Lattice Energy & Hydration Energy - Entropy: how far?

Factors that affect lattice energy:

The magnitude of lattice energy depends on the forces acting on the ions. In a lattice, each ion is surrounded by a number of ions of opposite charge, resulting in strong forces of attraction and some forces of repulsion.

According to Coulomb’s law [F=kQ₁Q₂/r²] where Q=charge on ion and r= sum of the radii

The strength of these forces of attraction and repulsion depend on:

·         The product of the charges on the ions – the greater the product of the charges, the greater the forces and the more exothermic the lattice energy

·         The sum of the radii of the cation and the anion – the larger the sum, the smaller the forces hence the less exothermic the lattice energy

·         The extent of covalency – the greater the extent of covalency, the greater the forces and the more exothermic the lattice energy

Example question: Explain why the lattice energy of NaF is more exothermic than the lattice energy of KCl.

 First of all, identify the differences and similarities between these two compounds.

The ionic lattice of NaF contains the ions: Na⁺ and F^-

The ionic lattice of KCl contains the ions:  K⁺   and Cl^-

Similarities: The product of the charges is the same for both compounds.

Differences: Na⁺ has as a smaller ionic radius than K⁺ and F^- is smaller than Cl^-.

What is the conclusion?

Therefore, the forces between the sodium ions and the fluoride ions are stronger than those between potassium ions and chloride ions, so the lattice energy for NaF is more exothermic than for KCl.

Lattice energy/ kJ mol -1
Halides	LiCl                -846	NaCl                    -771
Oxides	Li2O               -2814	Na2O                   -2478
	BeO               -4444	MgO                   -3890
Sulfates	Li2S                -2500	BeS                     -3832
Hydroxides		Mg(OH)2                   -2842

§  The magnitude of the lattice energy steadily decreases (becomes less exothermic; less heat given off) down a group of the periodic table as the size of the cation increase

§  The magnitude of the lattice energy steadily decreases down the group as the size of the anion increases

§  The magnitude of the lattice increases (becomes more exothermic; more energy released) as the charge on either or both the cation and the anion increases.

Factors that affect hydration energy:

Similar to lattice energy, forces between the ions and water molecules surrounding then depend on:

§  Charge on the ion

The greater the charge on the ion, the greater is the force. The hydration energies become more exothermic as the charge on the ion increases.

§  The radius of the ions

The smaller the radius, the greater is the force. The hydration energies become less exothermic as the radius of the ions in a group increases.