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.
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:
-------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 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.
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.
Using the cycle shown above and the formula, we have
A positive value indicates an endothermic reaction. However, the value is small, and depending on the source of data, the estimated value may change. This value of 19 kJ/mol is too high compared to the value given earlier for NaCl of 3.88 kJ/mol, due to a high value of lattice energy used.
The enthalpy of hydration for KCl is estimated to be
The enthalpy of solvation given above is -17.22. The two values here indicates that the solvation is an endothermic reaction or change. Should temperature decrease or increase when KCl dissolves?
Identify the type of interactions in the solvation process.
Toluene and benzene form ideal solution in that total vapor pressure of the solution is the sum of the partial pressures of benzene and toluene. These two compounds are so much a like that their molecules disperse into each other. The major driving force for solution is entropy.
The ion-dipole interaction is stronger between Ca2+ and water molecules than between Na+ and water molecules.
What are their electronic configuration? The sodium ion has the same electronic configuration as Ne, but the chloride ion has the same electronic configuration as Ar, which is larger than Ne. Which one release more energy when hydrated?
Among the substances listed in the table above, KNO3 absorbed the most energy per mole when dissolved.