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.
2.3:
Energetics of Solution Formation
The formation of a solution is an example of a spontaneous process, which is a process that occurs under specified conditions without energy from some external source.
When the strengths of the intermolecular forces of attraction between solute and solvent species in a solution are no different than those present in the separated components, the solution is formed with no accompanying energy change. Formation of the solution requires the solute–solute and solvent–solvent electrostatic forces to be overcome completely as attractive forces are established between the solute and solvent molecules. If the electrostatic forces within the solute are significantly greater than the solvation forces, the dissolution process is significantly endothermic and the compound may not dissolve to an appreciable extent. On the other hand, if the solvation forces are much stronger than the compound’s electrostatic forces, the dissolution is significantly exothermic and the compound may be highly soluble.
In the process of dissolution, an internal energy change often, but not always, occurs as heat is absorbed or evolved. An increase in matter dispersal always results when a solution forms from the uniform distribution of solute molecules throughout a solvent. Spontaneous solution formation is favored, but not guaranteed, by exothermic dissolution processes. While many soluble compounds do, indeed, dissolve with the release of heat, some dissolve endothermically. Endothermic dissolutions require greater energy input to separate the solute species than is recovered when the solutes are solvated, but they are spontaneous nonetheless due to the increase in disorder that accompanies the formation of the solution.
This text is adapted from Openstax, Chemistry 2e, Section 11.1: The Dissolution Process.