Edexcel A-Level Chemistry AS Paper 1, November 2020: Question 8
8 marks · Medium difficulty · Open Response
Explain the thermal stability and decomposition reactions of Group 1 and Group 2 nitrates and carbonates.
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Question text
8 This question is about the thermal stability of Group 1 and Group 2 nitrates and carbonates.
(a) Complete the equations for the thermal decomposition of sodium nitrate, NaNO3,
and for the thermal decomposition of calcium nitrate, Ca(NO3)2.
State symbols are not required.
(2)
NaNO3 →
Ca(NO3)2 →
(b) The thermal stability of Group 1 nitrates increases down the group.
The decomposition temperatures of some Group 1 nitrates are shown.
Decomposition temperature
Name Formula
/ K
sodium nitrate NaNO3 653
potassium nitrate KNO3 673
caesium nitrate CsNO3 687
Explain why the thermal stability of caesium nitrate is greater than that of
sodium nitrate.
(3)
(c) Calcium carbonate is thermally decomposed during the manufacture of cement.
20 (i) Write an equation, including state symbols, for the thermal decomposition of
calcium carbonate.*P62306A02024*
(1)
(ii) Name all the types of bond present in calcium carbonate.
(1)
(iii) Give a reason, in terms of the bonding, why a high decomposition
temperature is required.
(1)
(Total for Question 8 = 8 marks)
Mark scheme
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How to answer it
Thermal Stability of Group 1 and Group 2 Nitrates and Carbonates
What this question tests
This question assesses your understanding of the trends in thermal stability across Groups 1 and 2, writing thermal decomposition equations, identifying bonding types (ionic vs covalent within polyatomic ions), and applying the concepts of cationic polarising power and charge density to explain chemical stability.
Thermal Decomposition Equations
Complete equations for the thermal decomposition of sodium nitrate and calcium nitrate.
✅ Correct Answer
- Sodium nitrate: NaNO₃ → NaNO₂ + ½O₂ (or balanced with whole numbers: 2NaNO₃ → 2NaNO₂ + O₂ )
- Calcium nitrate: Ca(NO₃)₂ → CaO + 2NO₂ + ½O₂ (or balanced with whole numbers: 2Ca(NO₃)₂ → 2CaO + 4NO₂ + O₂ )
❌ Common Errors
- decomposing Group 1 nitrates all the way to the metal oxide (like Group 2 does). Group 1 nitrates decompose to form the nitrite and oxygen.
- Forgetting that Group 2 nitrates decompose into the metal oxide, nitrogen dioxide gas, and oxygen gas.
Explaining Trends in Thermal Stability
Explain why the thermal stability of caesium nitrate is greater than that of sodium nitrate. (3 marks)
✅ Correct Answer (3-Mark Breakdown)
- Point 1: Thermal stability depends on the polarising power / charge density of the cation (or statement that Na⁺ causes more distortion / Cs⁺ causes less distortion).
- Point 2: The sodium ion ( Na⁺ ) is a smaller cation and has a greater charge density than caesium ( Cs⁺ ).
- Point 3: Therefore, Na⁺ distorts/weakens the electron clouds of the bonds in the nitrate ion to a greater extent than Cs⁺ , making it easier to decompose.
🧠 Exam Technique
Always structure your comparison answers clearly by referring to both ions mentioned in the question. Use clear chemical terminology: polarising power, charge density, ionic radius, and distortion of electron cloud.
Decomposition of Calcium Carbonate
Write an equation, including state symbols, for the thermal decomposition of calcium carbonate. (1 mark)
✅ Correct Answer
CaCO₃(s) → CaO(s) + CO₂(g)
❌ Common Errors
Omitting state symbols or getting them wrong (e.g., writing (aq) for solids or gases) will lose you the mark since state symbols were explicitly requested here.
Bonding Types in Calcium Carbonate
Name all the types of bond present in calcium carbonate. (1 mark)
✅ Correct Answer
Ionic and covalent (bonding).
💡 Key Knowledge
Calcium carbonate contains ionic bonding between the Ca²⁺ cation and the CO₃²⁻ anion, and covalent bonding holding the carbon and oxygen atoms together inside the polyatomic carbonate ion ( CO₃²⁻ ).
High Decomposition Temperature
Give a reason, in terms of the bonding, why a high decomposition temperature is required. (1 mark)
✅ Correct Answer
There are strong covalent bonds within the carbonate ion ( CO₃²⁻ / C-O / C=O bonds) that require a large amount of energy to break during thermal decomposition.
❌ Common Errors
Do not say that the ionic bonds between Ca²⁺ and CO₃²⁻ are what need to be broken for decomposition; examiners explicitly note to ignore comments claiming the ionic attractions between the ions are the strong bonds being broken in this specific context.
Topics
Inorganic Chemistry · Physical Chemistry · Topic 4: Inorganic Chemistry and the Periodic Table · Topic 2: Bonding and Structure
Question and mark scheme from the Edexcel A-Level Chemistry examination, AS Paper 1, November 2020. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.