Edexcel A-Level Chemistry Paper 3, June 2023: Question 2

11 marks · Medium difficulty · Short Open Response

Describe and explain the reactions, trends, solubility, and thermal decomposition of Group 1 and Group 2 compounds, including oxidation number changes.

Practise this question

Question

Question text

2 This question is about some reactions of the elements of Group 1 and Group 2 of the

Periodic Table and their compounds.

(a) Group 2 metals react vigorously with oxygen when heated.

(i) Write the equation for the reaction between magnesium and oxygen.

Include state symbols.

(1)

(ii) Explain the trend in reactivity of the elements down Group 2.

(3)

(b) (i) State the trend in solubility of the Group 2 sulfates.

(1)

(ii) Explain why the reaction between calcium and excess sulfuric acid stops

before all of the metal has reacted.

(2)

*P71914A0332*

the products.

(i) Describe the test, with the positive result, for oxygen.

(1)

(ii) Write the equation for this decomposition.

State symbols are not required.

(1)

(iii) Show, by reference to oxidation number changes, that the decomposition of

sodium nitrate is a redox reaction.

(2)

(Total for Question 2 = 11 marks)

Mark scheme

Show the mark scheme

Question

Answer Additional Guidance Mark

Number

Example of equation (1)

2(a)(i) • balanced equation and state symbols 2Mg(s) + O2(g) → 2MgO(s)

Accept multiples

Question

Answer Additional Guidance Mark

Number

An explanation that makes reference to the following points: (3)

2(a)(ii)

• reactivity increases Allow specific references of reactivity trend or

and that ‘barium is the most reactive’

since ionisation energy decreases (1) Allow reactivity increases and easier to lose

electron(s)

Allow reactivity increases and reduced

attraction between the nucleus and the (outer)

electron(s)

(because)

• outer/valence electrons are further from the nucleus (1) Allow atomic radius increases down the group

Ignore just more electron shells

Do not award ionic radius

Do not award reference to charge density

• (electrons) more shielded Ignore reference to increasing nuclear charge

(electrons) more repelled by inner electron shells (1)

If response refers to trend going up Group 2

then allow reverse argument

Question

Additional Guidance Mark

Number

An answer that makes reference to the following point: Accept reverse argument (1)

2(b)(i)

• solubility decreases down the group Allow answers which compare specific

sulfates such as ‘barium sulfate is less soluble

than magnesium sulfate’

Question

Answer Additional Guidance Mark

Number

An explanation that makes reference to the following points: Mark independently (2)

2(b)(ii)

• the (calcium) sulfate / CaSO4 formed is insoluble/ Allow calcium sulfate solid/precipitate/ppt/ppte

CaSO4 sparingly soluble (1) formed

Ignore just calcium sulfate is less soluble

If name and formula given then both must be

correct

• (the reaction stops because) the layer/barrier formed Allow the layer/barrier prevents the sulfuric

(on the surface) prevents further reaction (1) acid from coming into contact with the calcium

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

2(c)(i)

• relights a glowing splint Allow relit/rekindle/reignite

Do not award relights a lit splint

Do not award reference to ‘squeaky pop’ or

similar description of hydrogen result

Question

Answer Additional Guidance Mark

Number

Example of equation (1)

2(c)(ii)

• balanced equation NaNO3 → NaNO2 + ½O2

Allow multiples

Ignore state symbols even if incorrect

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following points: Accept an annotated equation (2)

2(c)(iii)

• oxygen is oxidised from −2 to 0 (in O2) (1)

• nitrogen is reduced from +5 to +3 (in NaNO2) (1)

Allow (1) for these four correct oxidation

numbers which may be annotated on the

equation

(Total Question 2 = 11 marks)

How to answer it

Reactions of Group 1 and Group 2 Elements

📌 What this question tests

This question assesses your understanding of periodicity across the s-block elements (Groups 1 and 2). Key themes include writing balanced chemical equations with state symbols, explaining periodic trends in reactivity and solubility using atomic properties (ionisation energy, shielding, atomic radius), applying observations to practical scenarios (passivation layers), and tracking oxidation number changes in redox thermal decomposition reactions.

Question 2(a)(i)

Magnesium and Oxygen Reaction

✅ Correct Answer

2Mg(s) + O₂(g) → 2MgO(s)

(Accept multiples such as 1 Mg + 0.5 O₂ → 1 MgO)

❌ Common Errors

Omitting state symbols or writing incorrect ones (e.g., writing MgO(aq) ).
Failing to write oxygen as a diatomic molecule ( O₂ ).

Marks available: 1 | Requires balanced equation AND correct state symbols.

Question 2(a)(ii)

Trend in Reactivity Down Group 2

✅ Correct Answer

Reactivity increases down the group.
First and second ionisation energies decrease down the group.
Outer electrons are further from the nucleus (increased atomic radius/distance).
Increased shielding and repulsion from inner electron shells.

🧠 Exam Technique

To secure all 3 marks, you must link the macro observation (reactivity increases) directly to the micro atomic properties (ionisation energy, distance, shielding). Never mention "increasing nuclear charge" without balancing it with shielding, as nuclear charge increases down the group yet ionisation energy still falls.

Marks available: 3 | 1 mark for stating reactivity increases + lower IE; 1 mark for distance/radius; 1 mark for shielding/inner shell repulsion.

Question 2(b)(i)

Solubility Trend of Group 2 Sulfates

✅ Correct Answer

Solubility decreases down the group (e.g., Barium sulfate is insoluble while magnesium sulfate is soluble).

💡 Key Knowledge

Remember the opposite trends in Group 2: hydroxide solubility increases down the group, whereas sulfate solubility decreases down the group.

Marks available: 1

Question 2(b)(ii)

Reaction of Calcium with Sulfuric Acid

✅ Correct Answer

Calcium sulfate ( CaSO₄ ) is formed, which is insoluble or sparingly soluble.
A solid layer/barrier forms on the surface of the metal, preventing further contact between the calcium and sulfuric acid.

❌ Common Errors

Saying the acid runs out (the question explicitly states sulfuric acid is in excess).
Writing an incorrect formula for calcium sulfate.

Marks available: 2 | 1 mark for identifying that CaSO₄ is insoluble/sparingly soluble; 1 mark for explaining the surface layer halts the reaction.

Question 2(c)(i)

Test for Oxygen Gas

✅ Correct Answer

Add a glowing splint. Result: The splint relights (or rekindles/reignites).

❌ Common Errors

Using a lit splint (that is the test for hydrogen, which gives a squeaky pop).

Marks available: 1

Question 2(c)(ii)

Thermal Decomposition of Sodium Nitrate

✅ Correct Answer

NaNO₃ → NaNO₂ + 0.5 O₂

(Alternatively: 2NaNO₃ → 2NaNO₂ + O₂ )

🧠 Exam Technique

Group 1 nitrates (except lithium) thermally decompose into a nitrite and oxygen gas, unlike Group 2 nitrates which decompose into a metal oxide, nitrogen dioxide, and oxygen.

Marks available: 1 | State symbols are not required by the prompt.

Question 2(c)(iii)

Proving Redox via Oxidation Numbers

✅ Correct Answer

Oxygen changes oxidation number from -2 in NaNO₃ (or NaNO₂ ) to 0 in O₂ (therefore oxidised).
Nitrogen changes oxidation number from +5 in NaNO₃ to +3 in NaNO₂ (therefore reduced).

🧠 Exam Technique

To prove a reaction is redox, you must explicitly state both the species that is oxidised and the species that is reduced, along with their initial and final oxidation states.

Marks available: 2 | 1 mark for oxygen oxidation number change + oxidation statement; 1 mark for nitrogen oxidation number change + reduction statement.

Topics

Inorganic Chemistry · Physical Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 3: Redox I

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.