Hydration
Definition and meaning of Hydration in chemistry.
Hydration is the process in which water molecules surround and bind to ions, molecules, or polar groups in aqueous solution, typically through ion-dipole or hydrogen-bonding interactions.
In more detail
When an ionic or polar compound dissolves in water, the partially negative oxygen atoms of water orient toward cations while the partially positive hydrogen atoms orient toward anions, forming a hydration shell (or shell of hydration) around each ion. This process releases energy, called the hydration enthalpy, because new attractive interactions replace the ion-ion lattice forces being broken. Hydration enthalpy helps offset the lattice enthalpy that must be overcome for an ionic solid like table salt to dissolve, so the energetically favorable hydration of the separated ions is an important contributor to the solubility of many ionic compounds; however, dissolution also depends strongly on the entropy increase from dispersing ions into solution, since the overall enthalpy of solution for compounds like NaCl is close to zero. Smaller, more highly charged ions have larger hydration numbers and more negative hydration enthalpies because their concentrated charge attracts water molecules more strongly.
Key facts
| Field | Physical Chemistry |
|---|---|
| Driving force | Ion-dipole attraction and hydrogen bonding |
| Related quantity | Hydration enthalpy (ΔH_hyd), always negative (exothermic) |
| Trend | Smaller, more highly charged ions have larger hydration shells and more negative hydration enthalpies |
When sodium chloride (NaCl) dissolves in water, each Na+ ion becomes surrounded by water molecules with their oxygen atoms pointing inward, while each Cl- ion is surrounded by water molecules with their hydrogen atoms pointing inward, forming separate hydration shells.
Frequently asked questions
Is hydration the same as solvation?
Hydration is a specific type of solvation in which the solvent is water. Solvation is the general term for any solvent surrounding a solute.
Why does hydration release energy?
Hydration releases energy because forming new attractive interactions between water dipoles and ions is energetically favorable, releasing more energy than is required to separate the ions from their original lattice or bonding arrangement.