Substances with higher lattice energy are less soluble than substances with lower lattice energy. Even though LiCl has higher lattice energy than KCl , why does LiCl dissolve exothermically (spontaneously) in water whereas KCl dissolves endothermically ?
A reaction is exothermic if the products are at a lower energy than the reactants. When LiCl dissolves in water, it ionizes to form Li+ and Cl-. The hydration of these two ions with high charge density, particularly the Li+, is exothermic because of the high lattice energy of LiCl(s). The more energy absorbed by the crystal lattice upon formation, the more that is released when the lattice is broken apart.
The bond between KCl is very strong ionic bond which requires lot of energy to break. Hence, dissolution of KCl is an endothermic reaction.