OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), June 2024: Question 20

11 marks · Hard difficulty · Extended Response

Discuss the uses and displacement reactions of halogens, determine a reaction mechanism step and rate equation, and explain the melting points of Period 3 elements in terms of bonding and structure.

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Question

Question text

20 This question is about elements in the periodic table.

(a) Chlorine has many uses.

(i) Chlorine is used to treat water in large-scale water treatment plants.

Suggest why chlorine is added to water in large-scale water treatment plants.

… [1]

(ii) Sea water contains aqueous bromide ions.

Chlorine is used to extract bromine from sea water.

Construct the ionic equation for this reaction and explain why chlorine is suitable for this

extraction of bromine but iodine is not.

Equation …

Explanation …

… [2]

(b) Fluorine, F2, reacts with nitrogen dioxide, NO2, to form nitryl fluoride, NO2F, as shown in

Reaction 20.1 below.

F2 + 2NO2 2NO2F Reaction 20.1

The mechanism for this reaction involves two steps. Step 1 is the ‘slow’ step and Step 2 is the

‘fast’ step.

The equation for Step 2 is shown below:

F + NO2 NO2F

Suggest the equation for Step 1 and the rate equation for Reaction 20.1.

Equation for Step 1: …

Rate equation for Reaction 20.1: …

[2]

(c)* The table shows the melting points of some of the elements in Period 3 of the periodic table.

Element Al Si P4 S8

Melting point /°°C 660 1410 44 119

Explain the melting points in terms of bonding and structure. [6]

Extra answer space if required.

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

20 (a) (i) killing bacteria ✓ 1 ALLOW killing microorganisms / microbes / sterilises water

IGNORE ‘removing’ bacteria

(ii) Cl +2Br– → Br + 2Cl– ✓ 2 DO NOT ALLOW full equation

IGNORE state symbols

Chlorine is more reactive than bromine CARE with endings (e.g. ide and ine)

AND iodine is less reactive than bromine ALLOW ORA

OR ALLOW reactivity Cl > Br > I

chlorine is a stronger oxidising agent than bromine AND

iodine is a weaker oxidising agent than bromine.✓ ALLOW bromide is a stronger reducing agent than chloride

AND bromide is a weaker reducing agent than iodide

IGNORE displacement

IGNORE references to down the group.

IGNORE all comparisons of electron structure/electron

affinity

(b) Equation for Step 1 2 Mark independently

F2 + NO2 → F + NO2F

Rate Equation ALLOW rate = k [NO2][F2]

Rate = k [F2][NO2] Care – k must be included.

(c)* Please refer to the marking instructions on page 5 of this mark 6 Indicative scientific points may include:

scheme for guidance on how to mark this question. 25

Level 3 (5–6 marks) ALLOW minor omissions as we are looking for a holistic

Describe the types of structure and bonding of all four approach to LoR marking.

elements AND explains most of the differences in melting

points in terms of the relative strengths of the forces between

Al (Giant metallic)

the particles.

• Giant metallic structure/lattice

There is a well-developed line of reasoning which is clear and • Strong metallic bonding

logically structured. The information presented is relevant and • Electrostatic attraction between (positive) metal

substantiated. ions/cations and delocalised electrons

Level 2 (3–4 marks) • A lot of energy needed to break bonds

Si (Giant covalent)

Attempt to describe the types of bonding of three elements • Each Si atom forms 4 bonds / bonds with 4 other Si

AND explains most of the differences in melting points in terms atoms

of the relative strengths of the forces between the particles. • Giant covalent structure/lattice

OR • Strong covalent bonds between atoms

Describe in detail and bonding of two of the three types of

• Between shared pair of electrons and adjacent

structure AND explains most of the differences in melting

points in terms of the relative strengths of the forces between nuclei.

the particles. • Most energy needed to break bonds

There is a line of reasoning presented with some structure. P, S (Simple covalent)

The information presented is relevant and supported by some • Simple covalent / molecular structure/lattice

evidence.

• Strong covalent bonds between atoms

Level 1 (1–2 marks) • Weak induced dipole–dipole interactions between

molecules*

Attempt to describe the bonding of two elements AND • Least energy to overcome the forces

explains most of the differences in melting points in terms of • Melting point of S > P

the relative strengths of the forces between the particles.

OR • More electrons

Describes in detail the bonding of one of the three types of • Stronger induced dipole–dipole interactions

structure AND explains the melting point in terms of the • DO NOT ALLOW breaks BONDS

strength of the forces between the particles. • IGNORE van der Waals’ (VDW)

The information is basic and communicated in an unstructured *ALLOW London (dispersion) forces for induced dipole–

way. The information is supported by limited evidence and the26

dipole interactions.

relationship to the evidence may not be clear.

0 marks Aspects of the communication statement might typically not

No response or no response worthy of credit. have been met when irrelevant information (e.g. ionisation

energies, ionic radius etc) have been included.

How to answer it

Elements in the Periodic Table Study Guide

What this question tests

This comprehensive question assesses your understanding of halogen chemistry (reactivity, displacement, and water treatment), kinetics (deducing rate equations and reaction mechanisms from multi-step reactions), and periodicity (explaining trends in melting points across Period 3 based on bonding and structure types).

Question 20 (a)(i) - Chlorine in Water Treatment

Suggest why chlorine is added to water in large-scale water treatment plants.

✅ Correct Answer

Killing bacteria (or microbes / sterilising water).

❌ Common Errors

Writing that chlorine is used for "removing" bacteria instead of killing or sterilising them.

Mark: [1 mark]

Question 20 (a)(ii) - Halogen Displacement

Construct the ionic equation for the extraction of bromine from sea water and explain the reactivity trends.

✅ Correct Answer

Cl₂ + 2Br⁻ → Br₂ + 2Cl⁻

Explanation: Chlorine is more reactive than bromine, AND iodine is less reactive than bromine (or reference to relative oxidising strengths: chlorine is a stronger oxidising agent than bromine, and iodine is a weaker oxidising agent than bromine).

💡 Key Knowledge

Halogen reactivity decreases down Group 7 (F > Cl > Br > I).
Oxidising power decreases down the group; halogens act as oxidising agents by gaining electrons.

❌ Common Errors

Writing full equations containing spectator ions instead of the required ionic equation. Confusing element and ion terminology (e.g., saying "bromine" is a reducing agent instead of "bromide ions").

Mark: [2 marks]

Question 20 (b) - Kinetics & Mechanisms

Suggest the equation for Step 1 and the overall rate equation for Reaction 20.1.

✅ Correct Answer

Step 1 Equation: F₂ + NO₂ → F + NO₂F

Rate Equation: Rate = k[F₂][NO₂] (or Rate = k[NO₂][F₂] )

🧠 Exam Technique

To find Step 1, subtract the known Step 2 equation ( F + NO₂ → NO₂F ) from the overall equation ( F₂ + 2NO₂ → 2NO₂F ). The rate-determining step is the slow step (Step 1), meaning its reactants dictate the rate equation!

❌ Common Errors

Omitting the rate constant k in the rate equation, or accidentally including species from the fast step.

Mark: [2 marks]

Question 20 (c)* - Periodicity & Melting Points

Explain the melting points of Period 3 elements (Al, Si, P, S) in terms of bonding and structure. (6-mark Level of Response question)

💡 Key Scientific Points Required

Aluminium (Al): Giant metallic structure. Strong metallic bonding due to attraction between positive metal ions/cations and delocalised electrons. Requires a lot of energy to break.
Silicon (Si): Giant covalent structure. Strong covalent bonds between atoms (each Si forms 4 covalent bonds). Most energy needed to break bonds.
Phosphorus (P₄) & Sulfur (S₈): Simple molecular structures. Held together by weak induced dipole-dipole interactions (London forces). Least energy needed to overcome forces.
Comparing P₄ and S₈: S₈ has a higher melting point than P₄ because S₈ molecules are larger with more electrons, resulting in stronger induced dipole-dipole interactions.

🧠 Examiner Strategy & Level 3 Guidance

To achieve a Level 3 (5–6 marks) response, you must structure your answer logically, addressing all three distinct types of bonding/structure present across these four elements (metallic, giant covalent, and simple molecular) and accurately link the strength of forces to the energy required to melt them.

❌ Common Errors to Avoid

Crucial Trap: Saying that melting simple molecular lattices "breaks covalent bonds" (covalent bonds within P₄ or S₈ molecules do not break during melting; only the weak intermolecular forces between molecules are overcome!). Do not include irrelevant information like ionisation energies or atomic radii.

Mark: [6 marks]

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · Module 5: Physical chemistry and transition elements · 2.2 Electrons, bonding and structure · 3.1 The periodic table · 3.2 Physical chemistry · 5.1 Rates, equilibrium and pH

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), June 2024. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.