Question

1. 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
a) U/2 b) U c) 40 d) 2U​

Answer 1

⟶ AnSwer :-

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

(a) U/2

(b) U

☑ (c) 4 U

(d) 2U

Option (c) 4U is right.

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Explaination :-

$\sf lattice \: energy = \dfrac{product \: of \: charge}{inter \: ionic \: distance}$

In NaCl the product of charges = 1 × 1 ; In CaO product of charges = 2 × 2 = 4 while the inter Ionic distance is almost same in both . Thus, lattice energy of CaO is almost four times the lattice energy of NaCl.

Answer 2

Answer:

LATTICE ENERGY = -q1 q2/ r²

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 Ca2+ and Or ions is 2 each. therefore, the lattice energy of CaO is four times the lattice energy of NaCl, i.e., 4U.