A-Level Chemistry October/November 2024 Q1(d): Explain why compound E is much more soluble than magnesium hydroxide.

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  Compound E is a Group 2 hydroxide further down the group than magnesium. As you descend Group 2 from Mg to E, the ionic radius of the M²⁺ cation increases.

  1. Due to the larger ionic radius and lower charge density of the cation in compound E, both the lattice enthalpy (∆H_latt) and the hydration enthalpy (∆H_hyd) become less exothermic compared to magnesium.

  2. The magnitude of the decrease in lattice enthalpy is greater than the magnitude of the decrease in hydration enthalpy down the group.

  3. The enthalpy of solution (∆H_sol) is the net result of these two opposing energy changes. As the lattice enthalpy decreases more significantly, the overall ∆H_sol becomes more exothermic (or less endothermic) down the group. A more exothermic enthalpy of solution means the dissolving process is more energetically favourable, leading to greater solubility. Therefore, compound E is more soluble than Mg(OH)₂.

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  How the marks are awarded

  • M1 — The model answer correctly states that both lattice enthalpy and hydration enthalpy become less exothermic down the group from magnesium.
  • M2 — This mark is for correctly comparing the rates of change, stating that the lattice enthalpy changes (decreases in magnitude) more than the hydration enthalpy.
  • M3 — The final mark is awarded for linking the relative changes in enthalpy to the overall enthalpy of solution, concluding that it becomes more exothermic, which causes the increase in solubility.

  Common mistakes

  • Stating that hydration enthalpy changes more than lattice enthalpy. This is a common confusion, as it is true for other solubility trends (e.g., Group 1 halides) but not for Group 2 hydroxides/sulfates.
  • Only mentioning the change in lattice enthalpy and ignoring the change in hydration enthalpy, or vice versa.
  • Confusing the enthalpy signs, for example stating that solubility increases because the enthalpy of solution becomes 'more negative' but then incorrectly describing the changes in LE and H_hyd that would lead to a less negative value.
  • Simply stating that solubility increases down the group without providing an explanation based on enthalpy changes.

  Examiner tip: For questions on solubility trends, always explain the outcome by comparing the relative magnitudes of change of both the lattice enthalpy and the hydration enthalpies.

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