Answer to Question #58723 in Inorganic Chemistry for F.Z

Question

Hi! I have a question about Lattice Energy (enthalpy)

I know that when making a bond the reaction is exothermic (positive)
and breaking a bond the reaction is endothermic (negative).

Lattice enthalpy or lattice formation enthalpy is defined as the enthalpy of formation of 1 mole of an ionic compound from gaseous ions under standard conditions, which is endothermic (negative).

My question is ..
Why lattice formation is negative? It should be positive when making the ionic lattice because we are releasing energy.

Is bond making/breaking different than lattice making/breaking?

Answer 1

The positive sign of change of an enthalpy means that reaction endothermic, i.e. happens to absorption of heat. Therefore at destruction of a crystal lattice (endothermic process) ΔН>0.
The negative sign of change of an enthalpy means that reaction exothermic, i.e. happens to allocation of heat. Therefore at formation of a crystal lattice (exothermic process) ΔН<0.