Edexcel A-Level Chemistry AS Paper 1, June 2023: Question 3

12 marks · Medium difficulty · Open Response

Assess properties, bonding, formulas, and reactivity of Group 7 elements and their compounds including oxidation states, dot-and-cross diagrams, empirical formula calculations, bond enthalpy trends, and displacement reactions.

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Question

Question text

3 This question is about elements in Group 7.

(a) Group 7 elements react with many metals.

(i) What happens to the atoms of a metal and the Group 7 element when they

react together?

(1)

Metal Group 7 element

A gains electrons to form a positive ion loses electrons to form a negative ion

B increases in oxidation number gains electrons to form a negative ion

C loses electrons to form a negative ion decreases in oxidation number

D loses electrons to form a positive ion increases in oxidation number

(ii) Which is not true when calcium reacts with bromine?

(1)

A bromine reacts less vigorously than chlorine

B during the reaction bromine oxidises the calcium

C the percentage by mass of calcium in the product is 33%

D the product gives a brick‑red flame test

(iii) The equation for the reaction between aluminium and bromine is shown.

2Al + 3Br2 → Al2Br6

The compound is a dimer made up of two covalently bonded monomers

joined by dative covalent bonds.

Complete the dot‑and‑cross diagram, showing outer shell electrons only, of

this dimer.

Use • for bromine electrons and × for aluminium electrons.

(2)

Br Br Br

Al Al

Br Br Br

(iv)Platinum metal reacts with fluorine to give a compound of platinum with an*P71926A0628*

oxidation number of +6.

Give the formula of this compound of platinum.

(1)

(v) Chromium reacts with iodine.

The only product contains 12.0% chromium by mass.

Calculate the empirical formula of the chromium iodide formed.

You must show your working.

(2)

(b) Chlorine, bromine and iodine are three of the Group 7 elements. The bond length

and bond enthalpies in molecules of these elements are shown in the table.

Bond Bond length / nm Bond enthalpy / kJ mol–1

Cl Cl 0.199 243.4

Br Br 0.228 192.9

I I 0.267 151.2

(i) Explain how the trend in bond length relates to the trend in bond strength on

descending the group.

(2)

(ii) The relative reactivity of chlorine and iodine can be demonstrated using

the reaction between one of these elements and the potassium salt of the 7

other halogen. *P71926A0728*

Describe an experiment to show how this is deduced and give the expected

observation for the reaction that occurs.

Include an ionic equation for this reaction.

State symbols are not required.

(3)

(Total for Question 3 = 12 marks)

Mark scheme

Show the mark scheme

Question

Answer Mark

Number

3(a)(i) gains electrons to form a negative (1)

The only correct answer is B ( increases in oxidation number )

ion

A is not correct because metals lose electrons and Group 7 elements gain them

C is not correct because metals lose electrons to form a positive ion

D is not correct because Group 7 elements decrease in oxidation number

Question

Answer Mark

Number

3(a)(ii) (1)

The only correct answer is C (the percentage by mass of calcium in the product is 33%)

A is not correct because bromine does react less vigorously than chlorine under the same conditions

B is not correct because bromine does oxidise the calcium

D is not correct because calcium ions do give a brick-red flame test

Question Answer Additional Guidance Mark

Number

3(a)(iii) An answer that makes reference to the following Example of diagram (2)

points:

• two dative covalent bonds from a (1)

bridging bromine to an aluminium

• all the remaining electrons correct (1)

Dots and crosses may be swapped

Ignore inner shells

If all dots or all crosses allow M1 only

Number

3(a)(iv) (1)

• PtF6 Do not award PF6 / PtFl6

Do not award PTF6

Number

3(a)(v) An answer that makes reference to the following (2)

points:

• calculation of percentage of iodine and (1) 100 - 12 = 88 (%)

moles of iodine

88 ÷ 126.9 = 0.69346 (mol) (ans 1) Must see either sum or answer

• calculation of moles of chromium and CrI (1) 12 ÷ 52 = 0.23077 / 0.231 / 2.3077 x 10‒1 / 2.31 x 10‒1 (ans 2)

Must see either sum or answer

(ans 1) ÷ (ans 2) = 3.0050

So CrI3

Correct answer with no working or some correct working scores 1

Number

3(b)(i) An answer that makes reference to the following (2)

points:

• going down the group as the bond length Ignore bond enthalpy decreases

increases, the bond strength decreases / less Ignore comments about reactivity

energy is required to break the bonds (1)

• bonding electrons are further from the Allow outer electrons instead of bonding electrons

nucleus / more shielded Award poorer / weaker overlap between bonding orbitals

so less attracted / less tightly held / less Allow reverse argument

energy needed to separate (1) Allow atomic radius increases

Ignore comments about electrons being removed

Do not award ionic radius increases

Number

3(b)(ii) An answer that makes reference to the following Single reactions between a halogen and halide, other than the (3)

points: expected answer can score max 2 for any two correct points.

Series of 2 or more tests can score full marks if the relative

reactivity of iodine and chlorine are shown, but do not award a

marking point for a mistake in any of the tests.

• description of test-tube reaction (1) Solutions of chlorine and potassium iodide mixed (in test tube)

Allow chlorine water

Allow addition only one reactant as a solution

Allow pale green for chlorine solution

Ignore addition of organic solvent to confirm iodine present

If state symbols are given in the equation allow this as evidence the

reaction is in solution

• expected observation and inference (1) Expected result brown solution (containing iodine) and inference

of relative reactivity based on observed result

Allow red-brown, orange-brown instead of brown

Do not award purple solution of aqueous iodine

• equation (1) Equation to match the stated reagents in the form

Cl + 2I‒ → I + 2Cl‒

Ignore state symbols even if incorrect

Ignore non-ionic equations as working, even if incorrect

(Total for Question 3 = 12 marks)

How to answer it

Group 7 Elements & Halogen Chemistry Study Guide

What this question tests

This assessment targets your core understanding of Group 7 (halogens) and period 3/s-block redox chemistry. Key skills tested include interpreting electron transfer and changes in oxidation numbers, deducing covalent/dative bonding structures (dot-and-cross diagrams), performing empirical formula calculations, explaining covalent bond length versus bond enthalpy trends down a group, and describing displacement test-tube reactions with corresponding ionic equations.

Question 3(a)(i)

Redox in Metal-Halogen Reactions

✅ Correct Answer

B: Metal increases in oxidation number, Group 7 element gains electrons to form a negative ion.

❌ Common Errors

Students frequently confuse the direction of electron transfer or incorrectly assume metals gain electrons. Remember that metals are reducing agents that lose electrons (increasing in oxidation number), while halogens are oxidizing agents that gain electrons.

Question 3(a)(ii)

Reactivity & Properties of Calcium and Bromine

✅ Correct Answer

C (the percentage by mass of calcium in the product is 33%).

💡 Key Knowledge

Calcium bromide (CaBr₂) has a molar mass of roughly 40.1 + (2 × 79.9) = 199.9 g mol⁻¹ . The percentage by mass of Ca is (40.1 / 199.9) × 100 = 20% , making statement C mathematically false and therefore the correct "not true" choice.

Question 3(a)(iii)

Dot-and-Cross Diagram for Al₂Br₆ Dimer

✅ Correct Answer & Structure

Show two bridging bromine atoms each forming a dative covalent bond to an aluminium atom.
All remaining outer shell electrons correctly paired/placed for terminal bromines and aluminium atoms.

🧠 Exam Technique

Use dots ( • ) for bromine outer electrons and crosses ( × ) for aluminium outer electrons. Ensure the arrows/dative overlaps clearly originate from the lone pairs on the bridging bromine atoms pointing into the electron-deficient aluminium centers.

Question 3(a)(iv)

Formula of Platinum Fluoride

✅ Correct Answer

PtF₆

❌ Common Errors

Do not write PF₆ or sub-optimal indices like PTFl₆ . The oxidation number of platinum is +6, and each fluoride ion is -1, requiring six fluoride atoms per platinum atom.

Question 3(a)(v)

Empirical Formula Calculation

📐 Step-by-Step Calculation

Find percentage of iodine: 100 - 12.0 = 88.0%
Calculate moles of elements (per 100g):
Moles of Cr = 12.0 / 52.0 = 0.2307 mol
Moles of I = 88.0 / 126.9 = 0.6935 mol
Find simplest whole-number ratio:
Cr : I = 0.2307 / 0.2307 : 0.6935 / 0.2307
Ratio = 1 : 3.005 (rounds to 3)
Final Empirical Formula: CrI₃

❌ Calculation Traps

Always show intermediate division steps or clear mole values to secure M1 and M2. Missing the percentage subtraction step or rounding too early will lose consequential marks.

Question 3(b)(i)

Trend in Bond Length and Strength

✅ Marking Points (2 Marks)

Point 1: Going down the group, bond length increases, meaning bond strength decreases / less energy is required to break the bonds.
Point 2: Bonding electrons are further from the nucleus / experience more shielding, resulting in less attraction and less energy needed to separate them.

🧠 Examiner Commentary

Examiners heavily reward explicit references to atomic radius and shielding. Do not mention "ionic radius" or discuss overall elemental reactivity here—keep laser-focused on bond length and distance from nucleus.

Question 3(b)(ii)

Halogen Displacement Experiment

✅ Expected Observations & Equation

Test description: Mix chlorine water (or chlorine gas) with an aqueous solution of potassium iodide.
Observation: Solution turns brown (or red-brown / orange-brown), confirming iodine is displaced.
Ionic Equation: Cl₂ + 2I⁻ → I₂ + 2Cl⁻

❌ Common Errors & Guidance

State symbols are explicitly not required by the prompt, but if you include them incorrectly, ignore them. Ensure your ionic equation balances both atoms and charges ( 2I⁻ to produce neutral I₂ ).

Topics

Inorganic Chemistry · Physical Chemistry · Organic Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 2: Bonding and Structure · Topic 3: Redox I · Topic 5: Formulae, Equations and Amounts of Substance · Topic 15: Transition Metals

Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.