AQA A-Level Chemistry Paper 2, June 2025: Question 3

20 marks · Hard difficulty · Long Answer

Analyze the structure, properties, synthesis, and mechanisms of various polymers including PVC, nylon 6,6, and nylon 6, and explain the effect of plasticisers using intermolecular forces.

Practise this question

Question

An eight-part exam question about polymers, starting with PVC structure and monomer identification. It includes questions on the biodegradability and modification of PVC using plasticisers like DIBP and DEHA. It features reaction equations and mechanisms, including the formation of DIBP, the structure of DEHA, a table showing DEHA absorption in different foods, and incomplete mechanisms for the formation of nylon 6,6 and nylon 6, requiring students to draw intermediates and curly arrows.
Question text

03 This question is about polymers.

03.1 A section of the polymer PVC is shown.

Name the monomer used to make PVC.

Draw the structure of the monomer used to make PVC.

[2 marks]

Monomer name

Structure

03.2 State why PVC is not biodegradable.

[1 mark]

03.3 Diisobutyl phthalate (DIBP) is a plasticiser added to PVC to modify its properties.

State how the properties of PVC are modified by the addition of DIBP.

[1 mark]

03.4 An incomplete equation for the reaction used to make DIBP is shown.

Complete the equation.

Give the name of the mechanism for this reaction.

Deduce the empirical formula of DIBP.

[4 marks]

Name of mechanism

Empirical formula of DIBP

03.5 A different plasticiser, DEHA, is added to PVC used for food packaging.

The structure of DEHA is

Identify the organic compound that would react with hexanedioic acid to form DEHA.

[1 mark]

03.6 DEHA can be absorbed into food from PVC packaging.

In an investigation, 1 kg samples of three different foods are stored in PVC packaging

for 30 days.

Table 1 shows the water content, fat content and mass of DEHA in each type of food

after 30 days.

Table 1

Mass of DEHA in food

Food Water content (%) Fat content (%)

*07* after 30 days / mg

Milk 87 3 2.83

Cheese 45 26 10.88

Butter 16 80 24.56

Use your knowledge of intermolecular forces to explain these results.

[3 marks]

03.7 The polymer nylon 6,6 can be made from hexanedioyl dichloride and

hexane-1,6-diamine.

Figure 2 shows an incomplete mechanism for the first steps in this reaction.

Figure 2

Draw the missing intermediate in the box in Figure 2.

Complete Figure 2 by adding the curly arrows and any relevant lone pairs of electrons

involved in these steps in the mechanism.

[4 marks]

03.8 The polymer nylon 6 can be made by heating caprolactam with water.

Figure 3 shows the species involved in the first three steps of the mechanism for

this reaction.

Figure 3

Complete Figure 3 by adding the curly arrows and any relevant lone pairs of electrons

involved in these steps in the mechanism.

[4 marks]

Mark scheme

Show the mark scheme The mark scheme for question 3, detailing the answers for parts 3.1 to 3.8. It shows the structure of chloroethene, explanations for PVC's non-biodegradability and the effect of plasticisers, the completed equation and mechanism name (nucleophilic addition-elimination) for DIBP formation, the structure of 2-ethylhexan-1-ol, and detailed marking points for the mechanisms of nylon 6,6 and nylon 6, including specific curly arrow placements and intermediate structures.

Question Answers Additional comments/Guidelines Mark

M1 Chloroethene Allow vinyl chloride OR chloroethylene

03.1

M2 (2 x AO1)

Carbon Carbon bonds are non polar or (too) strong or not attacked

by nucleophiles

03.2 Or

(1 x AO2)

Carbon Carbon bonds cannot be hydrolysed

(Makes the PVC) softer / more flexible Must use comparative language

03.3 OR

(1 x AO1)

Lowers the glass transition temperature

– A-LEVEL CHEMISTRY – 7405/2 –

03.4 (1 x AO1,

M1 either formula of alcohol or water 3 x AO2)

M2 for balanced equation

M3 (Nucleophilic) Addition–elimination

M4 C8H11O2

2-ethylhexan-1-ol

Or suitable structural formula

eg CH3CH2CH2CH2CH(CH2CH3)CH2OH or

03.5

(1 x AO2)

– A-LEVEL CHEMISTRY – 7405/2 –

M1 The more fat is present, the larger the mass DEHA (is absorbed M1 allow converse for water content

from plastic to the food)

M2 DEHA (and oils / fats / fatty acids) contain long alkyl chains M2 DEHA doesn’t have any O-H (groups)

03.6 OR DEHA is non-polar OR DEHA hydrophobic (1 x AO1,

2 x AO3)

M3 (Relatively strong / many) van der Waals’ / London / dispersion M3 Water/milk doesn’t form H bonds with DEHA

forces are formed between DEHA and fats/butter

– A-LEVEL CHEMISTRY – 7405/2 –

03.7

(4 x AO1)

M1 Arrow from lone pair on N to C

M2 Arrow from C=O bond to O

M3 Structure including –ve on O and +ve on N

M4 Three arrows;

lone pair on O to C–O bond, C–Cl bond to Cl and N–H bond to N

– A-LEVEL CHEMISTRY – 7405/2 –

03.8

(4 x AO2)

Two arrows in step 1

M1 Arrow from lone pair on O to H

M2 Arrow from O–H bond to O

One arrow in step 2

M3 Arrow from C=O to O

One arrow in step 3

M4 Arrow from lone pair on O of :OH– to C+

How to answer it

AQA A-Level Chemistry Study Guide: Polymers & Mechanisms

What this question tests

This comprehensive question assesses your understanding of addition and condensation polymers (PVC, Nylon 6,6, and Nylon 6). Key skills tested include:

  • Identifying and drawing monomer structures from polymer chains.
  • Explaining polymer properties (biodegradability and the role of plasticisers) using intermolecular forces.
  • Balancing complex organic equations and calculating empirical formulas.
  • Drawing step-by-step organic mechanisms involving nucleophilic addition-elimination and ring-opening polymerisation.

Part 3.1: PVC Monomer Name & Structure

Identifying the building block of Poly(vinyl chloride)

Correct Answers

Monomer Name: Chloroethene OR vinyl chloride OR chloroethylene

Structure:

H H \ / C = C / \ H Cl

[2 Marks] 1 mark for correct name, 1 mark for fully displayed structure showing the C=C double bond.

Exam Technique

When asked to "draw the structure of the monomer", always show the C=C double bond explicitly. Do not draw the polymer repeating unit with single bonds and square brackets!

Part 3.2: Biodegradability of PVC

Why addition polymers persist in the environment

Correct Answer

The Carbon-Carbon (C-C) bonds in the polymer backbone are non-polar, very strong, and cannot be hydrolysed (or are not attacked by nucleophiles).

[1 Mark] Must specify C-C bonds.

Common Errors

✗ "It has strong bonds."

Why it loses marks: This is too vague. You must explicitly state that the C-C bonds in the main chain are the ones that resist chemical or biological breakdown.

Part 3.3: Modifying Properties with Plasticisers

How DIBP alters PVC's physical structure

Correct Answer

Makes the PVC softer and more flexible OR lowers the glass transition temperature.

[1 Mark] Must use comparative language.

Key Knowledge

Plasticiser molecules get between polymer chains, pushing them apart. This weakens the intermolecular van der Waals forces, allowing the chains to slide over each other more easily.

Part 3.4: Synthesis of DIBP & Empirical Formula

Esterification equation, mechanism, and molecular simplification

Correct Answers

  • Left Box (Reactant): 2 (CH₃)₂CHCH₂OH (2-methylpropan-1-ol / isobutanol)
  • Right Box (Product): 2 H₂O
  • Mechanism Name: (Nucleophilic) Addition-elimination
  • Empirical Formula: C₈H₁₁O₂
[4 Marks] 1 mark for reactant/product formulas, 1 mark for balancing, 1 for mechanism name, 1 for empirical formula.

Calculation Steps: Empirical Formula

  1. Count the atoms in DIBP:
    • Benzene ring core: C₆H₄
    • Two ester groups: -COO- × 2 = C₂O₄
    • Two isobutyl chains: -CH₂CH(CH₃)₂ × 2 = C₈H₁₈
  2. Sum them up:
    C₆H₄ + C₂O₄ + C₈H₁₈ = C₁₆H₂₂O₄ (Molecular Formula)
  3. Simplify to the simplest whole-number ratio:
    Divide all subscripts by 2 → C₈H₁₁O₂

Part 3.5: Identifying the Alcohol for DEHA

Working backwards from a complex ester

Correct Answer

2-ethylhexan-1-ol

Structural formula accepted: CH₃CH₂CH₂CH₂CH(CH₂CH₃)CH₂OH

[1 Mark] IUPAC name or clear structural drawing.

Key Knowledge

DEHA is a diester formed from hexanedioic acid (a 6-carbon dicarboxylic acid). By "snipping" the ester bonds ( C-O single bonds), you can identify the alcohol component as an 8-carbon chain with an ethyl branch at position 2.

Part 3.6: Intermolecular Forces & Food Packaging

Explaining why DEHA migrates into fatty foods

Correct Answer / Mark Scheme

  • M1: The higher the fat content of the food, the greater the mass of DEHA absorbed (or converse for water content).
  • M2: DEHA and fats/oils both contain long non-polar alkyl chains (are hydrophobic).
  • M3: Relatively strong/many van der Waals forces (London dispersion forces) form between DEHA and the fats.
[3 Marks] 1 mark per bullet point.

Exam Technique

To score full marks on intermolecular forces questions:

  1. State the observed trend from the data table.
  2. Identify the structural features (e.g., "long non-polar hydrocarbon chains").
  3. Name the specific intermolecular force involved (van der Waals / London forces).

Part 3.7: Nylon 6,6 Mechanism

Nucleophilic addition-elimination intermediate and curly arrows

How to Draw the Intermediate & Arrows

1. The Intermediate Structure:

  • The carbonyl carbon of the hexanedioyl group becomes tetrahedral.
  • It is single-bonded to an oxygen with a negative charge ( O⁻ ) and three lone pairs.
  • It is still bonded to the chlorine ( Cl ) atom.
  • It is bonded to the nitrogen of the amine group, which now carries a positive charge ( N⁺ ) and is bonded to two hydrogens.

2. Curly Arrows:

  • Step 1: Arrow from the lone pair on the nitrogen of H₂N-(CH₂)₆-NH₂ to the carbonyl carbon. Arrow from the C=O double bond to the oxygen atom.
  • Step 2 (on Intermediate): Arrow from a lone pair on O⁻ back to the C-O bond (reforming C=O ). Arrow from the C-Cl bond to the chlorine atom. Arrow from one of the N-H bonds to the N⁺ atom.
[4 Marks] M1: Arrow from N lone pair to C; M2: Arrow from C=O to O; M3: Correct intermediate structure with charges; M4: Three elimination arrows.

Common Pitfalls

✗ Forgetting charges in the intermediate.

Always ensure your intermediate is overall neutral if your starting materials were neutral! The negative charge on the oxygen ( O⁻ ) and the positive charge on the nitrogen ( N⁺ ) balance each other out.

Part 3.8: Nylon 6 Ring-Opening Mechanism

Curly arrows for acid-catalysed polymerisation

Curly Arrow Guide

Step 1 (Protonation):

  • Arrow 1: From the lone pair on the carbonyl oxygen of caprolactam to one of the hydrogens on the hydronium ion ( H₃O⁺ ).
  • Arrow 2: From the O-H bond of H₃O⁺ to the oxygen atom of H₃O⁺ .

Step 2 (Resonance/Polarisation):

  • Arrow 3: From the C=O double bond to the protonated oxygen atom ( O⁺ ). This leaves a carbocation ( C⁺ ).

Step 3 (Nucleophilic Attack):

  • Arrow 4: From the lone pair on the oxygen of the water molecule ( H₂O ) to the carbocation carbon ( C⁺ ).
[4 Marks] 1 mark for each correct arrow.

Exam Technique: Curly Arrows

Curly arrows represent the movement of a pair of electrons. They must start precisely from a lone pair or a covalent bond, and point directly to the atom forming the new bond.

Topics

Organic Chemistry · Physical Chemistry · 3.3.12 Polymers · 3.3.4 Alkenes · 3.3.9 Carboxylic Acids and Derivatives · 3.3.1 Introduction to Organic Chemistry · 3.1.3 Bonding · 3.1.2 Amount of Substance

Question and mark scheme from the AQA A-Level Chemistry examination, Paper 2, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.