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NEET2010ChemistryThermodynamics (C)Actual

CaO and NaCl have the same crystal structure and approximately the same ionic radii. If U is the lattice energy of NaCl , the approximate lattice energy of CaO is

Options

  1. AU / 2
  2. BU
  3. C2 U
  4. D4 U

Correct answer

D. 4 U

Step-by-step solution

Lattice energy =- q₁ q₂ r^2 where q₁ and q₂ are charges on ions and r is the distance between them. Since interionic distances in CaO and NaCl are similar, (larger cation has smaller anion and vice versa) therefore, r is almost the same. Therefore, lattice energy depends only on charge. Since the magnitude of charge on Na ⁺ and Cl ⁻ ions is unity and that on Ca ²⁺ and O ²⁻ ions is 2 each, therefore, the lattice energy of CaO is four times the lattice energy of NaCl , i.e., 4 U .

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