Question

Which has larger lattice energy among $ZnO$ and $NaCl$?
A.$ZnO > NaCl$
B.$ZnO < NaCl$
C.$ZnO = NaCl$
D.None of these

Answer 1

Hint: Lattice energy is the energy released when the ions of opposite charge in their gas phase come together to form a solid phase. Lattice energy is directly proportional to the charge on the ions. So the more the charge of the ions the more will be the lattice energy of that crystal.

Complete step by step answer:
Here we have to find which has larger lattice energy among $ZnO$ and$NaCl$. The lattice energy is the energy released when ions of gaseous phase combine together to form an ionic solid. As we have known lattice energy is directly proportional to the charge on the ions.
$ \Rightarrow {\text{L}}{\text{.E}}{\text{.}} \propto {\text{Charge}}$ -- (i)
As the charge of the ion increases, charge density also increases. So the ions form a strong electrostatic force of attraction which results in a stronger bond. So more energy will be released during formation of lattice structure.
First we will break the compounds into their respective ions-
$ \Rightarrow ZnO \rightleftharpoons Z{n^{2 + }} + {O^{2 - }}$
$ \Rightarrow NaCl \rightleftharpoons N{a^ + } + C{l^ - }$
 Here we can see that the given compounds break into their respective ions. In zinc oxide, the positive and negative ions both carry two charges. In sodium chloride, the positive and negative ions both carry only one charge.
So according to eq. (i) the lattice energy of Zinc oxide will be higher as its ions carry two charges. The lattice energy of Sodium chloride will be less than that of zinc oxide as ions of sodium chloride carry only one charge.

Hence the correct answer is A.
Note:
Lattice energy also depends on the size of the ions. Lattice energy is inversely proportional to the size of the ions. So as the size of the ions increases, the lattice energy of the compound decreases. So if when comparing there is the same charge on both the negative and positive ions of both compounds then we can check the size of the ions to see which of the compounds has larger lattice energy.