The physical properties of ionic substances
4 marks · NormalExplain why a large amount of energy is required to melt an ionic compound such as sodium chloride.
Compare the melting points of sodium chloride and potassium chloride. Both are ionic, but sodium chloride melts at 801 °C and potassium chloride at 770 °C. Suggest a reason for this difference.
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(a) Because strong electrostatic forces of attraction between oppositely charged ions in the lattice require a lot of energy to break.
(b) Potassium ions are larger than sodium ions, which means the electrostatic forces of attraction between the ions in potassium chloride are weaker, so less energy is needed to separate them.
Also accept: Potassium ions are larger than sodium ions, so lattice is weaker
Common mistakes: Only stating 'because it is ionic' without mentioning force strength; Wrong description of size or charge
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Marking points
M1 (a) · 1 mark · Mentions strong electrostatic forces of attraction between oppositely charged ions.
M2 (a) · 1 mark · Links large amount of energy required to overcoming these forces to melt the compound.
B1 (b) · 1 mark · Recognises potassium chloride has a lower melting point than sodium chloride.
B2 (b) · 1 mark · Explains lower melting point is due to larger size of potassium ions, meaning weaker forces in the lattice.
Worked solution
a) Strong electrostatic forces of attraction between Na+ and Cl- ions in sodium chloride require a lot of energy to overcome when melting the solid.
b) Potassium ions are larger than sodium ions, so the distance between K+ and Cl- ions is greater than between Na+ and Cl-. This reduces the attraction, so potassium chloride melts at a lower temperature.