Edexcel A-Level Chemistry AS Paper 2, June 2023: Question 3

18 marks · Medium difficulty · Open Response

Answer questions about alkenes, including their general formula, bonding, reactions such as hydration and oxidation, polymerisation, mechanism of electrophilic addition, and polymer disposal.

Practise this question

Question

Question text

3 This question is about alkenes and some of their reactions.

(a) Alkenes have the same general formula as cycloalkanes.

Explain why this general formula is different from that of alkanes.

Include the general formulae in your answer.

(3)

(b) Draw the skeletal formula of cyclohexene.

(1)

electron clouds.

Include labels for all the bond types within the molecule.

(3)

(d) The reaction scheme shows some of the reactions of but‑1‑ene.

excess KMnO / H+

CH3CH2CH CH2 C

steam/H3PO4

A + B

A and B are structural isomers. A is butan‑1‑ol.

(i) Draw the fully displayed*P71927A0728*formula of isomerB.

(1)

(ii) Draw the structural formula of product C, which is formed by the reaction of

but‑1‑ene with potassium manganate(VII) in acid conditions.

(1)

(e) The industrial manufacture of margarine involves the addition of hydrogen to

an alkene.

Which catalyst is used for this process?

(1)

A iron

B nickel

C phosphoric acid

D sodium hydroxide

(f ) Polymers can be produced by the addition polymerisation of alkenes.

8 Poly(propene) can be made from propene.

Draw the structure of poly(propene), showing*P71927A0828*tworepeat units.

(1)

(g) Propene reacts with hydrogen bromide to form a mixture of products.

(i) Complete the mechanism of the reaction of propene with hydrogen bromide

to form 2‑bromopropane.

Include curly arrows and any relevant lone pairs and dipoles.

(4)

H CH3

C C

H H

*P71927A0928*

(ii) What is the type and mechanism for the reaction of hydrogen bromide

with propene?

(1)

A electrophilic addition

B electrophilic substitution

C nucleophilic addition

D nucleophilic substitution

(h) Various methods are used to reduce the problem of plastic waste, including the

incineration, recycling and introduction of biodegradable polymers.

(i) Describe how the toxic gas hydrogen chloride produced during the*P71927A01028*

incineration of poly(chloroethene) could be removed.

(1)

(ii) Give a reason why some polymers cannot be recycled.

(1)

(Total for Question 3 = 18 marks)

Mark scheme

Show the mark scheme

Question

Acceptable Answer Additional Guidance Mark

Number

3(a) An answer that makes reference to the following points: (3)

• general formula for an alkene and/or cycloalkane General formula of alkenes and/or cycloalkanes = CnH2n

and

general formula of an alkane (1) General formula of alkanes = CnH2n+2

• alkenes have a double bond

(with two less hydrogens) (1)

• cycloalkanes have carbons in a ring

(resulting in two less hydrogens) (1)

Question

Acceptable Answer Additional Guidance Mark

Number

3(b) Example of skeletal formula (1)

• skeletal formula of cyclohexene

Ignore bond lengths and bond angles

Ignore position of C=C

Do not award any other type of formula

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Acceptable Answer Additional Guidance Mark

Number

3(c) An answer that makes reference to the following points: Example of diagram (3)

• /sigma bond drawn and identified between both (1)

C atoms

• (labelled) diagram identifying π/pi bond above (1)

and below /sigma bond between C atoms

• C−H bonds drawn and at least one identified as a (1)

bond

and π bonds may be shown as overlap of individual orbitals

Allow π bonds shown as banana shaped and bonds shown as

lozenge-shaped

If a double bond is drawn between the two carbon atoms then

M1 is lost but M2 can still be scored from appropriate pi bond

labelled drawing

Rescue mark - If no other mark is awarded and lines are shown

instead of shapes, allow (1) for identification of and π bonds

Question

Acceptable Answer Additional Guidance Mark

Number

3(d)(i) Example of displayed formula (1)

• fully displayed formula of isomer B (butan-2-ol)

Allow O−H shown as OH and ignore connectivity unless

shown horizontally as −HO

Do not award methyl and ethyl groups to be shown as CH3 and

C2H5

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Acceptable Answer Additional Guidance Mark

Number

3(d)(ii) Example of structural formula (1)

• structural formula of butane-1,2-diol

CH3CH2CH(OH)CH2OH

Allow partially displayed formula eg

CH3CH2CH CH2

OH OH

Allow C2H5

Ignore connectivity of vertical OH groups

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Answer Mark

Number

3(e) The only correct answer is B (nickel) (1)

A is not correct because iron is used in the Haber process

C is not correct because a heterogeneous catalyst is needed for this reaction

D is not correct because a heterogeneous catalyst is needed for this reaction

Question

Acceptable Answer Additional Guidance Mark

Number

3(f) Example of suitable diagram (1)

• poly(propene) showing two repeat units

Allow diagram with square or round brackets

Allow diagram with n or n/2

Allow isotactic or syndiotactic structures

Allow displayed formula

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Acceptable Answer Additional Guidance

Number Mark

3(g)(i) Example of mechanism (4)

• curly arrow from double bond to H of HBr

and

dipole on HBr (1)

• curly arrow from H−Br bond to Br (or just

beyond Br) (1)

• intermediate (1)

• lone pair on Br−

and

curly arrow from lone pair to C+ (1)

Formation of 1-bromopropane can score M1, M2 and M4

Question

Answer Mark

Number

3(g)(ii) The only correct answer is A (electrophilic addition) (1)

B is not correct because this is an addition reaction not a substitution

C is not correct because this is electrophilic not nucleophilic

D is not correct because this is electrophilic not nucleophilic and addition not substitution

Question

Acceptable Answer Additional Guidance Mark

Number

3(h)(i) An answer that makes reference to the following point: (1)

• scrubbing with alkali / pass through any alkaline Allow reference just to passing over an alkali

solution eg NaOH(aq) /over solid base e.g. CaO Ignore neutralisation without some specified alkali

Ignore reference to dissolving in water

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Acceptable Answer Additional Guidance Mark

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3(h)(ii) Possible reasons (1)

• any one reason Polymer degrades during heating (for recycling)

Heavy pigmentation/ colour of plastic to be recycled

Thermoset / 3D polymers cannot be recycled (because of their structure)

It’s difficult to separate polymers into types

Do not award reference to breaking of the long polymer chains

COMMENT

Allow reference to difficulty to break down because of the strong bonds

(Total for Question 3 = 18 marks)

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Acceptable Answer Additional Guidance Mark

Number

How to answer it

Study Guide: Alkenes and Their Reactions

What this question tests

This comprehensive AS Chemistry question assesses your understanding of alkene bonding (sigma and pi bonds), general formulae, skeletal/structural/displayed formulas, electrophilic addition mechanisms involving hydrogen halides, industrial hydrogenation catalysts, polymerisation structure, and environmental aspects of polymer disposal.

Question Part (a)

General Formulae Comparison

✅ Correct Answer

Alkenes and cycloalkanes share the same general formula: CnH₂n . Alkanes have the general formula CnH₂n₊₂ . Alkenes contain a double bond (giving them two fewer hydrogens than alkanes), while cycloalkanes contain a ring structure (also resulting in two fewer hydrogens).

💡 Key Knowledge

Degree of unsaturation (Index of Hydrogen Deficiency): Each ring or double bond removes 2 hydrogen atoms from the corresponding alkane formula.

Mark breakdown: 3 marks total (1 mark for stating the general formulae correctly, 1 mark for linking alkenes to the double bond/fewer hydrogens, 1 mark for linking cycloalkanes to the ring structure).

Question Part (b)

Skeletal Formula of Cyclohexene

✅ Correct Answer

A six-membered ring drawn as a hexagon, containing one double bond inside the ring.

❌ Common Errors

Drawing a fully displayed or structural formula instead of a skeletal one, or forgetting to include the double bond inside the ring (which would make it cyclohexane).

Mark breakdown: 1 mark for the correct skeletal formula.

Question Part (c)

Orbital Overlap in Ethene

✅ Correct Answer

A clear diagram showing:

Sigma (σ) bonds formed by linear overlap of atomic orbitals directly between carbon atoms and between carbon and hydrogen atoms.
A Pi (π) bond formed by sideways overlap of p orbitals, positioned above and below the plane of the C-C sigma bond.

🧠 Exam Technique

Ensure labels explicitly identify both the sigma bonds between carbon-carbon/carbon-hydrogen and the pi bond above and below the carbon-carbon axis.

Mark breakdown: 3 marks (1 for σ bond between C atoms, 1 for π bond above and below, 1 for C-H σ bonds).

Question Part (d)

Reaction Scheme: Isomers and Oxidation

✅ Correct Answer

(i) Isomer B (butane-2-ol): Fully displayed formula showing a 4-carbon chain with an -OH group attached to carbon 2, and all individual C-H and O-H bonds drawn out.
(ii) Product C: Structural formula for butane-1,2-diol: CH₂(OH)CH(OH)CH₂CH₃ (or equivalent clear structural/partially displayed representation).

❌ Common Errors

Condensing groups incorrectly (e.g., writing CH₃ instead of showing bonds in fully displayed formulas, or omitting hydrogen atoms on carbon chains).

Mark breakdown: 2 marks total (1 mark for part i, 1 mark for part ii).

Question Part (e)

Catalyst for Margarine Manufacture

✅ Correct Answer

B (nickel)

💡 Key Knowledge

Nickel is used as a heterogeneous catalyst for the hydrogenation of double bonds in unsaturated vegetable oils to manufacture margarine. Iron is used in the Haber process.

Mark breakdown: 1 mark.

Question Part (f)

Poly(propene) Structure

✅ Correct Answer

Draw the polymer backbone as -CH₂-CH(CH₃)-CH₂-CH(CH₃)- with open bonds extending outside brackets or clear repeating units showing at least two complete repeat units.

🧠 Exam Technique

Make sure the main carbon chain has single bonds only, with the methyl ( -CH₃ ) side chains correctly positioned on alternate carbons.

Mark breakdown: 1 mark for showing two correct repeat units.

Question Part (g)

Mechanism of Electrophilic Addition

✅ Correct Answer

(i) Mechanism:

Curly arrow from the C=C double bond to the partially positive hydrogen atom (H) of H-Br.
Partial charges ( δ+ on H, δ- on Br) shown on H-Br, with a curly arrow breaking the H-Br bond towards Br.
Carbocation intermediate structure formed with positive charge on the correct carbon.
Lone pair of electrons on the bromide ion ( Br⁻ ) with a curly arrow pointing towards the positive carbon center.

(ii) Type and Mechanism: A (electrophilic addition)

❌ Common Errors

Starting curly arrows from atoms rather than from the bond electrons, or placing partial charges incorrectly. Remember that electrophiles are electron-pair acceptors.

Mark breakdown: 5 marks total (4 for the mechanism steps, 1 for identifying electrophilic addition).

Question Part (h)

Waste Disposal and Polymer Recycling

✅ Correct Answer

(i) Scrubbing acidic gases with an alkali (e.g., passing flue gases over solid calcium oxide or through aqueous sodium hydroxide).
(ii) Any valid reason why polymers cannot be recycled, such as polymer degradation during heating, heavy pigmentation, being thermosetting plastics, or difficulty sorting mixed polymer types.

💡 Key Knowledge

Incineration of chlorinated polymers like poly(chloroethene) / PVC releases toxic hydrogen chloride gas ( HCl ), which must be neutralized to prevent environmental harm.

Mark breakdown: 2 marks total (1 mark for part i, 1 mark for part ii).

Topics

Organic Chemistry · Core Practicals · Physical Chemistry · Topic 6: Organic Chemistry I · Topic 2: Bonding and Structure

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 2, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.