Question

In each row, pick the compound with the bigger lattice energy. Note: lattice energy is always greater than zero. Which compound has the bigger lattice energy? Cas СаCl Mgl MgBr2 BeF2 LiF

Answer 1

Lattice energy = k. q₁ × q₂ / r

Lattice energy is proportional to the product of charges of the cation and anion present in the molecule; and is inversely proportional to the distance between cation and anion in the molecule.

This means more the value of product of charges, more is the lattice energy; and lesser is the ionic radius, more is the lattice energy.

Also, if the cation or the anion are bigger in size, naturally the distance between them (distance from the center of cation to the center of anion) increases within a molecule.

CaCl₂ vs CaS

Here the product of the charges is same in both case (+2 × -2 = -4). However, the radius of Cl is greater than S. So, CaS will have higher value of lattice energy.

(*Ca is the element on left side of periodic table in group 4, while S is on right side in group 3. Size increases down the group and decreases from left to right in periods)

MgBr₂ vs MgI₂

Here also charge is same but radius of Iodine is greater than Br. So, MgBr₂ will have higher lattice energy.

LiF vs BeF₂

Here the product of charges for LiF = +1 × -1 = -1

the product of charges for BeF₂ = +2 × -2 = -4

So, charge for BeF₂ is 4 times greater than LiF.

Also, radius of Li is greater than radius of Be. Hence, BeF₂ will have higher lattice energy.