2.3 : Energetics of Solution Formation

When a solute and solvent mix to form a solution, there are three major forces between their constituent molecules, or intermolecular forces; namely, attractions between the solute molecules, attractions between the solvent molecules, and attractions between the solvent and the solute molecules.

For a solute to dissolve in a solvent, solute–solute interactions between solute particles must be disrupted to allow more points of interaction between the solute and solvent particles.

Solvent–solvent interactions between solvent particles must be disrupted to accommodate the solute particles between the solvent molecules.

Solvent–solute interactions between solvent and solute particles must be established so that the substances can mix.

The extent to which a solute can dissolve in a solvent depends on how strong these three types of interactions are compared to each other.

If the solvent–solute interactions are strong enough to overcome the solute–solute and solvent–solvent interactions, then the solute will readily dissolve in the solvent.

Solubility depends both on the intermolecular forces between the solute and solvent molecules and on the tendency to mix, which is driven by an increase in entropy of the system.

Solution formation does not lower the potential energy of atoms; instead, it distributes their kinetic energy over a larger volume. This dispersal of energy increases the entropy of each phase, making solution formation a spontaneous process.

The disruption of the solute–solute and solvent–solvent particle interactions requires an input of energy to overcome the attractive forces between them, making these steps endothermic in nature.

Whereas the mixing of solute and solvent particles is an exothermic step because the attractive interactions between solute particles and solvent particles release energy.

The net enthalpy change of the solution is the sum of the enthalpy changes in each step. If the net enthalpy change is negative, the process is exothermic, whereas if the net enthalpy change is positive, the process is endothermic.