🟣 This is Higher Level (HL) content.
📘 Lattice Enthalpy (IB Definition)
The enthalpy change when one mole of a solid ionic compound is completely separated into gaseous ions at infinite separation. Always positive (endothermic dissociation).
The 5 Components
| Step | Process | Sign |
|---|---|---|
| Atomisation (metal) | Solid metal → gaseous atoms | + (endo) |
| Atomisation (non-metal) | ½X₂(g) → X(g). Half bond dissociation | + (endo) |
| Ionisation energy | M(g) → M⁺(g) + e⁻ (can be 1st + 2nd for M²⁺) | + (endo) |
| Electron affinity | X(g) + e⁻ → X⁻(g) | Usually − (exo) |
| Enthalpy of formation | Elements → ionic solid (the "shortcut" route) | Usually − (exo) |
Worked Example. MgCl₂
Born-Haber Cycle for MgCl₂
Data: ΔHf⦵ = −641 | ΔHat(Mg) = +148 | IE₁ + IE₂ = +2189 | ΔHat(Cl₂) = +242 (for 2 mol Cl) | EA(Cl) × 2 = −698
Cycle: −641 = 148 + 2189 + 242 − 698 − ΔHlatt
∴ ΔHlatt = +2522 kJ mol⁻¹
Theoretical vs Experimental
If experimental > theoretical → the compound has significant covalent character (Fajans' rules: small, highly charged cation polarises the anion's electron cloud).
NaCl: close match (purely ionic). AgI: large discrepancy (significant covalent character).
⚠️ Examiner Traps
- For MgCl₂: multiply EA(Cl) by 2 and include both IE₁ and IE₂
- 2nd EA of O²⁻ is endothermic (+844 kJ mol⁻¹). Students always get the sign wrong
- State symbols on every species are mandatory for full marks
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