The change in lattice energy refers to which of the following?

• A. The enthalpy change when 1 mole of an ionic compound is dissolved in water
• B. The enthalpy change when a solid lattice forms from gaseous ions
• C. Enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions
• D. The energy change when a molecule rotates

Answer: (C)

Lattice energy is the energy released when gaseous ions come together to form a solid ionic lattice. Describing it as the enthalpy change when 1 mole of an ionic compound is formed from its gaseous ions under standard conditions matches this idea exactly, because the formation of the crystal from dispersed ions is the process that releases energy (exothermic).

This contrasts with dissolution, which involves breaking the lattice apart and then solvating the ions in water, not forming the solid lattice. Rotational energy of a molecule is unrelated to lattice formation. Remember that the lattice energy (often taken as a positive magnitude) grows larger with higher charges and smaller ions, reflecting a stronger ionic lattice.