Crystal Lattice Energy
Definition and meaning of Crystal Lattice Energy in chemistry.
Crystal lattice energy is the energy released when one mole of an ionic solid forms from its constituent ions in the gaseous state, or equivalently, the energy required to separate that solid lattice back into gaseous ions.
In more detail
Lattice energy reflects the strength of the electrostatic (Coulombic) attraction between oppositely charged ions packed in a crystal, so it grows larger with higher ionic charges and shrinks as ionic radii (and thus ion separation) increase. Because the direct formation of a solid from isolated gaseous ions cannot be measured experimentally, lattice energy is instead calculated indirectly using a Born-Haber cycle, which applies Hess's law to combine measurable quantities like sublimation, ionization, bond dissociation, electron affinity, and enthalpy of formation. Higher lattice energies correspond to harder, higher-melting ionic solids with lower solubility, since more energy is needed to pull the ions apart.
Key facts
| Field | Physical Chemistry |
|---|---|
| Typical Units | kJ/mol |
| Governing Relationship | Coulomb's law: proportional to (charge1 × charge2) / distance between ion centers |
| Calculated Via | Born-Haber cycle (Hess's law) |
The lattice energy of sodium chloride (NaCl) is about -787 kJ/mol: this is the energy released when one mole of gaseous Na⁺ and Cl⁻ ions come together to form solid NaCl.
Frequently asked questions
Why can't lattice energy be measured directly?
Forming a solid crystal directly from a cloud of separated gaseous ions isn't a process that can be run or observed in the lab, so chemists calculate lattice energy indirectly using a Born-Haber cycle, summing other measurable enthalpy changes via Hess's law.
Why does magnesium oxide (MgO) have a much larger lattice energy than NaCl?
MgO is built from Mg2+ and O2- ions, whereas NaCl has singly charged Na+ and Cl- ions. Since Coulombic attraction scales with the product of the ionic charges, doubling both charges increases the attractive energy roughly fourfold, giving MgO a much higher lattice energy (about -3795 kJ/mol) than NaCl.