the lattice energy for nacl crystals is equal to -787 kj/mol. the enthalpies of solvation for na and cl- ions are equal to -424 kj/mol and -359 kj/mol respectively. for the process of making the nacl aqueous solution, the q value (the heat exchanged between the system i.e. nacl and the surroundings i.e. water) will have a negative sign.
