The+Solution+Process

__**The Solution Process**__

Solutions are formed when one substance is uniformly dispersed throughout another. All solutions, except gas mixtures, involve condensed substances. The same intermolecular forces that were spoken of in Chapter 11 exist between solute and solvent particles. Ion-dipole forces operate between ionic substances in water; while Dispersion forces are dominate between nonpolar solvents and solutes. A major factor in determining whether a solution forms is the strength of the intermolecular force between the solute and solvent molecules.

Solutions are created when attractive forces between the solute and solvent particles are in magnitude comparable to the forces that exist between the solute’s particles or solvent’s particles. An example of this is the fact that NaCl dissolves readily in water because attractive interactions between ions and polar water are great enough to overcome the lattice energy of NaCl(s).

Interactions between solute and solvent molecules in which the solvent molecules surround the solute molecules are known as **solvation**. When the solvent is water, they are also known as **hydration**.




 * Energy Changes and Solution Formation**

NaCl dissolves in water because of the fact that water molecules have an attraction sufficient for and ions to overcome attraction of the ions for one another in the crystal. To form an aqueous solution of NaCl, water molecules also have to separate in order to create spaces that will be occupied by the ions. This process is shown in the picture below. Because of these three steps, we are able to think of solution formation as having three components. Overall enthalpy change in a solution is, therefore, a sum of three terms:





The process of separating solute particles is endothermic because there is an input of energy required in order to overcome attractive interactions. The separation of solvent molecules also requires energy. The third part comes from the attractive interactions between solvent and solute and is exothermic. Thus, the three enthalpy terms can add to positive or negative sums.


 * Solution Formation, Spontaneity, and Disorder:**

When two substances are mixed and dissolving occurs without any extra input from outside the system, the dissolving occurs **__spontaneously__**. Processes in which the energy content of the system decreases tend to occur spontaneously. Spontaneous processes tend to be exothermic, but some processes that occur spontaneously do not result in lower energy for a system or may even be endothermic. The spontaneous processes that do not result in lower energy for a system are characterized by an increase in the disorder of the system.

[|http://www.ualberta.ca/~jplambec/che/p102/p02055g1.gif]

Formation of a homogeneous solution has increased disorder because the molecules of each substance are now mixed and distributed in a volume twice as large as that which they occupied before mixing. The amount of disorder in the system is given by a thermodynamic quantity called **__entropy__**. Processes in which the entropy of the system increases tend to occur spontaneously.

[|http://www.ualberta.ca/~jplambec/che/p102/p02055g1.gif]

When molecules of different types are brought together, mixing and an increase in entropy occur spontaneously unless the molecules are restrained by sufficiently strong intermolecular forces or by physical barriers. Thus, gases spontaneously mix and expand unless restrained by their containers because intermolecular forces are too weak to restrain the molecules, and sodium chloride does not spontaneously dissolve in gasoline because strong ionic forces hold sodium chloride ions together.


 * The solution process involves two factors:**
 * 1) **A change in enthalpy**
 * 2) **A change in entropy**

In most cases formation of solutions is favored by the increase in entropy that accompanies mixing. A solution will form unless solute-solute or solvent-solvent interactions are too strong relative to the solute-solvent interactions.

__**Sources**__ The Book; __Chemistry: The Central Science__, 9th ed. by Brown, LeMay, and Bursten http://nobel.scas.bcit.ca/chem3303/units/solution/images/ioniclattice_solvation.gif