A-Level Chemistry October/November 2024 Q2(a): Predict and explain the variation in enthalpy change of hydration for the ions F¯, Cl-,…

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  The enthalpy change of hydration becomes less exothermic (less negative) down the group from F⁻ to I⁻.

  Explanation:

  1. Going down the group from F⁻ to I⁻, the number of electron shells increases, so the ionic radius increases.
  2. Although all ions have the same -1 charge, the larger ionic radius means the charge is spread over a larger volume, leading to a lower charge density.
  3. This results in a weaker electrostatic attraction between the negative ion and the δ+ hydrogen atoms of polar water molecules. Therefore, the ion-dipole forces of attraction are weaker.
  4. As hydration is an exothermic process, weaker forces being formed means less energy is released. Consequently, the enthalpy change of hydration becomes less exothermic.

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  How the marks are awarded

  • B1 — Stating that the enthalpy change of hydration becomes less exothermic/less negative AND that this is due to the increase in ionic radius down the group.
  • B1 — Explaining that the larger radius leads to a decreased attraction to water molecules, specifically described as weaker ion-dipole forces.

  Common mistakes

  • Stating the trend is 'decreasing' which is ambiguous; you must specify 'less exothermic' or 'less negative'.
  • Forgetting to mention the increase in ionic radius as the primary reason for the trend.
  • Incorrectly stating that the charge on the ion changes down the group.
  • Failing to link the larger size to the weaker forces of attraction with water molecules, thus missing the second mark.

  Examiner tip: For questions on periodic trends, always link the change in a fundamental property like atomic/ionic radius or nuclear charge to the resulting change in electrostatic forces of attraction.

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