Question

For an ionic compound, the magnitude of the lattice energy (ΔHo lattice) is proportional to the strength of the interactions between the cations and anions in the solid. List and describe two factors that influence the magnitude of ΔHo lattice for an ionic compound.

Answer 1

The two factors are:

1. The charges on the ions : Describing with an example, lattice enthalpy of magnesium oxide is much greater than that of sodium chloride because in magnesium oxide, 2+ ions are attracting 2- ions, but in sodium chloride, the attraction is between 1+ and 1- ions.

2. The radius of the ions : From the same above example, oxide ions are smaller than chloride ions, thus in oxides, ions are closer together in lattice, and increases the strength of the attractions. Similarly, in periodic table as going down Group 7 from flourine to iodine, lattice enthalpies of their sodium salts decreases as the negative ions gets bigger. And due to this, distance between the centeres of the oppositly charged ions becomes greater, finally increasing the attraction.