Question

The relation between the magnitudes of lattice energy of crystal and its formation energy is:
A.Lattice energy $ > $ formation energy
B.Lattice energy $ = $ formation energy
C.Lattice energy $ < $ formation energy
D.None of the above

Answer 1

Hint: Lattice energy: It is defined as the energy required to separate one mole of ionic solid into gaseous ions.
Formation energy: It is defined as the energy required or released to generate molecules from the given set of molecules or ions.

Complete answer:
Atoms are not so stable they have high energies so to acquire stability they form molecules in which the same atoms or different atoms combine to each other to attain the stability. Because the higher the energy of the atom or molecule the less stable will be that atom or molecule.
Exothermic reaction: The reactions in which energy in the form of heat and light is released after the reaction, is known as exothermic reactions. In these reactions $Q$ (symbol of energy) is written on the product side. Although, a small amount of energy is required to start a reaction but in these reactions the given energy is greater than the released energy.
Endothermic reaction: The reactions in which the energy is required to start the reaction, are known as endothermic reactions. In these reactions $Q$ (symbol of energy) is written on the reactant side.
Now let us talk about lattice energy and formational energy.
When different atoms or elements are linked to each other by different bonds then they form molecules or compounds.
Lattice energy: It is defined as the energy required to separate one mole of ionic solid into gaseous ions. The value of lattice energy is positive quantity.
Formation energy: It is defined as the energy required or released to generate molecules from the given set of molecules or ions. It has negative value but equal to the value of lattice energy.

So option B is correct i.e. the relation between the magnitudes of lattice energy of crystal and its formation energy is Lattice energy $ = $ formation energy.

Note:
The negative energy is defined as the exothermic reaction i.e. the reaction in which heat is evolved during the reaction. The positive energy is defined as the endothermic reaction i.e. the reaction in which heat is required for the reaction.