Alkanes & Alkenes
Alkanes and alkenes are two important homologous series of hydrocarbons. Alkanes are saturated, meaning they only have single C-C bonds, and are generally unreactive. Alkenes are unsaturated, containing at least one C=C double bond, which makes them more reactive than alkanes. They undergo addition reactions.
Full topic guide: the detailed syllabus page with worked examples and common mistakes lives at studyvector.co.uk/gcse/chemistry/organic-chemistry/alkanes-alkenes.
Topic preview: Alkanes & Alkenes
Sample stems from the StudyVector question bank (AQA · Edexcel · OCR) — not generic filler text.
More questions are being linked to this topic. You can still start adaptive practice after you create a free account.
Coverage and provenance
What this page is based on
StudyVector does not present unsupported question coverage as complete. Read how questions are selected and reviewed.
Topic explanation
Alkanes and alkenes are two important homologous series of hydrocarbons. Alkanes are saturated, meaning they only have single C-C bonds, and are generally unreactive. Alkenes are unsaturated, containing at least one C=C double bond, which makes them more reactive than alkanes. They undergo addition reactions.
Alkanes & Alkenes is easiest to revise when it is treated as a precise exam behaviour, not a loose note-taking category. In GCSE 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 & Alkenes 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 & Alkenes 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 & Alkenes question appears in GCSE 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 & Alkenes 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 & Alkenes, 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
Ethene (an alkene, C₂H₄) reacts with bromine (Br₂) in an addition reaction. The C=C double bond breaks, and a bromine atom adds to each carbon atom, forming dibromoethane (C₂H₄Br₂). The orange bromine water is decolourised.
Example 2
Identify the task before answering
Question type: a Alkanes & Alkenes prompt asks for a clear response in GCSE Chemistry. Step 1: underline the command word. Step 2: name the exact part of Alkanes & Alkenes 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 & Alkenes improves faster when feedback creates a specific retry, not another passive reading session.
Next revision routes from this subject
Good topic pages should lead naturally into the next useful page. Use these links to stay inside the same strand or jump into the next topic area without starting your search again.
Stay in the same topic area
Explore the wider subject map
Common mistakes
- Confusing the reactions of alkanes and alkenes. Alkanes undergo combustion and substitution reactions (e.g., with halogens in UV light), while alkenes undergo addition reactions at the double bond.
- Forgetting the test to distinguish between an alkane and an alkene. Alkenes will decolourise bromine water (turn it from orange to colourless), while alkanes will not.
- Drawing the structure of an alkene with the wrong number of hydrogen atoms. Remember that each carbon atom can only form a total of four bonds.
Exam board notes
The chemistry of alkanes and alkenes is a major part of the organic chemistry section for all boards. You must know their structures, general formulas, and characteristic reactions, including the bromine water test. Cracking is also a key industrial process to understand.
FAQs
What is cracking?
Cracking is the process of breaking down large, less useful hydrocarbon molecules (long-chain alkanes) into smaller, more useful ones, including alkanes and alkenes. This is done using high temperatures and a catalyst.
Are alkanes or alkenes more useful?
Both are useful. Alkanes are excellent fuels. Alkenes are more reactive and are used to make polymers (plastics) and other important organic chemicals.
More on StudyVector
Full practice set
The complete adaptive question bank for this topic — personalised to your weak areas — is available after you sign in. Your session can start on this topic immediately.