Ionic Bond

Question

Lattice enthalpy of NaCl is 786 kJ mol⁻¹. Which of the following factors most significantly contributes to the high lattice enthalpy of NaCl compared to KCl?

RankGuru · Accepted Answer

1. **Recall the Lattice Enthalpy Trend**: Lattice enthalpy depends inversely on the interionic distance. According to the Born–Landé equation, $U \propto \frac{1}{r_+ + r_-}$, where $r_+$ and $r_-$ are the cation and anion radii respectively.
2. **Compare Ionic Radii**: Na⁺ has ionic radius ≈ 95 pm while K⁺ has ionic radius ≈ 133 pm. Since the anion (Cl⁻, 181 pm) is the same, the interionic distance is smaller in NaCl than in KCl.
3. **Effect on Lattice Enthalpy**: Smaller interionic distance leads to stronger electrostatic attraction between the ions, resulting in higher lattice enthalpy.
4. **Why Not Other Options?**
   - Electronegativity of the cation is not the primary factor determining lattice energy; it is the charge and size.
   - Number of electrons in Na⁺ is actually fewer than in K⁺, and electron count doesn't directly determine lattice enthalpy.
   - Ionisation enthalpy of Cl pertains to removing an electron from Cl, which is unrelated.
5. **Conclusion**: The smaller ionic radius of Na⁺ shortens the interionic distance and increases the lattice enthalpy.

Ionic Bond

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