OCR A-Level Chemistry Periodic table, elements and physical chemistry (01), November 2020: Question 21

9 marks · Medium difficulty · Structured Questions

Explain the trend in reactivity of halogens using displacement reactions, state a benefit and risk of water treatment with chlorine, and determine the molecular formula of a bromine-fluorine compound from gas volume data.

Practise this question

Question

Question text

21 This question is about halogens.

(a) A student adds a solution of bromine in an organic solvent to two test tubes.

The student adds aqueous sodium chloride to one test tube, and aqueous sodium iodide to

the other test tube.

The student shakes the mixtures, allows them to settle, and records the colour of the organic

layer in each mixture.

Sodium halide Colour of organic layer

Sodium chloride orange

Sodium iodide violet

Explain how the student’s results provide evidence for the trend in reactivity of the halogens

down group 17(7) and write an ionic equation for any reaction that takes place.

Use your chemical knowledge to explain the trend in reactivity.

… [5]

(b) Chlorine is used in water treatment.

State one benefit and one risk of using chlorine in water treatment.

Benefit …

Risk …

[1]

1.26 g of compound A is heated to 80 °C.

The volume of gas produced is 0.209 dm3.

Under the conditions used, 1 mol of gas molecules has a volume of 29.0 dm3.

Determine the molecular formula of compound A.

molecular formula = … [3]

Mark scheme

Show the mark scheme

Question Answer Marks Guidance

element

21 (a) Interpretation of Results 5 2.3× 1 Results can be interpreted anywhere in answer.

Orange contains bromine AND no reaction

AND violet contains iodine

ALLOW multiples, e.g. ½Br2 + I– → Br– + ½I2

Ionic equation 2.6×1 IGNORE other halogen/halide equations

Br2 + 2I– → 2Br– + I2

IGNORE state symbols

Reactivity (down the group) ALLOW ORA

Reactivity decreases 1.1×3

AND DO NOT ALLOW idea of losing

oxidising power decreases electrons/ionisation energy

OR gains electrons less easily

OR forms negative ion/1– ion less easily IGNORE chlorine is the most electronegative

OR less energy released when electron gained

IGNORE explanations in terms of displacement

OR more negative electron affinity

Size/shells/shielding (down the group)

Greater atomic radius

OR more shells

OR more shielding

Attraction (down the group)

Less nuclear attraction down the group

20 AO

element

(b) Benefit AND risk required for ONE mark 1 1.1 ALLOW kills micro-organisms

OR kills pathogens OR kills viruses OR

sterilises/disinfects water

Benefit: kills bacteria

AND

Risk: toxic/poisonous IGNORE antiseptic, reduces risk of disease,

OR forms chlorinated hydrocarbons cleans water

OR forms carcinogens/toxic compounds

IGNORE ‘harmful’/’dangerous’

IGNORE chlorine is carcinogenic/

dangerous for health/causes breathing problems

0.209

n(A) = = 0.00721 (mol)

29 2.2×2 ALLOW 2SF 0.0072 up to calculator value

0.0072068965517

1.26

Mr = = 174.8 ALLOW 175 up to calculator value 174.8325359

0.00721

Molecular formula = BrF5 3.2 ALLOW F5Br

Formula is dependent on Mr ALLOW ECF that matches calculated Mr

Total 9

How to answer it

Properties of Halogens & Gas Calculations Study Guide

What this question tests

This question assesses your knowledge of Group 7 chemistry, specifically displacement reactions, trends in oxidising ability and atomic structure down the group, the practical applications and risks of chlorine in water treatment, and stoichiometric gas calculations to determine molecular formulas.

Question 27 (a)

Halogen Displacement & Trends in Reactivity

✅ Correct Answer

Interpretation: Orange layer = bromine (no reaction with NaCl); violet layer = iodine (reaction occurred with NaI).
Ionic Equation: Br₂ + 2I⁻ → 2Br⁻ + I₂
Trend: Reactivity decreases down the group.
Atomic Structure: Atomic radius increases, more shells, and increased shielding.
Attraction: Less nuclear attraction for incoming electrons.

💡 Key Knowledge

Halogens dissolve in non-polar organic solvents (like cyclohexane) to give distinct colours: Cl₂ is pale green/colourless, Br₂ is orange, and I₂ is violet.
A more reactive halogen displaces a less reactive halide from solution. Here, bromine displaces iodide ions because bromine is more reactive than iodine.

🧠 Exam Technique

To secure all 5 marks here, structure your answer logically: first state what the observations mean, write the balanced ionic equation, state the overall trend, and finally explain *why* using shielding and nuclear attraction. Do not skip steps!

❌ Common Errors

Stating that atoms "lose electrons" or referring to "ionisation energy" (halogens gain electrons to form negative ions, which relates to electron affinity).
Confusing the colours of the halogens in organic layers.
Writing full molecular equations instead of requested ionic equations.

Mark Scheme Breakdown (5 Marks): 1 mark for interpreting organic layer colours; 1 mark for correct ionic equation; 1 mark for stating reactivity decreases; 1 mark for mentioning more shells/shielding/atomic radius; 1 mark for explaining decreased nuclear attraction on incoming electrons.

Question 27 (b)

Chlorine in Water Treatment

✅ Correct Answer

Benefit: Kills bacteria, pathogens, or micro-organisms (sterilises/disinfects water).
Risk: Toxic/poisonous, forms chlorinated hydrocarbons, or produces carcinogens.

❌ Common Errors

Vague answers like "it cleans water" or "it makes water safe" without specifying *how* (killing bacteria).
Stating that "chlorine is carcinogenic" directly (the risk is that it reacts with organic matter to form carcinogenic chlorinated compounds).

Mark Scheme Breakdown (1 Mark): Both a valid benefit and a valid risk must be stated to gain the single mark.

Question 27 (c)

Determining Molecular Formula from Gas Volume

📐 Step-by-Step Calculation

Find moles of gas A :
Using the molar volume given (29.0 dm³ mol⁻¹):
Moles = Volume ÷ Molar Volume = 0.209 ÷ 29.0 = 0.0072068... mol (or 0.00721 mol)
Calculate the molar mass ( Mᵣ ) of compound A :
Mᵣ = Mass ÷ Moles = 1.26 ÷ 0.0072068... = 174.8 g mol⁻¹
Determine the molecular formula:
Compound A contains only bromine (Ar = 79.9) and fluorine (Ar = 19.0).
Test combinations: BrF₅ has Mᵣ = 79.9 + (5 × 19.0) = 174.9 g mol⁻¹ .
Therefore, the molecular formula is BrF₅ (or F₅Br ).

🧠 Exam Technique

Always carry unrounded calculator values through multi-step calculations to prevent rounding errors at the final stage. Clearly lay out each step so the examiner can award partial credit if an arithmetic slip is made.

❌ Calculation Traps

Using standard molar gas volume (24.0 dm³ mol⁻¹) instead of the custom value provided in the question stem (29.0 dm³ mol⁻¹).
Inverting the moles formula (doing moles × volume instead of volume ÷ molar volume).

Mark Scheme Breakdown (3 Marks): 1 mark for calculating moles of A correctly; 1 mark for calculating the Mᵣ; 1 mark for deducing the correct molecular formula ( BrF₅ ).

Topics

Module 3: Periodic table and energy · Module 2: Foundations in chemistry · 3.1 The periodic table · 2.1 Atoms and reactions

Question and mark scheme from the OCR A-Level Chemistry examination, Periodic table, elements and physical chemistry (01), November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.