Is lattice enthalpy positive or negative? I've learnt that the lattice enthalpy (defined as the energy change from a solid ionic lattice to separate gaseous ions) is always positive, obviously. However, I've seen it explained as the opposite other places, so it's negative.
What is correct?
 A: When it is explained as opposite, it is usually the heat RELEASED when a crystal is formed, and since the word "released" already makes up for the sign of heat(i.e. if heat is -10 that means the heat released or evolved is +10) , it is taken as positive. Nonetheless, some books also present a negative value to it but it doesn't matter unless you are careful about it in calculations.
A: Lattice energy is defined as when 1 mole of cation and 1 mole of anion combined to form 1 mole of solid crystal, energy is released. The energy released is known as lattice energy because we know anion and cation has highewr energy than crystal lattice therefore when bond is formed between cation and anion energy is released.
That's why lattice energy is negative.
A: the lattice dissociation enthalpies are always positive and the lattice formation enthalpies are always negative. when 1 mole of lattice breaks down into its gaseous ions then energy is gained here hence +ve enthalpy but when gaseous constituent ions come together to form a lattice energy is released and hence enthalpy is -ve as exothermic reactions have negative enthalpy and endothermic reactions have positive enthalpies.
