Edexcel A-Level Chemistry Paper 1, June 2022: Question 3
9 marks · Medium difficulty · Synoptic Questions
Assess the properties, reactions, electronic configurations, electrode potentials, and boiling points of Group 7 elements and their compounds.
Practise this questionQuestion
Question text
3 This question is about the elements in Group 7 of the Periodic Table and some of
their compounds.
(a) What is the colour of iodine in the solid and gas states?
(1)
Colour of iodine solid Colour of iodine gas
A purple brown
B purple purple
C grey/black brown
D grey/black purple
(b) Which of these reactions occurs?
(1)
A Br2(aq) + 2NaCl(aq) → 2NaBr(aq) + Cl2(aq)
B Br2(aq) + 2NaF(aq) → 2NaBr(aq) + F2(aq)
C Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq)
D Cl2(aq) + 2NaF(aq) → 2NaCl(aq) + F2(aq)
(c) The halogens can form halide ions during reactions.
Complete the electronic configuration of the chloride ion.
(1)
1s2
(d) The standard electrode potentials for two half-equations involving bromine
are given.
– – d
Br2(aq) + 2e 2Br (aq) E = +1.09 V
+ – d
2HOBr(aq) + 2H (aq) + 2e Br2(aq) + 2H2O(l) E = +1.57 V
(i) Explain why the disproportionation of bromine in water is not
thermodynamically feasible under standard conditions.
d
Include the overall equation for the disproportionation and its Ecell value.
(3)
*P67093RA0632*
(ii) Bromine disproportionates in water to a small extent at 298K.
Give a possible reason why this reaction occurs.
(1)
(e) The hydrogen halides have the general formula HX, where X represents the
symbol of the halogen.
(i) Which diagram shows the trend in the boiling temperatures of the
hydrogen halides?
(1)
A B
300 300
Boiling 250 Boiling 250
temperature temperature
/ K 200 / K 200 7
*P67093RA0732*
150 150
HF HCl HBr HI HF HCl HBr HI
Hydrogen halide Hydrogen halide
C D
300 300
Boiling 250 Boiling 250
temperature temperature
/ K 200 / K 200
150 150
HF HCl HBr HI HF HCl HBr HI
Hydrogen halide Hydrogen halide
(ii) What type of reaction occurs when ammonia gas reacts with
hydrogen chloride gas?
(1)
A acid-base
B displacement
C redox
D substitution
(Total for Question 3 = 9 marks)
Mark scheme
Show the mark scheme
Question Answer Mark
number
3(a) (1)
The only correct answer is D (grey/black solid, purple gas)
A is incorrect because iodine solid is not purple and iodine gas is not brown
B is incorrect because iodine solid is not purple
C is incorrect because iodine gas is not brown
number
3(b) (1)
The only correct answer is C (Cl2(aq) + 2NaBr(aq) → 2NaCl(aq) + Br2(aq))
A is incorrect because bromine is less reactive than chlorine so no reaction occurs
B is incorrect because bromine is less reactive than fluorine so no reaction occurs
D is incorrect because chlorine is less reactive than fluorine so no reaction occurs
Question
Answer Additional Guidance Mark
Number
3(c) Example of electronic configuration (1)
• electronic (1s2)2s22p63s23p6
configuration of Allow 2p6 / 3p6 shown as e.g. 2p 22p 22p 2
x y z
chloride ion Ignore 1s2 repeated
Ignore working
Do not award correct answer if one or more incorrect answers are given
Question
Answer Additional Guidance Mark
Number
3(d)(i) An explanation that makes Example of equation (3)
reference to the following points:
• balanced equation (1) Br2(aq) + H2O(l) → HOBr(aq) + HBr(aq)
Allow multiples
Allow H+(aq) + Br−(aq) for HBr(aq)
Allow reversible arrows
Ignore state symbols even if incorrect
• calculation of Eo value (1) Eo = 1.09 – 1.57 = −0.48 (V)
cell cell
Allow correct answer without calculation
• Eo / answer is negative / <0
cell
and
the reaction is not
(thermodynamically)
feasible (1) Allow 3 marks for reverse argument
HOBr(aq) + HBr(aq) → Br2(aq) + H2O(aq) (1)
Eo = 1.57 − 1.09 = (+) 0.48 (V) (1)
cell
Eo is positive / >0 so the reverse of disproportionation is
cell
(thermodynamically) feasible (1)
Question
Answer Additional Guidance Mark
Number
3(d)(ii) An answer that makes reference to the following (1)
point:
• disproportionation is an equilibrium system Ignore just ‘non-standard conditions’
(and although K is very small, there is still a
small concentration of disproportionation Ignore references to activation energy / collision
products) theory
or Ignore H+ / ions from the water
excess water is used
or
concentration is not 1 mol dm−3
or
HOBr undergoes further disproportionation
number
3(e)(i) (1)
The only correct answer is C (graph C)
A is incorrect because HF has a much higher boiling temperature than expected due to hydrogen bonding
B is incorrect because these is an increase in boiling temperature from HCl to HI as the number of electrons
in the molecules increases so the London forces increase in strength
D is incorrect because HBr has a higher boiling temperature than HCl as there are more electrons in the
molecules
number
3(e)(ii) (1)
The only correct answer is A (acid-base)
B is incorrect there is no displacement taking place
C is incorrect because neither substance is oxidised or reduced
D is incorrect because there is no substitution taking place
(Total for Question 3 = 9 marks)
How to answer it
Edexcel A-Level Chemistry: Group 7 Elements & Compounds
What this question tests
This question assesses your knowledge of Group 7 (halogens) trends, physical states, displacement reactions, electronic configurations of halide ions, standard electrode potentials (E-cell calculations and thermodynamic feasibility), disproportionation equilibria, hydrogen halide boiling points (intermolecular forces), and acid-base reactions of hydrogen halides.
Physical States and Colours of Halogens
✅ Correct Answer: D
Iodine is a grey/black solid and forms a purple gas upon sublimation.
❌ Common Errors
Students frequently confuse solid iodine with purple (which is its vapour/solution colour in non-polar solvents) or mistake iodine gas for brown (which is the colour of aqueous iodine or I₃⁻ solutions).
Halogen Displacement Reactions
✅ Correct Answer: C
Reaction: Cl₂(aq) + 2NaBr(aq) -> 2NaCl(aq) + Br₂(aq)
💡 Key Knowledge
Halogen reactivity decreases down the group (F > Cl > Br > I). A more reactive halogen will displace a less reactive halide ion from its solution.
❌ Common Errors
Choosing options where less reactive halogens attempt to displace more reactive ones (e.g., trying to have bromine displace chloride or fluoride).
Electronic Configuration of a Halide Ion
✅ Correct Answer
Chloride ion ( Cl⁻ ): 1s² 2s² 2p⁶ 3s² 3p⁶
🧠 Exam Technique
Remember that a chloride ion has gained one electron compared to a chlorine atom, filling its outer 3p subshell to a stable octet (matching argon).
Electrode Potentials & Feasibility
📐 Calculation & Steps
- Step 1: Construct overall equation
Br₂(aq) + H₂O(l) -> HOBr(aq) + HBr(aq) - Step 2: Calculate E-cell value
E-cell = E(reduction) - E(oxidation)
E-cell = 1.09 V - 1.57 V = -0.48 V - Step 3: State feasibility
Since E-cell is negative (<0), the disproportionation is not thermodynamically feasible under standard conditions.
❌ Common Calculation Traps
Subtracting the electrode values in the wrong order resulting in a positive value. Always ensure you correctly identify the reduced and oxidized species from the half-equations provided.
Disproportionation Extent
💡 Key Knowledge
Even though standard cell potentials suggest non-feasibility, equilibria can still proceed to a very small extent because the equilibrium constant K, though tiny, is non-zero, meaning tiny concentrations of products still form, or non-standard conditions apply.
Hydrogen Halide Boiling Temperatures
✅ Correct Answer: A
Graph A correctly shows HF having an anomalously high boiling point due to hydrogen bonding, followed by an increase from HCl to HI due to increasing London forces as molecular size and number of electrons increase.
❌ Common Errors
Selecting graphs that miss hydrogen bonding in HF or incorrectly show HCl having a higher boiling point than HBr or HI.
Reaction of Ammonia with Hydrogen Chloride
✅ Correct Answer: A (acid-base)
Equation: NH₃(g) + HCl(g) -> NH₄Cl(s) . Ammonia acts as a Brønsted-Lowry base (proton acceptor) and HCl acts as a Brønsted-Lowry acid (proton donor).
🧠 Exam Technique
Do not confuse this classic proton transfer reaction with redox or displacement. No oxidation states change during the formation of ammonium chloride.
Topics
Physical Chemistry · Inorganic Chemistry · Topic 1: Atomic Structure and the Periodic Table · Topic 2: Bonding and Structure · Topic 14: Redox II · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2022. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.