Ionic Bond
Mediumchemistry
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?
Select the correct option:
Solution
Incorrect! Answer:
Smaller ionic radius of Na⁺ compared to K⁺
- Recall the Lattice Enthalpy Trend: Lattice enthalpy depends inversely on the interionic distance. According to the Born–Landé equation, U∝r++r−1, where r+ and r− are the cation and anion radii respectively.
- 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.
- Effect on Lattice Enthalpy: Smaller interionic distance leads to stronger electrostatic attraction between the ions, resulting in higher lattice enthalpy.
- 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.
- Conclusion: The smaller ionic radius of Na⁺ shortens the interionic distance and increases the lattice enthalpy.
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About This Question
- Subject
- chemistry
- Chapter
- chemical bonding and molecular structure
- Topic
- ionic bond
- Difficulty
- Medium
- Year
- 2025
Solution
Correct Answer:
Smaller ionic radius of Na⁺ compared to K⁺
- Recall the Lattice Enthalpy Trend: Lattice enthalpy depends inversely on the interionic distance. According to the Born–Landé equation, U∝r++r−1, where r+ and r− are the cation and anion radii respectively.
- 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.
- Effect on Lattice Enthalpy: Smaller interionic distance leads to stronger electrostatic attraction between the ions, resulting in higher lattice enthalpy.
- 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.
- Conclusion: The smaller ionic radius of Na⁺ shortens the interionic distance and increases the lattice enthalpy.
This medium difficulty chemistry question is from the chapter chemical bonding and molecular structure, covering the topic of ionic bond. It appeared in the 2025 exam.
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