When ionic solids dissolve in water theions that are adjacent to each other in the solid become surroundedby the water molecules (hydrated). The attraction force that occursbetween the ion and water is called an ion Ðdipole forces. Thepolar water molecules orient themselves so that the partiallycharged ends of the molecule are opposite the charge of the ions.So water molecules are oriented with their hydrogen atoms pointedat the anion and the oxygen atoms pointed at the cation. Thisprocess is called hydration. Hydration is more favored for smallions as compared to large ions. If this formation of the hydratedions were the only factor than we would expect all ionic compoundsto dissolve in water. However, that is not the case, and theproblem is that we need to condsider the other factors in thesolution process. the solution process is governed bythe solute-solute, solvent-solvent and solute-solventintermolecular attractive forces. So far we have only consideredhydration of the ions by the water molecules, that is thesoluteÐsolvent interactions. When we consider the soluteinteractions we begin to see some of the problems that can arise.The ions in a crystal are strongly attracted to each other and todissolve it is necessary to overcome the electrostatic attractionbetween the oppositely charged ions. The lattice energy of a solidis a measure of the strength of those electrostatic attractions.The lattice energy works to keep the ions in the solid state andionic compounds with large lattice energies are insoluble in waterwhile compounds with small lattice energies are soluble. Whenpotassium iodide dissolves in water the ions is the potassiumiodide solid must be separated; KI(s) -ÐH2O -->K+(g) + I-(g) ÆH1 = +632kJ/mol Now the gaseous ions are distributed inwater according to the equation; H2O(l) + K+(g) +I-(g) -Ð-> K+(aq) + IÐ(aq)ÆH2 + ÆH3 = -617 kJ The sum of these two equations yields theoverall solution process for KI dissolving in water. The value ofÆH2 + ÆH3 is call the hydrationenergy. In this example the hydration energy is not as large, inabsolute terms as the energy required to separate the ions in thesolute and the heat of solution isendothermic. Solution When ionic solids dissolve in water theions that are adjacent to each other in the solid become surroundedby the water molecules (hydrated). The attraction force that occursbetween the ion and water is called an ion Ðdipole forces. Thepolar water molecules orient themselves so that the partiallycharged ends of the molecule are opposite the charge of the ions.So water molecules are oriented with their hydrogen atoms pointedat the anion and the oxygen atoms pointed at the cation. Thisprocess is called hydration. Hydration is more favored for smallions as compared to large ions. If this formation of the hydratedions were the only factor than we would expect all ionic compoundsto dissolve in water. However, that is not the case, and theproblem is that we need to condsider the other factors in.

1. When ionic solids dissolve in water theions that are adjacent to each other in the solid become
surroundedby the water molecules (hydrated). The attraction force that occursbetween the ion
and water is called an ion Ðdipole forces. Thepolar water molecules orient themselves so that the
partiallycharged ends of the molecule are opposite the charge of the ions.So water molecules are
oriented with their hydrogen atoms pointedat the anion and the oxygen atoms pointed at the
cation. Thisprocess is called hydration. Hydration is more favored for smallions as compared to
large ions. If this formation of the hydratedions were the only factor than we would expect all
ionic compoundsto dissolve in water. However, that is not the case, and theproblem is that we
need to condsider the other factors in thesolution process. the solution process is governed bythe
solute-solute, solvent-solvent and solute-solventintermolecular attractive forces. So far we have
only consideredhydration of the ions by the water molecules, that is thesoluteÐsolvent
interactions. When we consider the soluteinteractions we begin to see some of the problems that
can arise.The ions in a crystal are strongly attracted to each other and todissolve it is necessary to
overcome the electrostatic attractionbetween the oppositely charged ions. The lattice energy of a
solidis a measure of the strength of those electrostatic attractions.The lattice energy works to
keep the ions in the solid state andionic compounds with large lattice energies are insoluble in
waterwhile compounds with small lattice energies are soluble. Whenpotassium iodide dissolves
in water the ions is the potassiumiodide solid must be separated; KI(s) -ÐH2O -->K+(g) + I-(g)
ÆH1 = +632kJ/mol Now the gaseous ions are distributed inwater according to the equation;
H2O(l) + K+(g) +I-(g) -Ð-> K+(aq) + IÐ(aq)ÆH2 + ÆH3 = -617 kJ The sum of these two
equations yields theoverall solution process for KI dissolving in water. The value ofÆH2 + ÆH3
is call the hydrationenergy. In this example the hydration energy is not as large, inabsolute terms
as the energy required to separate the ions in thesolute and the heat of solution isendothermic.
Solution
When ionic solids dissolve in water theions that are adjacent to each other in the solid become
surroundedby the water molecules (hydrated). The attraction force that occursbetween the ion
and water is called an ion Ðdipole forces. Thepolar water molecules orient themselves so that the
partiallycharged ends of the molecule are opposite the charge of the ions.So water molecules are
oriented with their hydrogen atoms pointedat the anion and the oxygen atoms pointed at the
cation. Thisprocess is called hydration. Hydration is more favored for smallions as compared to
large ions. If this formation of the hydratedions were the only factor than we would expect all
ionic compoundsto dissolve in water. However, that is not the case, and theproblem is that we
need to condsider the other factors in thesolution process. the solution process is governed bythe
solute-solute, solvent-solvent and solute-solventintermolecular attractive forces. So far we have
only consideredhydration of the ions by the water molecules, that is thesoluteÐsolvent

2. interactions. When we consider the soluteinteractions we begin to see some of the problems that
can arise.The ions in a crystal are strongly attracted to each other and todissolve it is necessary to
overcome the electrostatic attractionbetween the oppositely charged ions. The lattice energy of a
solidis a measure of the strength of those electrostatic attractions.The lattice energy works to
keep the ions in the solid state andionic compounds with large lattice energies are insoluble in
waterwhile compounds with small lattice energies are soluble. Whenpotassium iodide dissolves
in water the ions is the potassiumiodide solid must be separated; KI(s) -ÐH2O -->K+(g) + I-(g)
ÆH1 = +632kJ/mol Now the gaseous ions are distributed inwater according to the equation;
H2O(l) + K+(g) +I-(g) -Ð-> K+(aq) + IÐ(aq)ÆH2 + ÆH3 = -617 kJ The sum of these two
equations yields theoverall solution process for KI dissolving in water. The value ofÆH2 + ÆH3
is call the hydrationenergy. In this example the hydration energy is not as large, inabsolute terms
as the energy required to separate the ions in thesolute and the heat of solution isendothermic.