The binary oxides CaO and Al₂O₃ are often compared when discussing lattice strength

CHEMISTRY · Multi-part · A Level · Chemistry · Bonding · Bonding

Problem

The binary oxides CaO and Al₂O₃ are often compared when discussing lattice strength. Approximate ionic radii are provided below. Ion | Crystal radius / pm Ca²⁺ | 100 Al³⁺ | 67 O²⁻ | 140 Use these data to compare the magnitude of ionic interactions in the two oxides. (a) State which oxide, CaO or Al₂O₃, possesses the larger magnitude of lattice energy. Support your answer qualitatively. (b) Treating the electrostatic interaction energy as directly proportional to the product of the ionic charges and inversely proportional to the center-to-center distance between cation and anion, estimate the ratio ∣Elattice(Al2O3)∣∣Elattice(CaO)∣\dfrac{|E_{\text{lattice}}(\text{Al}_2\text{O}_3)|}{|E_{\text{lattice}}(\text{CaO})|}. Take the cation–anion separation as the sum of the tabulated radii and show your reasoning. (c) Explain how your result in part (b) helps account for the fact that Al₂O₃ has a significantly higher melting point than CaO.

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