A delicate balance determines whether the dissolving of a
salt crystal in water will absorb or give off heat. Energy
is required to pull apart the oppositely charged ions in the
crystal, and this is called the lattice energy, DHlat.
In contrast, energy is liberated when the separated ions attract
polar water molecules around themselves, and this is the heat
of hydration, DHhyd.
The observed heat effect on solution depends on which factor
predominates. Typical examples are as follows:
Predictions about heats of solution (DHsol)
become tricky, because 5% errors in theoretical calculations
of lattice and hydration energies will completely reverse
a prediction. This table makes it look as if the DHsol
values were obtained from DHlat