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A-Level Chemistry May/June 2025 Q1(b): State which of CaCO3 and BaCO3 decomposes at a lower temperature. Explain your answer.…
A-Level Chemistry · Paper 9701/41 · May/June 2025 · Question 1(b) · [2 marks]
State which of CaCO3 and BaCO3 decomposes at a lower temperature. Explain your answer. The compound that decomposes at a lower temperature is explanation
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The compound that decomposes at a lower temperature is CaCO₃.
Explanation:
The Ca²⁺ ion has a smaller ionic radius than the Ba²⁺ ion. This gives the Ca²⁺ ion a higher charge density and therefore a greater polarising power. As a result, the Ca²⁺ ion distorts the electron cloud of the large carbonate anion (CO₃²⁻) to a greater extent. This distortion weakens the covalent bonds within the carbonate ion, meaning less energy is required to break them, so decomposition occurs at a lower temperature.
How the marks are awarded
- M1 — Correctly identifying calcium carbonate (CaCO₃) and stating that the calcium ion (Ca²⁺) is smaller than the barium ion (Ba²⁺).
- M2 — Explaining that the Ca²⁺ ion causes greater distortion or polarisation of the carbonate anion's electron cloud.
Common mistakes
- Stating that BaCO₃ decomposes at a lower temperature, reversing the group trend.
- Giving a vague explanation based on 'reactivity' rather than ionic size and polarising power.
- Incorrectly describing the polarisation, e.g., stating that the carbonate anion polarises the cation.
- Referring to atoms (Ca and Ba) instead of ions (Ca²⁺ and Ba²⁺) when explaining the polarisation effect.
Examiner tip: For questions on the thermal stability of Group 2 compounds, always link the cation's size to its polarising power and then to the resulting distortion of the anion's electron cloud.
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