Enthalpy of Hydration, Hhyd

•The heat change when 1 mole of gaseous ions is hydrated in water.

•e.g:

  Na+(g) ¾® Na+(aq)    DHhyd = -406 kJ mol-1

  Cl-(g) ¾® Cl-(aq)        DHhyd = -363 kJ mol-1

Hydration

The formation of a solution involves the interaction of solute with solvent molecules. Many different liquids can be used as solvents for liquid solutions, and water is the most commonly used solvent. When water is used as the solvent, the dissolving process is called hydration.

The interaction between water molecules and sodium ion is illustrated as one of the diagram below. This is a typical ion-dipole interaction. At the molecular level, the ions interact with water molecules from all directions in a 3-dimensional space. This diagram depicts the concept of interaction only.

The above diagram also display hydrogen-bonding, dipole-dipole, ion-induced dipole, and dipole-induced dipole interactions. In the absence of these interactions, solvation takes place due to dispersion. Definitions of these terms are obvious from the diagrams. The meaning of the words used in the term also hints the nature of the interactions.

What is the energy of hydration?

Enthalpy of hydration, H_hyd, of an ion is the amount of energy released when a mole of the ion dissolves in a large amount of water forming an infinite dilute solution in the process, M^z+(g) + mH₂O ® M^z+(aq) where M^z+(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.

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Enthalpy of Hydration (H_hyd 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

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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, E_cryst. 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, H_lig. The processes related to these energies are shown below:
M^z+ + nL = ML_n^z+,             H_lig
ML_n^z+ + solvent = ML_n^z+ (solution),     H_disp 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, H_disp. Therefore, H_hyd = H_disp + H_lig. 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 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, H_solv, 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, H_hyd = H_{crystallization} + H_solv 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 H_cryst, and energy of crystallization, E_cryst referr 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 hight. 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, H_solv, is the energy released when one mole solid is dissolved in a solvent. Note that H_solv 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 H_solv. 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.

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Enthalpy of Solvation (H_solv 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

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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.