In
chemistry, hydration energy (also hydration enthalpy) is the amount of
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of hea ...
released when one
mole of
ions undergoes
hydration. Hydration energy is one component in the quantitative analysis of
solvation
Solvation (or dissolution) describes the interaction of a solvent with dissolved molecules. Both ionized and uncharged molecules interact strongly with a solvent, and the strength and nature of this interaction influence many properties of the ...
. It is a particular special case of
water
Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts as ...
.
[ The value of hydration energies is one of the most challenging aspects of structural prediction. Upon dissolving a salt in water, the cations and anions interact with the positive and negative ]dipoles
In physics, a dipole () is an electromagnetic phenomenon which occurs in two ways:
*An electric dipole deals with the separation of the positive and negative electric charges found in any electromagnetic system. A simple example of this system ...
of the water. The trade-off of these interactions vs those within the crystalline
A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macros ...
solid comprises the hydration energy.
Examples
If the hydration energy is greater than the lattice energy
In chemistry, the lattice energy is the energy change upon formation of one mole of a crystalline ionic compound from its constituent ions, which are assumed to initially be in the gaseous state. It is a measure of the cohesive forces that b ...
, then the enthalpy of solution is negative (heat
In thermodynamics, heat is defined as the form of energy crossing the boundary of a thermodynamic system by virtue of a temperature difference across the boundary. A thermodynamic system does not ''contain'' heat. Nevertheless, the term is ...
is released), otherwise it is positive (heat is absorbed).
The hydration energy should not be confused with solvation energy, which is the change in Gibb's free energy (not enthalpy) as solute in the gaseous state is dissolved.calcium chloride
Calcium chloride is an inorganic compound, a salt with the chemical formula . It is a white crystalline solid at room temperature, and it is highly soluble in water. It can be created by neutralising hydrochloric acid with calcium hydroxide.
Cal ...
) as a consequence of the large heat of hydration. However, the hexahydrate, CaCl2·6H2O cools the water upon dissolution. The latter happens because the hydration energy does not completely overcome the lattice energy, and the remainder has to be taken from the water in order to compensate the energy loss.
The hydration energies of the gaseous Li+, Na+, and Cs+ are respectively 520, 405, and 265 kJ/mol.
See also
* Enthalpy of solution
* Heat of dilution
* Hydrate
* Hydrational fluid
* Ionization energy
Ionization, or Ionisation is the process by which an atom or a molecule acquires a negative or positive Electric charge, charge by gaining or losing electrons, often in conjunction with other chemical changes. The resulting electrically charged a ...
References
Physical chemistry
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