A-Level Chemistry May/June 2024 Q3(b): The trend in the decomposition temperatures of the Group 2 ethanedioates is similar to…

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  CaC₂O₄ will decompose at the lower temperature.

  Explanation:

  The Ca²⁺ ion has a smaller ionic radius and therefore a higher charge density than the Ba²⁺ ion.

  Due to its higher charge density, the Ca²⁺ ion has a greater polarising power. It distorts the electron cloud of the large C₂O₄²⁻ anion more effectively. This polarisation weakens the covalent bonds within the ethanedioate anion, requiring less energy to break them upon heating.

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  How the marks are awarded

  • M1 — Correctly identifying CaC₂O₄ as the compound that decomposes at a lower temperature AND stating that the Ca²⁺ ion has a smaller ionic radius or a higher charge density.
  • M2 — Explaining that the higher charge density of the cation causes the C₂O₄²⁻ anion to become more polarised or distorted, which weakens the bonds within it.

  Common mistakes

  • Incorrectly stating that BaC₂O₄ decomposes at a lower temperature due to confusion about the trend.
  • Explaining the trend using vague terms like 'reactivity' instead of the precise concepts of charge density and ionic radius.
  • Correctly identifying the properties of the Ca²⁺ ion (high charge density) but failing to link this to the polarisation of the anion.
  • Stating that the cation is polarised, instead of correctly identifying that the cation polarises the anion.

  Examiner tip: For questions on the thermal stability of Group 2 compounds with large polyatomic anions, always link the cation's charge density to its ability to polarise the anion, which in turn weakens the anion's internal bonds.

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