OCR A-Level Chemistry AS Breadth in chemistry (01), June 2019: Question 23

8 marks · Medium difficulty · Structured Questions

Explain the trend in boiling points of the halogens and describe tests for ammonium and bromide ions in ammonium bromide including reagents, observations, and equations.

Practise this question

Question

Question 23 about halogens and halides. Part (a) includes a table of boiling points for halogens from fluorine to astatine (85 K to 503 K) and asks to explain the trend in boiling points in 3 marks. Part (b) asks to describe simple tests to identify the cation and anion in ammonium bromide, including reagents, expected observations, and relevant equations, worth 5 marks.
Question text

23 This question is about halogens and halides.

(a) The boiling points of the halogens are shown in the table.

Boiling point

Halogen

/ K

fluorine 85

chlorine 239

bromine 332

iodine 457

astatine 503

Explain the trend in boiling points of the halogens.

… [3]

(b) You are supplied with a sample of ammonium bromide.

Describe simple tests that would identify the cation and anion present in ammonium bromide.

Include reagents, expected observations and relevant equations.

… [5]

Mark scheme

Show the mark scheme Mark scheme for question 23. Part (a) awards marks for mentioning an increasing number of electrons down the group, London forces/induced dipole-dipole interactions, and more energy required to break these intermolecular forces (3 marks). Part (b) awards 2 marks for testing the bromide anion using silver nitrate (cream precipitate) and an ionic equation, and 3 marks for testing the ammonium cation using sodium/potassium hydroxide with heating, testing gas with pH/indicator paper to turn blue/purple, and a balanced equation.

AO

Question Answer Marks Guidance

element

23 (a) 3 AO1.1 FULL ANNOTATIONS MUST BE USED

×3 ----------------------------------------------------------------

Electrons (down group) ALLOW more electron shells

number of electrons increases

IGNORE ‘more shells’

OR more (electron) shielding

IGNORE comments about nuclear attraction,

ionisation energy, etc.

Type of intermolecular force (ANYWHERE)

induced dipole(–dipole) interactions

OR IGNORE van der Waals’ forces, vdw

London forces IGNORE abbreviations e.g. LDF, IDID

Link of energy with intermolecular forces (ANYWHERE) IGNORE less energy needed to break ‘bonds’

(Down group,) OR less energy needed to break ‘London forces’

more energy to break/overcome intermolecular forces Too vague – needs idea of ‘between molecules’

OR

more/stronger intermolecular forces IGNORE ‘covalent bonds’ between atoms

BUT response linking to breaking of covalent bonds

is a CON for last marking point ONLY.

10 AO

element

(b) 5 AO3.3 FULL ANNOTATIONS WITH TICKS, CROSSES,

×5 CON, etc. MUST BE USED

Test for Br– (anion) 2 marks ----------------------------------------------------------------

IGNORE confusion between cation and anion

Reagent AND observation IGNORE nitric acid

Silver nitrate/AgNO3 AND cream (precipitate) ALLOW ‘bromine’ for bromide in text

Equation IGNORE responses about solubility in NH3

Ag+ + Br– → AgBr

State symbols not required ALLOW full equation:

e.g. AgNO3 + NH4Br → AgBr + NH4NO3

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

ALLOW displacement by Cl2

Reagent Cl2/chlorine

AND

Observation Orange (solution)

ALLOW shade of orange

DO NOT ALLOW precipitate

Equation 2Br– + Cl → Br + 2Cl–

ALLOW full equation, e.g.

2NaBr + Cl2 → Br2 + 2NaCl

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

Test for NH + (cation) 3 marks

Reagent and conditions

(Heat with) NaOH/KOH/Ca(OH) /OH–/hydroxide

BUT NOT ammonia

Observation (Independent mark)

pH/indicator paper turns blue / purple / alkaline ALLOW full equation:

Equation i.e. NH4Br + NaOH → NaBr + NH3 + H2O

NH + + OH– → NH + H O

43 2

State symbols not required ALLOW NH4Br + NaOH → NaBr + NH4OH

Total 8

How to answer it

Halogens and Halides Exam Guide

What this question tests

This question assesses your understanding of Group 7 (halogens) physical trends, specifically boiling point variations linked to intermolecular forces, alongside practical analytical chemistry skills for identifying specific cations (ammonium) and anions (bromide) using chemical tests, ionic equations, and physical observations.

Question Part (a) - Boiling Point Trend [3 Marks]

Explain the trend in boiling points of the halogens.

✅ Correct Answer Structure

  • Point 1: Number of electrons increases down the group.
  • Point 2: Induced dipole-dipole interactions (London forces) become stronger.
  • Point 3: More energy is required to overcome/break these intermolecular forces between molecules.

💡 Key Knowledge

  • Halogens exist as simple diatomic molecules (X₂).
  • Boiling points increase down Group 7 from fluorine (85 K) to astatine (503 K) due to larger electron clouds setting up larger temporary dipoles.

🧠 Exam Technique

  • Break your answer down into three distinct logical statements: electrons → intermolecular forces → energy required.
  • Use precise terminology: refer specifically to intermolecular forces, not just "bonds".

❌ Common Errors

  • Describing covalent bonds breaking between atoms (penalised heavily!).
  • Using vague phrasing like "breaking bonds between molecules" without specifying intermolecular forces.
  • Mentioning nuclear attraction or ionisation energy, which are irrelevant here.
Mark breakdown: 1 mark for electrons increasing, 1 mark for mentioning induced dipole-dipole / London forces, 1 mark for linking stronger forces to more energy needed to overcome them.
Question Part (b) - Identification Tests [5 Marks]

Describe simple tests to identify the cation and anion in ammonium bromide. Include reagents, observations, and equations.

✅ Correct Answers & Mark Scheme

1. Test for Bromide Anion (Br⁻) [2 marks]:

  • Reagent: Silver nitrate solution ( AgNO₃ ) (+ nitric acid).
  • Observation: Cream precipitate formed.
  • Equation: Ag⁺ + Br⁻ → AgBr

2. Test for Ammonium Cation (NH₄⁺) [3 marks]:

  • Reagent & Conditions: Add sodium hydroxide ( NaOH ) and warm.
  • Observation: Damp red litmus paper turns blue (or indicator turns blue/purple / alkaline gas produced).
  • Equation: NH₄⁺ + OH⁻ → NH₃ + H₂O

💡 Key Knowledge

Halide precipitates with silver nitrate: Chloride = white, Bromide = cream, Iodide = yellow. For ammonium, heating with any alkali releases ammonia gas ( NH₃ ), which is alkaline and turns red litmus paper blue.

🧠 Exam Technique

  • Clearly structure your answer with subheadings for the anion test and cation test so the examiner can award marks quickly.
  • Always state both the reagent and the visual observation.

❌ Common Errors

  • Confusing the tests for cations and anions.
  • Forgetting to state that the mixture must be warmed during the ammonium test.
  • Writing incorrect ionic equations including spectator ions when ionic equations are expected (though full equations are usually accepted if balanced).
Mark breakdown: 2 marks for bromide (reagent + cream precipitate, ionic equation); 3 marks for ammonium (warm with OH⁻, indicator turns blue/alkaline gas, ionic equation).

Topics

Module 2: Foundations in chemistry · Module 3: Periodic table and energy · 2.2 Electrons, bonding and structure · 3.1 The periodic table

Question and mark scheme from the OCR A-Level Chemistry examination, AS Breadth in chemistry (01), June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.