A-Level Chemistry May/June 2024 Q1(a)(i): Describe the trend in the solubility of the hydroxides of magnesium, calcium and stront…

1 answer

• ▲0
  accepted ✓

  The solubility of the hydroxides increases down Group 2. Therefore, the order of increasing solubility is magnesium hydroxide < calcium hydroxide < strontium hydroxide.

  This trend can be explained by considering the enthalpy changes involved in dissolving:

  1. As you go down the group from Mg²⁺ to Sr²⁺, the ionic radius of the cation increases. This leads to a decrease in charge density.
  2. Due to the increasing ionic radius, both the lattice enthalpy ($\Delta H_{latt}$) and the enthalpy of hydration ($\Delta H_{hyd}$) become less exothermic (less negative) down the group.
  3. However, the lattice enthalpy decreases in magnitude more significantly than the enthalpy of hydration does.
  4. The enthalpy of solution ($\Delta H_{sol}$) is the overall energy change. Since the endothermic lattice enthalpy term decreases more than the exothermic hydration enthalpy term, the overall $\Delta H_{sol}$ becomes more exothermic (more negative) down the group.
  5. A more exothermic enthalpy of solution results in greater solubility.

  ---

  How the marks are awarded

  • M1 — Correctly stating the trend that solubility increases down the group, or listing the hydroxides in order of increasing solubility.
  • M2 — Stating that both lattice enthalpy and hydration enthalpy become less exothermic (or less negative) down the group.
  • M3 — Explaining that the lattice enthalpy changes more (decreases in magnitude more) than the hydration enthalpy.
  • M4 — Concluding that as a result of the relative changes, the overall enthalpy of solution becomes more exothermic (more negative) down the group.

  Common mistakes

  • Confusing the trend with that of Group 2 sulfates and incorrectly stating that solubility decreases down the group.
  • Failing to compare the relative change in lattice enthalpy vs hydration enthalpy, often only mentioning one of them or stating they both decrease without comparison.
  • Incorrectly stating that hydration enthalpy changes more than lattice enthalpy, which would lead to the opposite conclusion.
  • Using vague language like 'the energies decrease' without specifying which enthalpy changes are involved and whether they are becoming more or less exothermic.

  Examiner tip: For solubility trends of ionic compounds, always explain the outcome by comparing the relative magnitudes of change in both lattice enthalpy and hydration enthalpy down the group.

  ---

  AI-generated model answer, grounded in the official Cambridge mark scheme and reviewed by the MarkScheme team. Mark your own answer to this question →

  @markscheme_answers

Your answer

Sign in to answer this question.