AQA A-Level Chemistry: Alkenes (7405) – Practice Questions

These are original questions written for Marlbridge, for revision and practice on this content. They are not reproduced past-paper questions, and they do not replicate the exam’s exact structure, question count or mark tariffs – examination boards hold copyright in their own papers. Use these alongside the official past papers from your board or school.

These questions cover section 3.3.4 Alkenes of the AQA AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: 3.3.4.1 Structure, bonding and reactivity, 3.3.4.2 Addition reactions of alkenes and 3.3.4.3 Addition polymers. All of it is AS content, so nothing here is A-level only. The topic is examined on AS Paper 2 and A-level Paper 2, and A-level Paper 3 can test any content.

Learn the ideas first in the alkenes study guide and the alkenes revision notes. The course hub is AQA A-Level Chemistry, the printable checklist lists every specification point, and the free 10-minute diagnostics show where your gaps are. Mole calculations are practised further in the mole practice questions. Questions 7, 10 and 12 need a calculator. Use Ar: H = 1.0, C = 12.0, Br = 79.9.

Questions

1. Which carbocation is the most stable?

• A CH₃CH₂⁺
• B CH₃CH₂CH₂⁺
• C (CH₃)₂CH⁺
• D (CH₃)₂C⁺CH(CH₃)₂

[1]

2. Explain why ethene reacts with electrophiles but ethane does not. [3]

3. Propene reacts with bromine at room temperature.

(a) Name the type of mechanism. [1] (b) Outline the mechanism for this reaction. [4] (c) Name the organic product. [1]

4. Describe a chemical test that distinguishes hexane from hex-1-ene. Give the result for each compound. [3]

5. Pent-1-ene reacts with hydrogen bromide. Give the structural formulas of the major product and the minor product. [2]

6. Propene reacts with cold, concentrated sulfuric acid.

(a) Identify the electrophile. [1] (b) Outline the mechanism for the formation of the major product. [4]

7. A sample of 1.12 g of a non-cyclic alkene reacts with exactly 3.20 g of bromine, Br₂. One Br₂ molecule adds to each alkene molecule.

(a) Calculate the Mr of the alkene and deduce its molecular formula. [3] (b) Name two alkenes, which are structural isomers, that have this molecular formula. [2]

8. This question is about addition polymers.

(a) 1,1-dichloroethene is CH₂=CCl₂. Draw the repeating unit of the polymer it forms and name the polymer. [2] (b) A section of another polymer is –CH(CH₃)–CH(CH₃)–CH(CH₃)–CH(CH₃)–. Draw its repeating unit and name its monomer. [2] (c) Explain why the polymer in part (b) is unreactive. [2]

9. Poly(ethene) and poly(chloroethene) are both solids at room temperature.

(a) Explain why poly(ethene) is a solid even though only van der Waals forces act between its chains. [2] (b) Name one other type of intermolecular force between poly(chloroethene) chains and explain why it is present. [2] (c) Poly(chloroethene) is used both for rigid window frames and for flexible electrical cable insulation. Explain how the same polymer can suit both uses. [2]

10. A sample of poly(propene) has an average Mr of 2.10 × 10⁵.

(a) Calculate the average number of propene units in one chain. [2] (b) State the atom economy of addition polymerisation and give a reason. [1]

11. Hex-3-ene, CH₃CH₂CH=CHCH₂CH₃, and 2-methylpent-1-ene, CH₂=C(CH₃)CH₂CH₂CH₃, are isomers.

(a) Hex-3-ene gives only one product with hydrogen bromide. Name the product and explain why only one product forms. [2] (b) 2-methylpent-1-ene gives two products with hydrogen bromide. Name the major product and the minor product. [2] (c) Explain, with reference to the intermediates, why more of the major product forms. [3] (d) State why each alkene gives only one product with bromine. [1]

12. A student bubbles 0.700 g of ethene into excess bromine and obtains 3.99 g of pure 1,2-dibromoethane.

(a) Write an equation for the reaction. [1] (b) Calculate the percentage yield. [3] (c) State the atom economy of this reaction. [1] (d) Suggest two reasons why the percentage yield is less than 100%. [2]

Answers

1. D [1]. It is tertiary: three alkyl groups release electron density towards the positive carbon. [1] Examiner insight: C is the usual wrong choice; check how many carbon groups are bonded to the C⁺, not how many carbons the ion has in total.

2. The C=C double bond in ethene is a centre of high electron density [1]. Electrophiles are electron pair acceptors, so they are attracted to it and accept a pair of electrons [1]. Ethane is saturated: it has only single C–C and C–H bonds and no region of high electron density [1]. [3] Examiner insight: “Ethene has a double bond” alone does not explain anything; the phrase “high electron density” is what links the structure to the attack.

3. (a) Electrophilic addition [1] (b) Br₂ shown with an induced dipole, Brδ+–Brδ− (as it approaches the high electron density) [1]; arrow from the C=C bond to the δ+ Br and arrow from the Br–Br bond to the δ− Br [1]; carbocation CH₃C⁺HCH₂Br with Br⁻ [1]; arrow from a lone pair on Br⁻ to the C⁺ [1]. (c) 1,2-dibromopropane [1]. [6] Examiner insight: Arrows must be placed precisely: an arrow that starts from a carbon atom, rather than from the C=C bond, does not show electrons leaving the double bond and is unlikely to be credited.

4. Add bromine water and shake [1]. Hex-1-ene: orange to colourless [1]. Hexane: stays orange (no change) [1]. [3] Examiner insight: “Goes clear” is not the same as colourless, and a result is needed for both compounds, not just the alkene.

5. Major: CH₃CH₂CH₂CHBrCH₃ (2-bromopentane) [1]. Minor: CH₃CH₂CH₂CH₂CH₂Br (1-bromopentane) [1]. [2] Examiner insight: The question asks for structural formulas; a correct name with no formula may not be accepted when the command asks for the formula.

6. (a) A hydrogen of H₂SO₄ (Hδ+) [1] (b) Arrow from the C=C bond to an H of H₂SO₄ and arrow from that O–H bond to the O [1]; secondary carbocation CH₃C⁺HCH₃ [1]; hydrogensulfate ion with a lone pair on the negative O and an arrow from it to the C⁺ [1]; product CH₃CH(OSO₂OH)CH₃ [1]. [5] Examiner insight: The bond to carbon must come from the negative oxygen; showing the S, or the OH oxygen, bonding to carbon gives the wrong structure and loses the final marks.

7. (a) Mr(Br₂) = 159.8; n(Br₂) = 3.20 / 159.8 = 0.0200 mol [1]. n(alkene) = 0.0200 mol (1 : 1), so Mr = 1.12 / 0.0200 = 55.9, which is 56 to the nearest whole number [1]. CₙH₂ₙ: 14n = 56, n = 4, so C₄H₈ [1]. (b) Any two of: but-1-ene, but-2-ene, 2-methylpropene [1] [1]. [5] Examiner insight: An error in n(Br₂) can usually be carried forward to the Mr, but the formula mark needs a whole-number n consistent with CₙH₂ₙ.

8. (a) Repeating unit with C–C, two H on one carbon, two Cl on the other, extension bonds and n: –[CH₂–CCl₂]ₙ– [1]; poly(1,1-dichloroethene) [1]. (b) Repeating unit –[CH(CH₃)–CH(CH₃)]ₙ– [1]; monomer but-2-ene (CH₃CH=CHCH₃) [1]. (c) The chain is saturated, with no C=C bond for electrophiles to attack [1]. It has only strong, non-polar C–C and C–H bonds [1]. [6] Examiner insight: In (b) a three- or four-carbon repeating unit is wrong even if it repeats; the unit must be the two carbons that came from one monomer.

9. (a) The chains are very long, with very many electrons [1], so there are many van der Waals interactions along each chain and the total attraction between chains is large [1]. (b) Permanent dipole–dipole forces [1]; the C–Cl bond is polar because Cl is more electronegative than C [1]. (c) Window frames use unplasticised PVC, which is rigid [1]. For cable insulation a plasticiser is added; its molecules push the chains apart and weaken the forces between them, so the chains slide and the PVC is flexible [1]. [6] Examiner insight: In (a) talking about breaking covalent bonds when the polymer melts contradicts the answer; the forces are between chains, not within them.

10. (a) Mr(C₃H₆) = 3(12.0) + 6(1.0) = 42.0 [1]; 2.10 × 10⁵ / 42.0 = 5000 [1]. (b) 100%, because the polymer is the only product [1]. [3] Examiner insight: The monomer and the repeating unit have the same formula, so dividing by the monomer’s Mr is valid; using Mr(CH₂) = 14.0 gives an answer three times too large.

11. (a) 3-bromohexane [1]. Hex-3-ene is symmetrical, so both possible carbocations are identical and give the same product [1]. (b) Major: 2-bromo-2-methylpentane [1]. Minor: 1-bromo-2-methylpentane [1]. (c) The major product forms via a tertiary carbocation and the minor via a primary carbocation [1]. Alkyl groups release electron density and stabilise the positive charge; the tertiary carbocation has three, so it is more stable [1]. The more stable carbocation forms more readily, so more product forms through it [1]. (d) Both atoms that add are the same (Br), so either route gives the same product [1]. [8] Examiner insight: Part (c) needs the intermediates named as tertiary and primary; “the tertiary product is more stable” talks about the product, not the carbocation, and misses the point.

12. (a) C₂H₄ + Br₂ → C₂H₄Br₂ (CH₂=CH₂ + Br₂ → CH₂BrCH₂Br) [1] (b) n(C₂H₄) = 0.700 / 28.0 = 0.0250 mol [1]. Mr(C₂H₄Br₂) = 187.8; theoretical mass = 0.0250 × 187.8 = 4.695 g [1]. Yield = 3.99 / 4.695 × 100 = 85.0% [1]. (c) 100% [1] (d) Any two: some ethene passes through without reacting; product lost during transfer or purification; impurities in the ethene [1] [1]. [7] Examiner insight: Allow error carried forward from a wrong Mr in (b); give the yield to 3 significant figures to match the data.

Where marks are usually lost

• Curly arrows that start from an atom rather than from the C=C bond or a lone pair.
• Missing the induced dipole on Br₂ or the δ+/δ− on HBr.
• Carbocation + on a carbon that does not match the product given.
• Hydrogensulfate bonded through S or the OH oxygen.
• Explaining major products using the product’s stability instead of the carbocation’s.
• Saying Br₂ gives major and minor products.
• Repeating units longer than two main-chain carbons, or drawn with C=C.
• Describing van der Waals forces as covalent bonds that break on melting.
• Forgetting that plasticiser works by weakening forces between chains.
• Rounding Mr too early in a moles calculation.

Next steps

• Alkenes revision notes
• Alkenes study guide
• Alcohols study guide
• Bonding
• Mole practice questions
• AQA A-Level Chemistry hub
• Printable checklist
• All free 10-minute diagnostics
• Book a free trial class

Official syllabus

AQA, AS and A-level Chemistry specification (7404/7405), version 1.2, July 2026, for AS and A-level exams June 2016 onwards: section 3.3.4 Alkenes (3.3.4.1 Structure, bonding and reactivity, 3.3.4.2 Addition reactions of alkenes and 3.3.4.3 Addition polymers).