OCR A-Level Chemistry AS Depth in chemistry (02), June 2023: Question 3

15 marks · Medium difficulty · Structured Questions

Explain the relative reactivity of halogens, calculate volume of seawater needed to extract iodine, define disproportionation with oxidation numbers, outline the nucleophilic substitution mechanism for the alkaline hydrolysis of 2-bromopropane, and compare rates of hydrolysis of haloalkanes.

Practise this question

Question

Question 3 covers halogens and halogen compounds. Part (a) asks to explain the relative reactivity of chlorine and iodine and calculate the volume of seawater needed to manufacture 1.00 tonne of iodine, given a concentration of potassium iodide of 0.150 g dm^-3 (6 marks). Part (b) gives the equation for chlorine reacting with calcium hydroxide to form bleaching powder and asks to define disproportionation and use oxidation numbers (3 marks). Part (c) requires outlining the mechanism for the alkaline hydrolysis of 2-bromopropane using curly arrows, dipoles, lone pairs, and naming the mechanism (3 marks). Part (d) presents an experiment comparing rates of hydrolysis of 2-bromopropane and 2-iodopropane, asking to complete a table of formulas and colours of precipitates (2 marks) and predict which precipitates first with explanation (1 mark).
Question text

3 This question is about halogens and halogen compounds.

(a)* Seawater contains very small quantities of dissolved iodide ions.

The concentration of potassium iodide, KI, in seawater is 0.150 g dm–3.

Iodine can be extracted by bubbling chlorine gas through seawater.

Explain why chlorine is more reactive than iodine and determine the volume, in dm3, of

seawater that is needed to manufacture 1.00 tonne of iodine, I2. [6]

Additional answer space if required.

(b) Chlorine reacts with calcium hydroxide to form Ca(OCl)2, which is the active ingredient in

bleaching powder.

2Cl2 + 2Ca(OH)2 CaCl2 + Ca(OCl)2 + 2H2O

This is a disproportionation reaction.

State what is meant by disproportionation and use oxidation numbers to show that

disproportionation has taken place.

… [3]

(c) A student is studying the hydrolysis of haloalkanes.

The equation for the alkaline hydrolysis of 2-bromopropane, CH3CHBrCH3, is shown below.

CH CHBrCH + OH– CH CHOHCH + Br–

33 3 3

Use the curly arrow model to outline the mechanism for the alkaline hydrolysis of

2-bromopropane.

Show relevant dipoles and lone pairs, and name the mechanism.

name of mechanism … [3]

(d) The student sets up an experiment to compare the rates of hydrolysis of

2-bromopropane and 2-iodopropane.

The student uses the method below.

Step 1 Place two test tubes, both containing aqueous silver nitrate and ethanol, in a

water bath at 60 °C.

Step 2 Add five drops of 2-bromopropane to one test tube and five drops of

2-iodopropane to the other test tube.

Step 3 Record the time taken for a precipitate to appear in each test tube.

(i) Complete the table below to show the formula and colour of each precipitate formed.

Haloalkane Formula of precipitate Colour of precipitate

2-bromopropane

2-iodopropane

[2]

(ii) Predict which precipitate would form first and explain the difference in the rates of

hydrolysis of 2-bromopropane and 2-iodopropane.

… [1]

Mark scheme

Show the mark scheme Mark scheme for Question 3 detailing level-based answers for part (a) including relative reactivities and calculation steps. Part (b) awards marks for definition of disproportionation and correct oxidation numbers for chlorine in reactants and products. Part (c) gives accepted curly arrow directions, dipoles, and the name nucleophilic substitution. Part (d) lists AgBr (cream) and AgI (yellow) formulas and colours, followed by explanation of C-I bond strength.

AO

Question Answer Marks element Guidance

3 (a) Please refer to the marking instructions on page 4 of this 6 AO1.1 Indicative scientific points may include:

* mark scheme for guidance on how to mark this question. 2

Explanation of relative reactivities

Level 3 (5–6 marks) AO1.2 Comparison required throughout

A comprehensive explanation of relative reactivities of 1 • Chlorine gains electron more easily

chlorine and iodine OR forms negative ion more easily

AND AO2.6 OR attracts an electron (to its outer shell) more

Uses an appropriate method to calculate volume of 3

easily

seawater allowing for an acceptable error

• Because chlorine (atom) is smaller

There is a well-developed line of reasoning which is OR outer shell of chlorine less shielded/closer

clear an • Greater nuclear attraction (on chlorine electrons)

d logically structured. The information presented is • ORA

relevant and substantiated.

IGNORE ‘nuclear charge’ for ‘nuclear attraction’

Level 2 (3–4 marks)

A comprehensive explanation of relative reactivities of Determination of volume of seawater

chlorine and iodine • Cl (g) + 2I–(aq) → I (aq) + 2Cl–(aq)

AND OR molar ratio I–: I = 2:1

Some attempt at calculation 6

1 × 10

• n(I2) = 253.8 = 3940(.11…) (mol)

OR

Explanation of relative reactivities of chlorine and iodine • n(KI) = 3940(.11…) 2 = 7880.(22…) (mol)

containing most details 3 0.150

AND • n(KI) in 1 dm seawater = 166

A reasonable attempt at calculation = 9.036(144..) 10–4 (mol)

7880.(22..)

OR • Volume of seawater = –4

9.036(…) 10

Explanation of relative reactivities of chlorine and iodine

containing some details

AND = 8.72…… 106 dm3

Uses an appropriate method to calculate volume of

seawater allowing for an acceptable error

15 AO

There is a line of reasoning presented with some Alternative method:

structure. The information presented is relevant and

supported by some evidence. • m(KI) = 7880.(22…) x 166 (mol)

= 1.308(116627) x 106

Level 1 (1–2 marks) •

Explanation of relative reactivities of chlorine and iodine 6

containing some details 1.308(116627) 𝑥 10

Volume of seawater =

AND 0.15

Some attempt at the calculation

= 8.72…… 106 dm3

OR

Explanation of relative reactivities containing most

details Acceptable errors:

OR Volume of seawater obtained from:

A reasonable attempt at calculation • Use of 126.9 to find n(I ) giving 1.74….x 107

• Missing x 2 ratio giving 4.36….x 106

The information is basic and communicated in an

unstructured way. The information is supported by DO NOT ALLOW for L3 if both errors are present, please

limited evidence and the relationship to the evidence note it gives a volume of 8.72…… 106 dm3

may not be clear.

ALLOW minor slips in rounding, transcription errors, etc

0 marks throughout

No response or no response worthy of credit.

AO

3 (b) Disproportionation 3

Oxidation AND reduction of same element/chlorine AO1.1 IGNORE numbers around equation for oxidation

OR numbers

Chlorine/Cl/Cl2 has been oxidised AND

chlorine/Cl/Cl2 has been reduced IGNORE ‘species’ or ‘reactant’ for element

Oxidation ALLOW 1+ for +1 AND 1– for –1

from 0 in Cl to +1 in Ca(OCl) OR ClO- AO2.2 NOTE for chlorine/Cl from 0 only needs to be seen

22 2

once, does not need to be stated twice

Reduction

from 0 in Cl to –1 in CaCl OR Cl- AO2.2 ALLOW 1 mark for 3 oxidation numbers correct but no

mention of words oxidation/reduction: e.g.

0 in Cl2 AND –1 in CaCl2 AND +1 in Ca(OCl)2

ALLOW 1 mark for species missing

oxidised from 0 to +1

AND reduced (from 0) to –1

AO

Question Answer Marks Guidance

17 element

3 (c) Mechanism 3 1st curly arrow must

AO1.2 • go to the C of C–Br

Curly arrow from OH– to C atom of C−Br bond in AND

2-bromopropane✓ • start from, OR be traced back to any point across

AO2.1 width of lone pair on O of OH–

Dipole shown on C–Br bond, C + and Br −,

AND

• OR start from – charge on O of –OH ion

curly arrow from C−Br bond to Br atom ✓

AO1.1 (Lone pair NOT needed if curly arrow shown from O–)

2nd curly arrow must start from, OR be traced back to, any

part of C–Br bond and go to Br

---------------------------------------------------------------------

ALLOW SN1 mechanism for 2 curly arrow marks

Name

nucleophilic substitution ✓ First mark

Dipole shown on C–Br bond, C + and Br −,

AND curly arrow from C−Br bond to Br atom ✓

NOTE: Curly arrows can be straight, snake-like, etc.

but NOT double headed or half headed arrows

Second mark

Curly arrow from OH– AND to correct carbocation ✓

Curly arrow must come from lone pair on O of HO– OR OH–

OR from minus on O of HO– ion (no need to show lone pair

if curly came from negative charge)

18 AO

element

3 (d) (i) Haloalkane Formula Colour 2 AO1.1 ALLOW 1 mark if correct formula for both OR

2-bromopropane AgBr cream ✓ x 2 correct colour for both

2-iodopropane AgI yellow ✓

Formula AND colour required for each mark

(ii) AgI OR yellow (precipitate forms first) 1 AO2.3 ALLOW (precipitate from) 2-iodopropane

ALLOW ECF from incorrect formula or colour

ppt from 3(d)(ii)

AND

C–I bond is weaker (than C–Br bond) ✓ ALLOW C–I bond has a lower bond enthalpy

OR C–I bond is longer

ORA

IGNORE references to bond length, polarity

and electronegativity

How to answer it

Halogens and Halogen Compounds Study Guide

What this question tests

This exam question assesses your understanding of Group 7 (halogen) trends in reactivity, redox reactions, disproportionation, oxidation numbers, organic nucleophilic substitution mechanisms (curly arrow models), and practical rates of hydrolysis for haloalkanes linked to carbon-halogen bond enthalpies.

Question 3 (a) - 6 Marks

Reactivity Trends & Quantitative Calculation

💡 Key Knowledge

  • Chlorine gains electrons more easily than iodine because its outer shell experiences less shielding and a greater nuclear attraction.
  • Reacting ratio from equation: Cl₂ + 2I⁻ -> I₂ + 2Cl⁻ means 1 mole of I₂ is produced from 2 moles of KI.
  • Conversion factor: 1.00 tonne = 1,000,000 g .

📐 Step-by-Step Calculation

  1. Moles of Product: Find moles of I₂ in 1.00 tonne.
    n(I₂) = 1,000,000 g / 253.8 g mol⁻¹ = 3940.11 mol
  2. Reacting Ratio: Apply the 1:2 ratio to find moles of KI needed.
    n(KI) = 3940.11 × 2 = 7880.22 mol
  3. Volume of Seawater: Use concentration ( 0.150 g dm⁻³ ) to find volume.
    Mass of KI = 7880.22 mol × 166.0 g mol⁻¹ = 1.308×10⁶ g
    Volume = 1.308×10⁶ g / 0.150 g dm⁻³ = 8.72×10⁶ dm³

❌ Common Errors

  • Forgetting to multiply the moles of iodine by 2 to account for the stoichiometric ratio of iodide ions.
  • Using atomic mass of iodine ( 126.9 ) instead of molecular mass ( 253.8 ) for I₂.
  • Using vague terms like "nuclear charge" instead of "greater nuclear attraction" or "less shielding".
Mark Scheme: Level of response (Levels 1–3) based on chemical explanation of reactivity plus correct numerical calculation arriving at 8.72 × 10⁶ dm³.
Question 3 (b) - 3 Marks

Disproportionation & Oxidation Numbers

✅ Correct Answer

Disproportionation: The oxidation and reduction of the same element (chlorine) in a single reaction.

Oxidation numbers:
Chlorine in Cl₂ = 0
Chlorine in CaCl₂ = -1 (reduced)
Chlorine in Ca(OCl)₂ = +1 (oxidised)

🧠 Exam Technique

State both definitions clearly using keywords: same element is simultaneously oxidised and reduced. Explicitly link starting oxidation states ( 0 ) to both product states ( +1 and -1 ) to secure full marks.

Mark Scheme: 1 mark for defining disproportionation, 1 mark for showing chlorine is oxidised (0 to +1), 1 mark for showing chlorine is reduced (0 to -1).
Question 3 (c) - 3 Marks

Nucleophilic Substitution Mechanism

✅ Correct Answer

Name of mechanism: Nucleophilic substitution (SN2).

Curly arrows & Dipoles:

  • Partial charges shown as δ+ on C and δ- on Br.
  • First curly arrow starts from the lone pair (or negative charge) on the oxygen of the OH⁻ ion, pointing to the electron-deficient C atom attached to bromine.
  • Second curly arrow starts from the C–Br bond and points directly to the Br atom, showing heterolytic fission.

❌ Common Errors

  • Drawing curly arrows starting in empty space instead of explicitly starting from a lone pair or covalent bond.
  • Omitting partial charges ( δ+ / δ- ) on the polar carbon-bromine bond.
  • Arrow heads pointing the wrong way (must point towards the atom accepting the electron pair).
Mark Scheme: 1 mark for dipoles and first curly arrow from OH⁻ to C, 1 mark for second curly arrow from C–Br bond to Br, 1 mark for naming "nucleophilic substitution".
Question 3 (d) - 3 Marks (2 + 1)

Hydrolysis Rates & Precipitates

✅ Correct Answers

(i) Precipitates Table:

  • 2-bromopropane: Formula AgBr , Colour: cream
  • 2-iodopropane: Formula AgI , Colour: yellow

(ii) Rate Prediction: AgI (yellow precipitate) forms first because the C–I bond has a lower bond enthalpy / is weaker than the C–Br bond, requiring less energy to break.

💡 Key Knowledge

As you descend Group 7, atomic radius increases, overlapping orbitals become less effective, and carbon-halogen bond enthalpies decrease. Weaker bonds break faster during nucleophilic substitution.

Mark Scheme: (i) 2 marks for correct formulas and colours. (ii) 1 mark for identifying AgI / yellow forms first due to weaker C–I bond enthalpy.

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Module 4: Core organic chemistry · 2.1 Atoms and reactions · 3.1 The periodic table · 4.2 Alcohols, haloalkanes and analysis

Question and mark scheme from the OCR A-Level Chemistry examination, AS Depth in chemistry (02), June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.