2024 H2 Chem P1 Q8 — Chemical bonding Solution

The answer is D (potassium peroxide).
The magnitude of lattice energy is proportional to \(\frac{q_+ \times q_-}{r_+ + r_-}\). The least negative (least exothermic) lattice energy belongs to the compound with the smallest ionic charges and the largest ions.
• Cation: \(\text{K}^+\) has a smaller charge (+1) than \(\text{Ca}^{2+}\) (+2), and is also larger, so a potassium compound has a smaller magnitude.
• Anion: \(\text{O}_2^{2-}\) and \(\text{O}^{2-}\) have the same charge, but \(\text{O}_2^{2-}\) is larger, so a peroxide has a smaller magnitude than the oxide.
So potassium peroxide, \(\text{K}_2\text{O}_2\), has the least negative lattice energy.
• A is wrong: calcium oxide has the \(2+\) cation and the smaller \(\text{O}^{2-}\) anion, so it has the most negative lattice energy of the four.
• B is wrong: calcium peroxide contains \(\text{Ca}^{2+}\), whose larger charge makes its lattice energy more negative than that of potassium peroxide.
• C is wrong: potassium oxide contains the smaller \(\text{O}^{2-}\) ion, so its lattice energy is more negative than that of potassium peroxide.