# 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.

> Edexcel A-Level Chemistry (9CH0) — 13.1 Born-Haber cycles and entropy · Evaluate · 5 marks

> 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.

## Mark scheme (5 marks)

1. The solid lattice has particles in fixed, ordered positions (low entropy / low disorder)
2. When dissolved, ions are free to move and are more randomly distributed / disordered (higher entropy in solution)
3. Water molecules become ordered / arranged around the ions (hydration reduces entropy of the solvent)
4. The entropy increase from releasing ions into solution competes with / is partially offset by the entropy decrease from ordering water molecules
5. 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

- [entropy](https://www.gradenine.co.uk/glossary/entropy)
- [lattice](https://www.gradenine.co.uk/glossary/lattice)
- [standard entropy change](https://www.gradenine.co.uk/glossary/standard-entropy-change)

## Related

- [Revision notes for Edexcel A-Level Chemistry (9CH0)](https://www.gradenine.co.uk/learn)
- [How to answer "Evaluate" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
- [Practice this with AI marking (free)](https://www.gradenine.co.uk/start)

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Source: [GradeNine](https://www.gradenine.co.uk/q/calcium-fluoride-caf-forms-an-ionic-877c1647) · Published by Druglandscape Ltd.