Question

Which of the following compounds has the largest lattice energy?

NaCl

CaCl2

RbCl

MgCl2

Answer 1

Let us First consider the Born-Lande equation for computing Lattice Energy-

So from the equation,two things are clear which the Lattice Energy U depends on-

1. The Charge on the ions- Lattice Energy U is directlry proportional with the charges on the cations and anions
2. The distance of cation and anion-U is inversely proportional to the distance of the cation to anion i.e. the interionic distance.

As the anion is same in all four i.e. Cl-,the competition is between cations-

From the above discussion,it is clear that,

More the charge on cation=Greater U value

On the ground of requirement of +2 charge,NaCl and RbCl are knocked out as both the cations have +1 charges on them

Now between CaCl2 and MgCl2,consider the 2nd factor i.e. distance between cation and anion.

To account this,consider the position of Ca2+ and Mg2+ on Periodic table,as Mg is above Ca in the Alkaline earth metal group,it is quite evident that Mg2+ is smaller than Ca2+.As U depends inversely on size of cation,the smaller the ion the stronger the electrostatic attraction between them,leading to lesser distance between the cation and anion resulting the larger Lattice Energy.

Hence, MgCl2 is anticipated to have the highest Lattice Energy.