Question

Define lattice energy (ΔH^o lattice). The lattice energies (ΔH^o lattice) for sodium chloride and potassium chloride are -788 kJ and -717 kJ, respectively. Based on Coulomb’s Law, briefly explain why the lattice energy for potassium chloride is less than the lattice energy for sodium chloride.

Answer 1

coulombs law

q is the charge and r is the distance between two charges.

In NaCl, cations Na+ and Cl- are responsible to form electostatic attraction.

In KCl, K+ and Cl- are the ions that form electrostatic attraction.

Anion size and charge is same. We only need to look at cations.

Na+ has smaller size then K+ and therefore ionic radius beween NaCl is less than KCl.

r is the radius thta come in the denominator in coulomb law. Coulomic energy is inversely propertional to the radius and therfore NaCl has higer coulombic energy.

More is the coulombic energy, more is the lattice energy. and vice versa.

Therefor NaCl has higher lattice enrgy than KCl.