Edexcel A-Level Chemistry Paper 3, June 2018: Question 1
10 marks · Medium difficulty · Short Open Response
Describe the formation of London forces, explain the boiling point trend between chlorine and bromine, and describe displacement experiments to determine the reactivity of halogens.
Practise this questionQuestion
Question text
1 This question is about some halogens and their compounds.
(a) The intermolecular attractions between halogen molecules are London forces.
(i) Describe how London forces form between halogen molecules.
(3)
(ii) The boiling temperatures of chlorine and bromine are shown in the table.
Halogen Boiling temperature / °C
chlorine −34
bromine 59
Explain why bromine has a higher boiling temperature than chlorine.
(2)
(b) A student carries out experiments to determine the order of reactivity of three halogens:
bromine, chlorine and iodine.
The student is provided with aqueous solutions of the following five substances:
2 • bromine
• iodine *P52304A0232*
• potassium chloride
• potassium bromide
• potassium iodide.
The student has no access to chlorine gas or chlorine water.
The student uses cyclohexane, an organic solvent, to identify the halogen present
at the end of each experiment.
The student carries out the smallest number of experiments required to
determine the order of reactivity of the halogens.
Describe the experiments and the expected observations.
Include in your answer ionic equations for any reactions that occur.
State symbols are not required.
(5)
(Total for Question 1 = 10 marks)
Mark scheme
Show the mark scheme
Question
Acceptable Answers Additional Guidance Mark
Number
1(a)(i) An answer that makes reference to the following points: M1 & M3 could be scored for an appropriate (3)
diagram
setting up of the dipole
uneven distribution of electrons / Allow
(random) movement of electrons / (random) “Change in electron density”
fluctuations of electrons
(1)
type of dipole
(results in an) instantaneous dipole / temporary dipole Allow
(in the first molecule) “transient dipole” / “oscillating dipole”
(1) Do not award for “permanent dipole”
induction of a second dipole
causes/induces a (second) dipole on another molecule Allow
(1) neighbouring molecule / adjacent molecule
Do not award for “permanent dipole”
Question
Acceptable Answers Additional Guidance Mark
Number
1(a)(ii) An explanation that makes reference to the following Allow reverse arguments (2)
points: Allow correct formulae
relative number of electrons
bromine has more electrons (than chlorine) / bromine Bromine has 35/70 electrons and chlorine has
has one more shell of electrons (than chlorine) 17/34 electrons
(1)
Ignore comments about protons, molecular
mass etc
Do not award “more outer shells”
relative strength of intermolecular forces
(so) bromine has stronger (London) forces (between Ignore comments about ‘points of contact’
molecules) / more (heat) energy is needed to overcome Allow more (London) forces
the London forces between bromine molecules / greater Allow “bonds between molecules”
temporary dipole – induced dipole forces
(1)
Award (0) marks overall if any implication that
covalent bonds are broken (on boiling)
Question
Acceptable Answers Additional Guidance Mark
Number
1(b) An answer that makes reference to the following points: Ignore any reference to any additional reactions, (5)
mixing of 1st pair of solutions e.g. with silver nitrate
mix Br2 with KCl Award mark if correct ionic equation is given
(1)
mixing of 2nd pair of solutions
mix Br2 with KI
or
mix I2 with KBr
(1)
colours of halogen (in cyclohexane) Ignore colours before the addition of
cyclohexane
colour seen for experiment 1/ bromine is orange / Do not award brown
yellow
and
colour seen for experiment 2/ iodine is purple / pink / Do not award red
violet / lilac
(1)
correct ionic equation
How to answer it
Halogens and Their Compounds
This question assesses your understanding of physical trends and chemical properties in Group 7 (halogens). Specifically, it targets intermolecular forces (London forces), factors influencing boiling temperatures, displacement reactions used to determine halogen reactivity, and the practical identification of halogens using organic solvents like cyclohexane.
Part (a)(i): Formation of London Forces
Describe how London forces form between halogen molecules. (3 marks)
✅ Correct Answer / Mark Scheme
- Point 1: Uneven distribution / random movement of electrons in a molecule (1 mark)
- Point 2: Creates an instantaneous (temporary) dipole in the first molecule (1 mark)
- Point 3: Induces a dipole in a neighbouring/adjacent molecule (1 mark)
💡 Key Knowledge
London forces (instantaneous dipole–induced dipole forces) exist between all simple covalent molecules. They arise due to the constant, random motion of electrons creating momentary imbalances in electron density.
🧠 Exam Technique
Be precise with your terminology. Use phrases like "uneven distribution of electrons" and "induced dipole in a neighbouring molecule". Memorise this 3-step sequence as it is heavily examined.
❌ Common Errors
- Mentioning permanent dipoles (halogens are non-polar molecules).
- Stating that covalent bonds break during boiling.
- Vague wording like "electrons move around" without linking it to a temporary or instantaneous dipole.
Part (a)(ii): Boiling Temperatures of Halogens
Explain why bromine has a higher boiling temperature than chlorine. (2 marks)
✅ Correct Answer / Mark Scheme
- Point 1: Bromine has more electrons (or more electron shells) than chlorine (1 mark)
- Point 2: Therefore, stronger London forces between bromine molecules require more energy to overcome (1 mark)
💡 Key Knowledge
As you go down Group 7, the number of electrons in the molecules increases. Larger electron clouds lead to greater polarisability and stronger instantaneous dipole–induced dipole forces, raising boiling temperatures.
🧠 Exam Technique
Always link molecular size / number of electrons directly to the strength of intermolecular forces, and finally to the energy required to overcome them. Never mention breaking covalent bonds when discussing boiling points of simple molecules!
❌ Common Errors
- Stating that bromine has "more outer shells" (it has more total shells/electrons, but the same number of outer shell electrons).
- Confusing intermolecular forces with covalent bonds inside the molecule.
- Referring to molecular mass or protons instead of electrons.
Part (b): Halogen Displacement Reactions
Describe the experiments, expected observations, and ionic equations to determine the order of reactivity of bromine, chlorine, and iodine (without access to chlorine gas/water). (5 marks)
✅ Correct Answer / Mark Scheme
- Experiment 1: Mix Br₂ with KCl (1 mark)
- Experiment 2: Mix Br₂ with KI or I₂ with KBr (1 mark)
- Colours in Cyclohexane: Bromine = orange/yellow; Iodine = purple/pink/violet (1 mark)
- Ionic Equations: Correct equations for any reacting pairs (up to 2 marks)
Br₂ + 2I⁻ ➔ I₂ + 2Br⁻
Cl₂ is not used since chlorine is unavailable, so Br₂ acts as the testing agent against Cl⁻, Br⁻, and I⁻.
💡 Key Knowledge
Reactivity of halogens decreases down the group (Cl₂ > Br₂ > I₂). A more reactive halogen will displace a less reactive halide ion from solution. Cyclohexane is an organic solvent used because halogens are much more soluble in it than in water, making distinct upper organic layers with clear characteristic colours.
🧠 Exam Technique
Read constraints carefully: the student has no access to chlorine. Therefore, bromine is your starting oxidant to test against chloride and iodide ions, requiring the smallest number of experiments.
❌ Common Errors
- Including unnecessary tests with chlorine or repeating redundant combinations.
- Getting organic solvent colours wrong (e.g., calling iodine red or brown).
- Including spectator ions in ionic equations.
Topics
Physical Chemistry · Inorganic Chemistry · Topic 2: Bonding and Structure · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2018. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.