Question

Which of the following has the highest lattice energy?
(a) $NaCl$
(b) $LiF$
(c) $KCl$
(d) $KF$

Answer 1

Hint: We have to find out what is lattice energy, how it is measured, how the lattice is attached, on which factors it depends upon, how the factors affect the lattice energy and then compare them with the given ionic solids.


Complete answer:
Let’s start with lattice energy:
Lattice energy: The energy required to convert one mole of an ionic solid into gaseous ionic compound or the energy released by a crystalline solid when ions are combined to make a compound is known as Lattice energy.
It can be measured by using the Born-Haber Cycle.
The lattice energy is expressed in the terms of Kilojoules per mole ( $KJmo{l^{ - 1}}$ ).
Factors affecting the Lattice energy:
(a) Charge on the ions
(b) Distance between the ions
(a) Charge on the ions: The lattices of the atoms are attached by the electrostatic forces of attraction. The electrostatic force of attraction is directly proportional to the magnitude of the charge held by the ions present in the ionic solid. Hence, larger the charge, larger the force of attraction and stronger the lattice, stronger the lattice stronger the energy required.
(b) Distance between the ions: As the lattices of the atoms are attached by the electrostatic forces of attraction. The distance between the ions decreases the electrostatic force of attraction. Hence, the distance between the ions is inversely proportional to the electrostatic force of attraction. Hence, smaller the size of the ions, greater the force of attraction, stronger the lattice stronger the energy required.
So, in the given options, cations and anions $Cl,F$ , all the atoms have same charge, as the cations are of the same group (Group 1), +1 charge and all the anions of the same charge as they are of same group (Group 17: Halogens), -1 charge. So, they don't differ by the charge on the ions factor.
So, we have to move on the second factor, distance between the ions, as in the given cations $Na,K,Li$ , $Li$ has the smallest size and in the anions $Cl,F$ , $F$ has the smallest size. Hence, the compound having strongest lattice energy will be $LiF$ .

Hence, the correct option is (b) $LiF$.

Note: Lattice energies are defined for ionic solids only. When we are calculating lattice energy, we can use Born-Haber Cycle and when we have to compare the Lattice energy of the ionic solid, we have to keep both the factors in mind.