Edexcel A-Level Chemistry Paper 1, June 2019: Question 4
8 marks · Medium difficulty · Short Open Response
Investigate the thermal decomposition of metal nitrates and carbonates, explaining the relative ease of decomposition based on ionic radius and polarization.
Practise this questionQuestion
Question text
4 Thermal decomposition is the breaking down of a substance by heat.
(a) An experiment was carried out to investigate the thermal decomposition of a
metal nitrate using the apparatus shown.
glowing splint
metal nitrate
test tube
heat
(i) The glowing splint is used as a test for one of the gases given off in this experiment.
Identify this gas and the positive result of the test.
(1)
(ii) Give the name and appearance of the other gas given off in this experiment
when a Group 2 nitrate is heated.
(1)
(iii) Write the equation for the decomposition if the Group 1 compound,
sodium nitrate, was used in this experiment.
State symbols are not required.
(1)
(iv) Describe the apparatus that would be used to compare the decomposition of
metal carbonates. Include how the rate of decomposition would be compared.
(2)
… *P58306A01028*
(b) Explain why magnesium carbonate decomposes much more readily on heating
than barium carbonate.
(3)
(Total for Question 4 = 8 marks)
Mark scheme
Show the mark scheme
How to answer it
Thermal Decomposition of Group 1 and Group 2 Compounds
What this question tests
This question assesses your knowledge of thermal stability trends down Groups 1 and 2, standard gas tests (oxygen and nitrogen dioxide), writing balanced decomposition equations, practical techniques for measuring rates of thermal decomposition, and the periodic trend in polarizing power (ionic radius and charge density).
Identifying Oxygen Gas
✅ Correct Answer
- Gas identity: Oxygen / O₂
- Test result: The glowing splint relights (or rekindles).
❌ Common Errors
- Writing just the letter O instead of O₂ .
- Confusing the test with hydrogen (which gives a squeaky pop) or carbon dioxide (which turns limewater cloudy).
Brown Gas Identification
✅ Correct Answer
- Gas name/formula: Nitrogen dioxide / NO₂
- Appearance: Brown gas (or brown fumes).
❌ Common Errors
- Stating NO or N₂O₄ .
- Describing the product as a liquid rather than a gas/fumes.
Thermal Decomposition of Sodium Nitrate
✅ Correct Answer
2NaNO₃ → 2NaNO₂ + O₂
Accept correct multiples. State symbols are ignored even if incorrect.
💡 Key Knowledge
Unlike Group 2 nitrates (which decompose fully to metal oxide, nitrogen dioxide, and oxygen), Group 1 nitrates like sodium and potassium nitrate thermally decompose only as far as the metal nitrite and oxygen gas.
Comparing Rates of Carbonate Decomposition
✅ Correct Answer
- Use a delivery tube to bubble the evolved gas through limewater.
- Compare the time taken for the limewater to go cloudy (or milky).
🧠 Exam Technique
Ensure your apparatus description is physically viable. Examiners heavily penalise setups with missing bungs, gaps around delivery tubes, or horizontal tubes dipping directly into limewater without a proper seal, as gas would escape.
Thermal Stability Trends in Group 2 Carbonates
✅ Correct Answer (3 Mark Points)
- The magnesium ion ( Mg²⁺ ) has a smaller ionic radius than the barium ion ( Ba²⁺ ) — or higher charge density.
- Therefore, the magnesium ion has a greater polarizing effect on the large carbonate anion ( CO₃²⁻ ) electron cloud.
- This polarization weakens the carbon-oxygen bonds within the carbonate ion, requiring less thermal energy to break them.
❌ Common Errors & Pitfalls
- Referring to the magnesium or barium atom instead of the ion/cation.
- Stating that ionic bonds between the metal and carbonate are weakened, rather than the covalent bonds within the carbonate ion itself.
Topics
Inorganic Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table
Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 1, June 2019. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.