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Hydration energy (also hydration enthalpy) is the amount of energy released when one mole of ions undergo hydration which is a special case of solvation. It is a special case of dissolution of energy, with the solvent being water.

For example, upon dissolving a salt in water, the outermost ions (those at the edge of the lattice) move away from the lattice and become covered with the neighbouring water molecules. If the hydration energy is equal to or greater than the lattice energy, then the salt is water-soluble. In salts for which the hydration energy is higher than the lattice energy, solvation occurs with a release of energy in the form of heat. For instance, CaCl2 (anhydrous calcium chloride) heats the water when dissolving. 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.

What is the energy of hydration?
Enthalpy of hydration, Hhyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinitely dilute solution in the process,
Mz+(g) + mH2O ® Mz+(aq)
where Mz+(aq) represents ions surrounded by water molecules and dispersed in the solution. The approximate hydration energies of some typical ions are listed here. The table illustrates the point that as the atomic numbers increases, so do the ionic size, leading to a decrease in absolute values of enthalpy of hydration.

Enthalpy of Hydration (Hhyd kJ/mol) of Some Typical Ions

Ion Hhyd Ion Hhyd Ion Hhyd
H+ -1130 Al3+ -4665 Fe3+ -4430
Li+ -520 Be2+ -2494 F -505
Na+ -406 Mg2+ -1921 Cl -363
K+ -322 Ca2+ -1577 Br -336
Rb+ -297 Sr2+ -1443 I -295
Cs+ -276 Ba2+ -1305 ClO4 -238
Cr2+ -1904 Mn2+ -1841 Fe2+ -1946
Co2+ -1996 Ni2+ -2105 Cu2+ -2100
Zn2+ -2046 Cd2+ -1807 Hg2+ -1824

From the above table, an estimate can be made for the hydration energy of sodium chloride. This amount is very close to the energy of crystallization, Ecryst.
The hydration energy of an ionic compound consists of two inseparable parts. The first part is the energy released when the solvent forms a coordination compound with the ions. This energy released is called the energy of ligation, Hlig. The processes related to these energies are shown below:

Mz+ + nL = MLnz+, Hlig
MLnz+ + solvent = MLnz+ (solution), Hdisp
The second part is to disperse the ions or hydrated ions into the solvent medium, which has a dielectric constant different from vacume. This amount of energy is called energy of dispersion, Hdisp. Therefore,
Hhyd = Hdisp + Hlig.
This idea is brought up just to point out that the formation of aqua complex ions is part of the hydration process, even though the two energies are not separable. When stronger coordination is made between the ions and other ligands, they replace the coordinated water molecules if they are present. In the presence of NH3 molecules, they replace the water of Cu(H2O)62+:
Cu(H2O)62+ + 6NH3 ® Cu(NH3)62+ + 6H2O

How is hydration energy related to the enthalpy of crystallization?

In the discussion of lattice energy, we consider the ions separated into a gas form whereas in the disolution process, the ions are also separated, but this time into ions dispersed in a medium with solvent molecules between ions. The medium or solvent has a dielectric constant. The molar enthalpy of solvation, Hsolv, is the energy released when one mole solid is dissolved in a solvent. This quantity, the enthalpy of crystallization, and energy of hydration forms a cycle. Taking the salt NaCl as an example, the following relationship is obvious,
Hhyd = Hcrystallization + Hsolv
from the following diagram.

   -------Na+(g)+Cl-(g)--------
         |                       |
         |                       |
         |                       |Hcryst
         |Hhyd                    |
         |                       ¯
         |                 ----NaCl(s)---
         |                       |
         |                       |Hsolv
         ¯                       ¯
         -------Na+(aq)+Cl-(aq)--------

The term enthalpy of crystallization is used in this diagram instead of lattice energy so that all the arrows point downward. Note that enthalpy of crystallization Hcryst, and energy of crystallization, Ecryst refer to the same quantity, and they are used interchangably.
The energies of solvation for some salts can be positive values, in these cases the temperatures of the solution decrease as the substances dissolve. The solvation is an endothermic reaction. The energy levels of solids and solutions reverse in order of height. The cycle is shown below.

  -------Na+(g)+Cl-(g)-------
         |                       |
         |                       |
         |                       |Hcryst
         |Hhyd                    |
         |                       |
         ¯                       |
         ---Na+(aq)+Cl-(aq )----    |
                            ­    |
                      Hsolv  |    |
                            |    ¯
                   ---------NaCl(s)--------

In these cases, the enthalpies of hydration are less negative than the enthalpies of crystallization.

What is enthalpy of solvation?

The molar enthalpy of solvation, Hsolv, is the energy released when one mole solid is dissolved in a solvent.
Note that Hsolv is also called molar heat of solution or molar energy of solution in some literature. Sometimes the enthalpy of hydration is also (mis)understood as Hsolv. When apply these values, make sure you understand the process involved. The following enthalpies of solvation are given in Chemistry by Radel and Navidi, West Publishing Co.

Enthalpy of Solvation (Hsolv kJ/mol) of Some Common Electrolytes

Substanc Hsolv Substance Hsolv
AlCl3(s) -373.63 H2SO4(l) -95.28
LiNO3(s) -2.51 LiCl(s) -37.03
NaNO3(s) 20.50 NaCl(s) 3.88
KNO3(s) 34.89 KCl(s) -17.22
NaOH(s) -44.51 NH4Cl(s) 14.77

These values indicates that when aluminum chloride and sulfuric acid are dissolved in water, much heat is released. Due to the very small value of enthalpies of solvation, the temperature changes are hardly noticed when LiNO3 and NaCl are dissolving.

For information on Bond Energy click here!

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