What is lattice enthalpy?
It is the amount of energy required to break one mole of crystal lattice(ionic solid) into its constituent ions in gaseous state.
Since energy is needed to break the crystal lattice hence this reaction is endothermic reaction. for example MX(s) is an ionic solid and breaks down to its constituent ions (M+(g) and X–(g)). The energy needed to break MX(s) is lattice enthalpy.
MX(s) –>M+(g) + X–(g)
What is a crystal lattice?
It is the three dimensional network of cations( Positive) and anions(negative) in an ionic solid.
For example sodium chloride(NaCl) is an ionic solid and has a crystal structure in which each sodium ion is surrounded by six chloride ions and each chloride ion is surrounded by six sodium ions.