The lattice enthalpy change when forming an ionic solid from gaseous ions is which?

• A. Exothermic
• B. Endothermic
• C. Zero
• D. Depends on the elements involved

Answer: (A)

When gaseous ions come together to form an ionic solid, energy is released as the ions arrange into a stable lattice. This release of energy means the lattice enthalpy is negative, so the process is exothermic. The lattice energy represents the energy required to break the solid into gaseous ions, and since forming the lattice gives off energy, its enthalpy is exothermic. The amount of energy depends on the ions’ charges and sizes (higher charges and smaller radii lead to a more exothermic lattice energy), but the sign stays negative. So, exothermic is the best description. Endothermic would imply energy input to form the lattice, zero would mean no energy change, and saying it depends on the elements would misstate the direction of the energy change.