Solvation
Summary
Solvation is the stabilizing attraction between solvent molecules and dissolved solute particles, which surround each particle. In water it is called hydration. Dissolving is favored when solute-solvent attractions are comparable to or stronger than solute-solute and solvent-solvent ones ("like dissolves like").
Detail
Meaning: Solvent molecules cluster around each solute particle and attract it, forming a solvation shell. For ions in water this is ion-dipole attraction: the oxygen end faces cations and the hydrogen ends face anions. Polar solutes interact through hydrogen bonding or dipole-dipole forces, and nonpolar solutes in nonpolar solvents through London dispersion forces.
Equation: ΔHsoln = ΔH1 + ΔH2 + ΔH3, where ΔH1 is the energy to separate solute particles (J/mol, positive), ΔH2 is the energy to separate solvent molecules (J/mol, positive), and ΔH3 is the enthalpy of solvation (J/mol, negative).
On the MCAT: Expect questions on whether a solute dissolves in a given solvent, or on whether dissolving warms or cools the solution. For ionic solids, compare lattice energy with hydration energy. Hydration is more exothermic for smaller ions and ions with higher charge.
Common confusion: Solvation is not dissociation. Dissociation is the separation of an ionic solid into ions, and solvation is the stabilization of those ions by solvent. Dissolving is also not always exothermic. Many salts dissolve endothermically, and the increase in entropy drives the process (ΔG = ΔH − TΔS).
Sources
- Chemistry: The Central Science
- Chemistry: Atoms First
- General Chemistry: Principles and Modern Applications
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.