Calcium fluoride, CaF₂, forms an ionic lattice. A student claims that the standard entropy change for the dissolving of calcium fluoride in water is negative. Evaluate this claim, using ideas about the number and arrangement of particles in the solid and in solution.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Calcium fluoride has a giant ionic lattice structure. When it dissolves in water, calcium ions and fluoride ions are released into solution and become surrounded by water molecules.
Model answer (5 marks)
The solid lattice of CaF₂ has the calcium and fluoride ions fixed in a highly ordered array, giving it a low entropy.
When CaF₂ dissolves, the Ca²⁺ and two F⁻ ions are released into the bulk solution. These ions are free to move throughout the solvent, increasing the number of accessible micro‑states and therefore the entropy of the solute.
However, each ion is surrounded by a hydration shell of water molecules. The water molecules become more ordered around the ions, which reduces the entropy of the solvent.
The overall standard entropy change is a balance between the large positive contribution from the increased disorder of the ions and the negative contribution from the ordering of water. In most cases the ion‑release term dominates, so ΔS° for the dissolution of CaF₂ is positive. Consequently, the student’s claim that ΔS° is negative is unlikely to be correct.
When CaF₂ dissolves, the Ca²⁺ and two F⁻ ions are released into the bulk solution. These ions are free to move throughout the solvent, increasing the number of accessible micro‑states and therefore the entropy of the solute.
However, each ion is surrounded by a hydration shell of water molecules. The water molecules become more ordered around the ions, which reduces the entropy of the solvent.
The overall standard entropy change is a balance between the large positive contribution from the increased disorder of the ions and the negative contribution from the ordering of water. In most cases the ion‑release term dominates, so ΔS° for the dissolution of CaF₂ is positive. Consequently, the student’s claim that ΔS° is negative is unlikely to be correct.
Examiner tips
- Use the command word ‘evaluate’ to discuss both the increase in disorder from ion release and the decrease from hydration. Show that the lattice has low entropy and the solution has higher entropy, then explain the competing hydration effect. Conclude that the net ΔS° is positive, matching the mark scheme.
- Include the key terms: ‘lattice’, ‘ordered’, ‘disordered’, ‘hydration’, ‘entropy change’.
- Keep the answer concise – 5 marks can be earned with a short paragraph, not an essay.
Mark scheme (5 marks)
- The solid lattice has particles in fixed, ordered positions (low entropy / low disorder)
- When dissolved, ions are free to move and are more randomly distributed / disordered (higher entropy in solution)
- Water molecules become ordered / arranged around the ions (hydration reduces entropy of the solvent)
- The entropy increase from releasing ions into solution competes with / is partially offset by the entropy decrease from ordering water molecules
- The student's claim is unlikely to be correct / the overall standard entropy change is likely positive, because releasing ions from a rigid lattice into solution typically produces a net increase in disorder
Key terms in this question
entropy · lattice · standard entropy change
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Evaluate" question →
- Decode the mark scheme abbreviations →
More Born-Haber cycles and entropy questions
- Explain why the standard enthalpy of atomisation of chlorine is an endothermic p…
- Explain why the standard entropy change for the formation of sodium chloride fro…
- Potassium bromide, KBr, is an ionic compound. A student constructs a Born-Haber …
- Explain why the lattice enthalpy of magnesium oxide is much more negative than t…