topic? This matters because many lost marks come from almost-correct answers that do not match the expected structure.
Use this guide as the first layer: understand the topic, look at the worked examples, complete the mini quiz, then move into full practice. The full StudyVector practice loop is designed to capture whether mistakes are caused by knowledge, method, language, or timing. That distinction is important. If the error is factual, you need reteaching. If the error is method-based, you need a worked retry. If the error is wording, you need command-word calibration. That is how Alkanes becomes a controlled revision target rather than another page in a folder.
Lost marks → repair task
Why marks are usually lost here
These are the error patterns StudyVector looks for after an attempt. The goal is not a generic explanation; it is one repair move and one follow-up question.
Unit, formula, or method slip
Examiner move: Select the correct method and keep units, substitutions, signs, and rounding visible.
Repair drill: Redo the calculation or method line slowly, naming the formula before substituting values.
Missing chain of reasoning
Examiner move: Show the link between point, method, evidence, and conclusion instead of jumping to the final line.
Repair drill: Write the missing because/therefore step, then retry one isomorphic question.
Command-word miss
Examiner move: Answer the action in the command word before adding extra detail.
Repair drill: 60-second rewrite: start the answer with explain, compare, evaluate, state, or calculate in mind.
Mini quiz
Use these checks before full practice. They test topic recognition, exam technique, and whether you can connect the explanation to a marked response.
1. What should you check first when a Alkanes question appears in A-Level Chemistry?
- A.The command word and the exact topic focus
- B.The longest paragraph in your notes
- C.A memorised answer from a different topic
2. Which revision action gives the strongest evidence that Alkanes is improving?
- A.Rereading the explanation twice
- B.Answering a timed exam-style question and reviewing lost marks
- C.Highlighting every key phrase in the topic notes
Sample questions
Topic-specific public question previews are still being reviewed. We keep them off public pages until the topic match is safe.
Exam tips
- Read the command word carefully — "explain" needs reasons; "state" expects a short fact.
- For Alkanes, show structured working even when you are practising multiple choice — it builds accuracy under time pressure.
- Mark yourself against the mark scheme style: one clear point per mark, in logical order.
- Come back to this topic after a day or two; short spaced reviews beat one long cram.
Worked examples
Example 1
Modelled exam response
Show the propagation steps for the reaction of methane with chlorine. Step 1: A chlorine radical attacks a methane molecule, abstracting a hydrogen atom to form a methyl radical and a molecule of hydrogen chloride. Cl- + CH4 -> -CH3 + HCl. Step 2: The methyl radical is also very reactive and attacks a chlorine molecule, forming chloromethane and a new chlorine radical. -CH3 + Cl2 -> CH3Cl + Cl-. This new chlorine radical can then repeat step 1, continuing the chain reaction.
Example 2
Identify the task before answering
Question type: a Alkanes prompt asks for a clear response in A-Level Chemistry. Step 1: underline the command word. Step 2: name the exact part of Alkanes being tested. Step 3: decide whether the mark scheme wants a definition, method, explanation, comparison, or calculation. Why it works: most weak answers fail before the content starts because they answer the topic generally rather than the exact exam task.
Example 3
Turn feedback into a repair task
Suppose your answer shows partial understanding but loses marks for precision. First, rewrite the missing mark as a short target: "I need to state the mechanism, unit, reason, or evidence explicitly." Then answer one similar question without notes. Finally, compare the second attempt with the first and check whether the same mark was recovered. Why it works: Alkanes improves faster when feedback creates a specific retry, not another passive reading session.
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Common mistakes
- Confusing the conditions for cracking with those for free-radical substitution. Cracking requires high temperatures and a catalyst, whereas free-radical substitution requires UV light.
- Incorrectly drawing the mechanism for free-radical substitution, for example by using double-headed arrows (which show movement of an electron pair) instead of single-headed arrows (for movement of a single electron).
- Forgetting that free-radical substitution can lead to a mixture of products, as substitution can occur at different positions on the carbon chain and multiple substitutions can happen.
Exam board notes
All boards cover the mechanism of free-radical substitution in detail. AQA often asks about the environmental impact of using fossil fuels and the role of catalytic converters. Edexcel may focus on the industrial processes of cracking and reforming. OCR questions frequently require students to predict the products of free-radical substitution and explain why a mixture of products is formed.
FAQs
Why are alkanes relatively unreactive?
Alkanes are unreactive because they only contain strong, non-polar C-C and C-H sigma bonds. There are no regions of high electron density to attract electrophiles, and the bonds require a large amount of energy to break.
What is the difference between thermal and catalytic cracking?
Thermal cracking uses high temperatures (around 700-1200K) and high pressures (up to 7000 kPa) and produces a high proportion of alkenes. Catalytic cracking uses a lower temperature and pressure but requires a zeolite catalyst, and tends to produce aromatic compounds and motor fuels.
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