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A-Level Chemistry May/June 2025 Q1(a)(ii): Describe the trend in the decomposition temperature of the Group 2 nitrates. Explain yo…
A-Level Chemistry · Paper 9701/42 · May/June 2025 · Question 1(a)(ii) · [3 marks]
Describe the trend in the decomposition temperature of the Group 2 nitrates. Explain your answer.
A full-marks model answer with a mark-by-mark examiner breakdown is below.
1 answer
- accepted ✓
The thermal decomposition temperature of the Group 2 nitrates increases down the group.
Explanation:
As you descend Group 2, the ionic radius of the M²⁺ cation increases. A larger cation has a lower charge density and therefore a lower polarising power. This means the larger cation causes less polarisation, or distortion, of the electron cloud of the large nitrate anion (NO₃⁻). Consequently, the covalent N-O bonds within the nitrate ion are weakened less, and more energy (in the form of a higher temperature) is required to break these bonds and decompose the compound.
How the marks are awarded
- M1 — Awarded for the first sentence, which correctly states the trend: 'The thermal decomposition temperature... increases down the group.'
- M2 — Awarded for linking the trend to the change in the cation, specifically stating that the 'ionic radius of the M²⁺ cation increases' down the group.
- M3 — Awarded for explaining the consequence of the larger cation size, stating that it causes 'less polarisation' of the nitrate anion, which means the N-O bonds are 'weakened less' and thus harder to break.
Common mistakes
- Stating the trend incorrectly, e.g., 'The decomposition temperature decreases down the group'.
- Confusing the effect of ionic radius on polarising power, e.g., 'The larger cation has a higher charge density and polarises the anion more'.
- Giving a vague explanation like 'the compound becomes more stable' without mentioning polarisation of the anion.
- Incorrectly discussing lattice energy (the strength of the ionic bond) instead of the polarisation of the anion (weakening of covalent bonds within the anion).
Examiner tip: For questions on the thermal stability of ionic compounds with polyatomic anions, always connect the cation's charge density (related to its size and charge) to its ability to polarise the anion.
AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →
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