WJEC A-Level Chemistry Unit 4, June 2025: Question 3

2 marks · Easy difficulty · Structured Questions

Calculate the mass of water required to react with bis(2-chloroethyl) sulfide and deduce the products from a nucleophilic substitution mechanism step.

Practise this question

Question

Question 3 presents a reaction equation: (ClCH2CH2)2S with Mr 159 reacts with 2 H2O to form (HOCH2CH2)2S and 2 HCl. Part (a) asks to calculate the minimum mass of water needed to destroy 477 tonnes of bis(2-chloroethyl) sulfide, worth 1 mark. Part (b) shows a nucleophilic attack mechanism with curly arrows: a water molecule's oxygen lone pair attacks a delta-positive carbon atom, the C-Cl bond breaks towards the chlorine atom, and an O-H bond breaks towards the oxygen atom. Students are asked to complete the equation showing the products of this stage, worth 1 mark.
Question text

3. Bis(2-chloroethyl) sulfide has been used as a mustard gas. One way of destroying this material

is by reacting it with hot water.

(ClCH2CH2)2S + 2H2O (HOCH2CH2)2S + 2HCl

Mr 159

(a) Calculate the minimum mass of water needed to destroy 477 tonnes of

bis(2-chloroethyl) sulfide. [1]

Mass = … tonnes

(b) Part of the mechanism is shown below.

H

δ+

C

δ–

Cl H

O

H H

Complete the equation showing the products of this stage of the mechanism. [1]

Mark scheme

Show the mark scheme Mark scheme for question 3: (a) awards 1 mark for the value 108. (b) awards 1 mark for showing the substituted alcohol product attached to the carbon, plus H+ and Cl-, with a note stating to accept HCl in place of both ions.

3 (a) 108 1 1

(b)

accept HCl in place of both ions

How to answer it

Hydrolysis & Stoichiometry of Bis(2-chloroethyl) Sulfide

What this question tests

This question assesses fundamental chemical competencies from halogenoalkane organic chemistry and basic stoichiometry:

  • Reacting mass calculations: Using balanced chemical equations and stoichiometric ratios to determine the mass of reactant required involving tonnes.
  • Nucleophilic substitution mechanisms: Predicting the products formed when water acts as a nucleophile to hydrolyse a primary halogenoalkane bond.
Part (a) • 1 Mark

Calculating Reacting Mass of Water

The equation provided is:

(ClCH₂CH₂)₂S + 2H₂O → (HOCH₂CH₂)₂S + 2HCl

Mr of (ClCH₂CH₂)₂S = 159. Calculate the minimum mass of water needed to destroy 477 tonnes of bis(2-chloroethyl) sulfide.

📐 Step-by-Step Calculation

  1. Calculate moles of bis(2-chloroethyl) sulfide:
    Moles = Mass / Mr
    Moles = 477 / 159 = 3.0 × 10⁶ mol (or 3 "tonne-moles")
  2. Apply the stoichiometric ratio:
    From the equation, 1 mole of (ClCH₂CH₂)₂S reacts with 2 moles of H₂O.
    Moles of H₂O = 3.0 × 2 = 6.0 × 10⁶ mol
  3. Calculate mass of water:
    Mr(H₂O) = (2 × 1.01) + 16.00 = 18.02 (or 18)
    Mass = 6.0 × 18 = 108 tonnes

✅ Correct Answer

108 tonnes

Mark Scheme:
• [1 Mark] for 108.

🧠 Exam Technique: Working with Tonnes

  • Keep units consistent: Since the question gives tonnes and asks for tonnes, you do not need to convert to grams and back again! Treat 1 tonne as 10⁶ g or simply calculate in standard mole units directly.
  • Always check the stoichiometric ratio: notice the 2 in front of H₂O in the balanced chemical equation.

❌ Common Errors

  • Missing the 1:2 ratio: Forgetting to multiply by 2, leading to an answer of 54 tonnes (3 × 18).
  • Unit conversion errors: Converting tonnes to grams ( × 10⁶ ) and making a power-of-ten mistake when converting back to tonnes.
Part (b) • 1 Mark

Products of Nucleophilic Substitution Stage

The diagram shows nucleophilic attack by the oxygen lone pair of a water molecule on the δ+ carbon atom, accompanied by heterolytic fission of the C–Cl bond (loss of chloride).

✅ Correct Answer

The products must account for all atoms and charges leaving this step:

–CH₂–OH + H⁺ + Cl⁻
(or –CH₂–OH + HCl)

Specifically, draw the remaining carbon skeleton with the hydroxyl group attached to the carbon:

R–CH₂–OH + H⁺ + Cl⁻

Mark Scheme:
• [1 Mark] for displaying the alcohol product –C(H)₂–OH together with H⁺ + Cl⁻ (or HCl ).

💡 Key Knowledge: Hydrolysis Mechanism

  • Mechanism Type: Nucleophilic substitution (SN2).
  • Role of H₂O: Water acts as a neutral nucleophile via the lone pair on oxygen.
  • Conservation of Charge: The initial reactants are neutral; the products must be neutral overall. The substitution displaces a chloride ion ( Cl⁻ ), and loss of a proton ( H⁺ ) from the coordinated water restores neutrality to the alcohol.

🧠 Visual Description of the Products

  • Retain the central carbon with its two hydrogen atoms ( –CH₂– ) and the bond going off to the rest of the molecule ( –CH₂S–... ).
  • Replace the leaving –Cl group with a neutral –O–H group (showing lone pairs on oxygen if desired).
  • Write + H⁺ + Cl⁻ or + HCl as the inorganic side products to balance mass and charge.

❌ Common Errors

  • Leaving off counter-ions: Writing only the organic product and forgetting the displaced chloride ( Cl⁻ ) or the acidic proton ( H⁺ ).
  • Incorrect charges: Drawing an un-deprotonated species –OH₂ without a positive formal charge, or forgetting charges on separated ions (e.g. writing H + Cl instead of H⁺ + Cl⁻ ).

Topics

Physical Chemistry · Organic Chemistry · 1.3 Chemical calculations · 2.4 Organic compounds · 2.6 Halogenoalkanes

Question and mark scheme from the WJEC A-Level Chemistry examination, Unit 4, June 2025. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.