Why is ionic bonding exothermic?
Ionic bonding is exothermic because the packing of oppositely charged ions into a crystal structure makes it extremely stable.
We can consider the formation of NaCl as occurring in steps.
- Na(s) → Na(g); ΔH = 107.3 kJ/mol
- Na(g) → Na⁺(g) + e⁻; ΔH = 495.8 kJ/mol
- ½Cl₂(g) → Cl(g); ΔH = 121.7 kJ/mol
- Cl(g)+ e⁻ → Cl⁻(g); ΔH = -348.8 kJ/mol
So it requires 376.0 kJ to convert 1 mol of Na and ½ mol of Cl₂ into 1 mol each of gaseous Na⁺ and Cl⁻ ions.
The lattice energy
NaCl(s) → Na⁺(g) + Cl⁻(g);
For the reverse reaction,
Na⁺(g) + Cl⁻(g) → NaCl(s); ΔH = -787.3 kJ/mol
So, forming the ions from Na(s) and ½Cl₂(g) requires the input of +376.0 kJ/mol.
When these ions incorporate into the salt lattice, they liberate -787.3 kJ/mol, for an overall exothermic release of -411.3 kJ/mol.
You can see that the energy released when the ions combine in the crystal makes ionic bonding exothermic.