Explain why calcium fluoride (CaF₂) has a much higher melting point than potassium fluoride (KF), even though both compounds contain fluoride ions.

IB DP Chemistry Higher Level (2023 syllabus) — S2.1 The ionic model · Explain · 4 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (4 marks)

CaF₂ contains Ca²⁺ ions whereas KF contains K⁺ ions, so the cation in CaF₂ has a greater charge.
Ca²⁺ is smaller than K⁺ (has a smaller ionic radius), so the interionic distance in CaF₂ is shorter.
Greater charge and smaller ionic radius give stronger electrostatic attraction between oppositely charged ions in CaF₂.
More energy is therefore required to overcome the stronger electrostatic attractions in CaF₂, giving a higher melting point.

Examiner tips

  • Use the charge and size of the cation to explain the difference.
  • Show the link between stronger attraction and higher melting point.
  • Keep the answer concise and use the exact terminology.
  • Use bullet points or short sentences to cover each point.

Mark scheme (4 marks)

  1. CaF₂ contains Ca²⁺ ions whereas KF contains K⁺ ions, so the cation in CaF₂ has a greater charge.
  2. Ca²⁺ is smaller than K⁺ (has a smaller ionic radius), so the interionic distance in CaF₂ is shorter.
  3. Greater charge and smaller ionic radius result in stronger electrostatic attraction between oppositely charged ions in CaF₂.
  4. More energy is therefore required to overcome the stronger electrostatic attractions in CaF₂, resulting in a higher melting point.

Key terms in this question

melting point

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