AP® Chemistry · Unit 9 · Topic 9.6
Free Energy of Dissolution
A salt dissolves on its own whenever delta G = delta H - T delta S comes out negative, and a big positive entropy change from the crystal falling apart into dispersed ions can outweigh an unfavorable, heat-absorbing delta H.
Dissolving a salt means pulling ions out of a rigid lattice and surrounding each one with water molecules. Breaking the lattice costs energy, that's an input. Hydration, the ions attracting water molecules around themselves, releases energy, that's a payoff. The enthalpy of dissolution is just the balance of these two: lattice energy in, hydration energy out. For some salts hydration wins and dissolving releases heat. For ammonium nitrate the lattice energy is larger, so the net process absorbs heat from the surroundings, which is why the water gets cold.
A positive delta H should make a process unfavorable, yet ammonium nitrate dissolves readily and completely. The missing piece is entropy. A crystal is ordered: ions locked in fixed positions. Once dissolved, those ions scatter through the solution, free to move independently, each one also disrupting the water structure around it. That is a large increase in disorder, a large positive delta S. Plug both into delta G = delta H - T delta S: even though delta H is positive, T delta S is large enough to make delta G negative overall. The entropy term, not the enthalpy term, is what drives this dissolution.
This is the same free energy logic from reactions applied to a physical process, and it sets up the electrochemistry half of the unit, where delta G connects to cell voltage instead of solubility. It also explains a practical pattern: since dissolving NH4NO3 is endothermic, heating the water adds energy that the process can absorb, so solubility increases with temperature. That is the opposite of what happens for a salt whose dissolution releases heat, where warming the water works against the process instead of helping it.
Key terms
- Lattice energy
- The energy required to separate ions from a solid crystal into isolated gas-phase ions.
- Hydration energy
- The energy released when water molecules surround and stabilize dissolved ions.
- Entropy (S)
- A measure of disorder or the number of ways particles can be arranged.
- Gibbs free energy (delta G)
- The quantity delta H minus T delta S that determines whether a process is spontaneous when negative.
Watch it explained
- How it worksHow do lattice breaking and hydration compete to set the enthalpy of dissolving?
- How it worksWalk me through why an endothermic dissolution can still be driven by entropy.
- Common mistakeWhy isn't every salt that dissolves releasing heat as it goes?
- Going furtherWhat happens to the solubility of an endothermically dissolving salt as you heat the water?
Check yourself
End of topic.