Heat of Solution
Definition and meaning of Heat of Solution in chemistry.
Heat of solution is the amount of heat absorbed or released when a specific quantity of solute dissolves completely in a solvent to form a solution at constant pressure.
In more detail
The value reflects a balance between the energy needed to separate solute particles (breaking the crystal lattice or intermolecular forces) and the energy released as solvent molecules surround and interact with the dissolved particles (solvation or hydration). If solvation releases more energy than is needed to separate the solute particles, dissolution is exothermic (ΔHsoln negative); if separation requires more energy than solvation supplies, it is endothermic (ΔHsoln positive). Chemists often report a molar heat of solution, extrapolated to infinite dilution so further dilution adds no additional heat change.
Key facts
| Symbol | ΔH_soln |
|---|---|
| Typical units | kJ/mol (or J/g) |
| Field | Physical Chemistry |
| Sign convention | Negative = exothermic; positive = endothermic |
Dissolving ammonium nitrate (NH4NO3) in water is endothermic (ΔHsoln = +25.7 kJ/mol), so the solution cools noticeably, the principle behind instant cold packs.
Frequently asked questions
Is heat of solution the same as heat of hydration?
No. Heat of hydration is specifically the energy released when ions become surrounded by water molecules. Heat of solution is the broader, net quantity that combines lattice (or cohesive) energy with solvation energy for any solvent, not just water.
What decides whether dissolving is exothermic or endothermic?
The competition between lattice energy (energy required to pull solute particles apart) and solvation energy (energy released when solvent molecules interact with the separated particles) determines the overall sign of ΔHsoln.