what determines if the heat of solution is endothermic or exothermic for salts?
Mostly the competition between the lattice energy (how much energy it takes to break the crystalline strucure) vs the heat of solvation of the ions.
To determine whether the heat of solution for a salt is endothermic or exothermic, you need to consider the competition between two factors: the lattice energy and the heat of solvation.
The lattice energy refers to the energy required to break the crystalline structure of the salt, separating the positive and negative ions. This process requires energy, as the forces holding the ions together in the crystal lattice are strong.
On the other hand, the heat of solvation refers to the energy released or absorbed when the separated ions are surrounded by solvent molecules and form hydrated ions. If energy is released during this process, it is called exothermic, and if energy is absorbed, it is called endothermic.
When you have a salt with a strong lattice energy, it requires a lot of energy to break the bonds in the crystal structure. In this case, the heat of solvation might not be able to overcome the energy required to break the lattice, resulting in an overall endothermic process. Conversely, if the heat of solvation is significantly greater than the lattice energy, then the process will be exothermic.
In summary, the heat of solution for salts is determined by the balance between the lattice energy required to break the crystal structure and the heat of solvation of the ions.