Edexcel A-Level Chemistry AS Paper 1, June 2019: Question 9

23 marks · Hard difficulty · Open Response

Assess various aspects of chlorine and its compounds including disproportionation, VSEPR shapes, redox balancing, separation techniques, mole calculations, equilibria, hydration of ions, intermolecular forces, and stoichiometry.

Practise this question

Question

Exam question in multiple parts about chlorine, potassium chlorate, chlorate(V) ion dot-and-cross diagram, solubility tables, swimming pool disinfection calculations, equilibrium effects, hydration of ions, intermolecular forces of ethanol and chloroethane, and calcium chlorate calculations.
Question text

9 This question is about chlorine and its compounds.

(a) Potassium chlorate(V) can be produced by passing chlorine gas into hot,

concentrated potassium hydroxide solution.

3Cl2 + 6KOH → 5KCl + KClO3 + 3H2O

(i) This reaction is an example of

(1)

A oxidation only

B reduction only

C disproportionation

D decomposition

(ii) A dot-and-cross diagram for the chlorate(V) ion (ClO–) is shown.

⎡ ⎡– Key

O

⎢ ⎢ = chlorine electrons

⎢O Cl O ⎢ = an added electron

⎢⎣ ⎢⎣ = oxygen electrons

Predict the shape and bond angle (O Cl O) of the chlorate(V) ion.

Justify your answer.

(4)

… *P55601A01724*

(b) (i) The following reaction occurs when potassium chlorate(V) is heated at a*P55601A01824*

suitable temperature.

Complete the equation by balancing it.

State symbols are not required.

… KClO3 → … KCl + … KClO4

(1)

(ii) The table shows some properties of potassium chloride and potassium chlorate(VII).

Potassium chloride Potassium chlorate(VII)

KCl KClO4

Solubility in water –1 –2

4.81 × 10 1.29 × 10

(mol/100 g)

Solubility in ethanol –4 –6

2.9 × 10 8.7 × 10

(mol/100 g)

Devise a brief method to show how the compounds produced in the

decomposition of potassium chlorate(V) could most effectively be separated.

Use information from the table.

(3)

(c) Chlorine gas, Cl2, can be dissolved in swimming pool water to disinfect it.

An Olympic-sized swimming pool contains about 2500 m3 of water.

The chlorine content is 2 ppm (parts per million) by mass.

Calculate the number of moles of chlorine, Cl2, in the swimming pool.

[One ppm is equivalent to 1 g of chlorine dissolved in 1 × 106 g of water.

Density of water = 1 g cm–3] 19

*P55601A01924* (3)

(d) When chlorine gas is dissolved in water, it reacts according to the equation

Cl2 + H2O → HCl + HClO

The chloric(I) acid (HClO) produced is much more effective as a disinfectant than

dissolved chlorine.

Chloric(I) acid is a weak acid and has little effect on the pH of the water.

Swimming pools usually have a chlorine content of 1 – 3 ppm.

Use the equation to explain one disadvantage of a chlorine content that is

much lower than 1 ppm and one disadvantage of a chlorine content that is much

higher than 3 ppm.

(4)

(e) In many swimming pools, sodium chlorate(I) has replaced chlorine gas as a*P55601A02024*

disinfectant.

Sodium chlorate(I) is an ionic compound. It is very soluble in water.

NaClO(aq) → Na+(aq) + ClO–(aq)

(i) Describe, using diagrams to illustrate your answer, the interactions between

each of the ions and the solvent when sodium chlorate(I) dissolves in water.

(2)

(ii) The displayed formulae of ethanol and chloroethane are shown.

H H H H

H C C O H H C C Cl

H H H H

ethanol chloroethane

Ethanol is very soluble in water whereas chloroethane is almost insoluble in water.

Explain this observation by comparing the types of intermolecular forces

formed by each of these molecules with water.

(2)

… *P55601A02124*

(f ) Calcium chlorate(I), Ca(ClO)2, can be used to disinfect drinking water.

The concentration of chlorate(I) ions required to disinfect water is about

5.6 × 10–6 mol dm–3.

Calculate the mass of calcium chlorate(I), in g, that should be added to 1000 dm3

of water to produce a chlorate(I) ion concentration of 5.6 × 10–6 mol dm–3.

(3)

*P55601A02224*

(Total for Question 9 = 23 marks)

Mark scheme

Show the mark scheme Mark scheme for the chlorine and compounds question, showing acceptable answers, marks allocated per point, and calculation guidance for each part (a) through (f).

How to answer it

Edexcel AS Chemistry: Chlorine and its Compounds

What this question tests

This comprehensive synoptic question assesses redox reactions (specifically disproportionation), VSEPR theory for predicting shapes and bond angles of ions, equation balancing, practical separation techniques based on solubility data, mole calculations involving parts per million (ppm), equilibrium and pH consequences in water treatment, ion-dipole hydration interactions, intermolecular forces governing solubility, and multi-step stoichiometry.

Question 9(a) - Disproportionation & VSEPR

Part (i) & (ii): Disproportionation and Shape of ClO₃⁻

✅ Correct Answers

  • 9(a)(i): C (disproportionation) (1 mark)
  • 9(a)(ii) Shape: Trigonal pyramidal (1 mark)
  • 9(a)(ii) Angle: 107° (Accept 106.5° to 107.5°, or 110°) (1 mark)
  • 9(a)(ii) Justification: 3 bond pairs, 1 lone pair; electron pairs repel to minimum separation; lone pair-bond pair repulsion > bond pair-bond pair repulsion. (2 marks)

💡 Key Knowledge

  • Disproportionation occurs when the same element is simultaneously oxidized and reduced in a single chemical reaction.
  • Chlorine in Cl₂ starts at oxidation state 0, and ends up in Cl⁻ (-1) and ClO₃⁻ (+5).
  • VSEPR rules: Lone pairs exert greater repulsive force than bonding pairs, reducing the standard tetrahedral angle (109.5°) down to 107°.

❌ Common Errors

  • Failing to explicitly *name* the shape (writing "pyramidal" without "trigonal" can miss the mark depending on strictness, though trigonal pyramidal is required).
  • Confusing total electron domains with lone pairs when explaining the angle.
Question 9(b) - Balancing Equations & Separation Techniques

Part (i) & (ii): Thermal Decomposition and Solubility Separation

✅ Correct Answers

  • 9(b)(i) Balanced equation: 4 KClO₃ → 1 KCl + 3 KClO₄ (1 mark)
  • 9(b)(ii) Separation: Use water only as a solvent (1 mark); add mixture to water to dissolve one component (1 mark); filter off the undissolved solid KClO₄ (1 mark).

🧠 Exam Technique

  • Check solubility table data carefully: KClO₄ has a drastically lower solubility in water compared to KCl , making filtration effective.
  • Ensure you specify water only as the solvent to gain full separation marks; mentioning arbitrary extra solvents can invalidate procedural marks in mark schemes.
Question 9(c) - PPM and Mole Calculations

Part (c): Chlorine Concentration in an Olympic Pool

📐 Step-by-Step Calculation

  1. Convert pool volume to cm³ (or mass in g):
    Volume = 2500 m³ = 2500 × 10⁶ cm³ = 2.5 × 10⁹ cm³ (since 1 m³ = 10⁶ cm³).
    Given density = 1 g cm⁻³, mass of water = 2.5 × 10⁹ g.
  2. Calculate mass of chlorine (Cl₂) in grams:
    Concentration = 2 ppm = 2 g per 1 × 10⁶ g of water.
    Mass of Cl₂ = (2 / 1×10⁶) × (2.5 × 10⁹) = 5.0 × 10³ g.
  3. Convert mass of chlorine to moles:
    Molar mass of Cl₂ = 35.5 × 2 = 71.0 g mol⁻¹.
    Moles of Cl₂ = 5.0 × 10³ / 71.0 = 70.4 mol (Accept 70 mol).
3 marks total. Awarded for volume conversion, mass calculation using ppm ratio, and final division by molar mass (71).
Question 9(d) - Equilibrium & Water Treatment Context

Part (d): Disadvantages of Incorrect Chlorine Content

✅ Correct Answers & Mark Scheme Points

  • Disadvantage 1 (Content < 1 ppm): HClO concentration will be too low (1 mark), making it ineffective as a disinfectant (1 mark).
  • Disadvantage 2 (Content > 3 ppm): HCl concentration will be too high (1 mark), leading to hazards like skin/eye irritation, corrosion, or excessively lowered pH (1 mark).

❌ Common Errors

  • Vague statements like "it is harmful" without specifying *why* high chlorine causes issues (e.g., increased acidity/HCl buildup).
  • Forgetting that dissolving chlorine produces hydrochloric acid ( HCl ), which directly impacts pool pH and swimmer safety.
Question 9(e) - Ion-Dipole Interactions & Intermolecular Forces

Part (i) & (ii): Hydration of Ions and Solubility of Ethanol vs Chloroethane

💡 Key Knowledge: 9(e)(i) Ion-Dipole

  • Both Na⁺ and ClO⁻ must be shown surrounded by multiple water molecules (>1).
  • Orientation matters: Water molecules near Na⁺ must have their partially negative oxygen (δ⁻) pointing toward the positive ion. Water molecules near ClO⁻ must have their partially positive hydrogens (δ⁺) pointing toward the negative ion.

💡 Key Knowledge: 9(e)(ii) Solubility Explanation

    ">
  • Ethanol (CH₃CH₂OH): Forms hydrogen bonds with water molecules due to the polar -OH group, making it completely miscible/soluble.
  • Chloroethane (CH₃CH₂Cl): Cannot form hydrogen bonds with water; it only forms permanent dipole-dipole attractions and London forces, which are insufficient to overcome the strong hydrogen bonding between water molecules, rendering it insoluble.
Question 9(f) - Multi-Step Solution Stoichiometry

Part (f): Mass of Calcium Chlorate(I) Required

📐 Step-by-Step Calculation

  1. Calculate moles of ClO⁻ required in 1000 dm³ of water:
    Concentration = 5.6 × 10⁻⁶ mol dm⁻³.
    Moles of ClO⁻ = 5.6 × 10⁻⁶ × 1000 = 5.6 × 10⁻³ mol.
  2. Determine moles of calcium chlorate(I) - Ca(ClO)₂ needed:
    Formula contains two chlorate ions per molecule: Ca(ClO)₂ .
    Moles of Ca(ClO)₂ = (5.6 × 10⁻³) / 2 = 2.8 × 10⁻³ mol.
  3. Calculate mass of Ca(ClO)₂:
    Molar mass of Ca(ClO)₂ = 40.1 + 2(35.5 + 16.0) = 40.1 + 103.0 = 143.1 g mol⁻¹.
    Mass = 2.8 × 10⁻³ mol × 143.1 g mol⁻¹ = 0.40 g (Accept 0.401 g).
3 marks total. Awarded for calculating moles of ClO⁻, scaling by the 1:2 stoichiometric ratio to find Ca(ClO)₂ moles, and multiplying by molar mass.

Topics

Inorganic Chemistry · Physical Chemistry · Organic Chemistry · Core Practicals · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 2: Bonding and Structure · Topic 5: Formulae, Equations and Amounts of Substance

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.