11.1 The Dissolution Process

Although water is heavier than sucrose, gravity does not cause them to "settle out" over time.

The word solution has come to mean an aqueous solution to many people because water is used so often as a solvent.

Solutions are homogeneity, that is, after a solution is mixed, it has the same composition at all points throughout.

The physical state of a solution is the same as that of a solvent, and the components of a solution are dispersed on a molecule scale.

Sometimes we stir a mixture to speed up the dissolution process, but this is not necessary; a homogeneity solution would form if we waited long enough.

The topic of spontaneity is important to the study of chemical thermodynamics and is treated more thoroughly in a later chapter of this text.

As heat is absorbed or evolved, internal energy can change in the dissolution process.

When the strengths of the intermolecular forces of attraction between solute and solvent species are the same as those present in the separated components, the solution is formed with no accompanying energy change.

An example of an ideal solution is a mixture of ideal gases, such as helium and argon, which closely approach ideal behavior.

The formation of this solution involves an increase in disorder since the helium and argon atoms occupy a volume twice as large as that which they occupied before mixing.

The disorder of the atoms of the two gases is increased when samples of helium and argon mix.

The components of liquid-liquid solutions experience attractive forces unlike a mixture of gases.
The dissolution process does not involve any increase or decrease in energy because the intermolecular attractive forces between the two substances are essentially the same.
The driving force needed to cause the formation of a solution can be provided by diffusion alone.
The relative magnitudes of intermolecular forces of attraction between solute and solvent species may prevent dissolution.

solute-solute, solvent-solvent, and solute-solvent are three types of intermolecular attractive forces.

The relative magnitudes of the energy changes associated with these stepwise processes determine whether the dissolution process will release or absorb energy.
Solutions don't form because the energy required to separate solute and solvent species is more than the energy released by solvation.

This schematic representation of dissolution shows a stepwise process involving the separation of solute and solvent species.

Hydrogen bonding is the dominant intermolecular attractive force present in liquid water; the nonpolar hydrocarbon molecules of cooking oils are not capable of hydrogen bonding, instead being held together by dispersion forces.

It would take overcoming the strong hydrogen bonding in water and the strong dispersion forces between oil and water to form an oil-water solution.

The energy released by solvation would not be very high since the polar water and non polar oil molecules wouldn't experience strong intermolecular attraction.

The solvation process is exothermic to compensate for the separations of solute and solvent molecule, because both substances are capable of hydrogen bonding.

Spontaneous dissolution solution formation is favored, but not guaranteed, by exothermic processes.

Some compounds do endothermically, dissolving with the release of heat.
A thin-walled plastic bag of water is sealed inside a larger bag of NH4NO3 to make an instant cold pack.
When the smaller bag is broken, a solution of NH4NO3 forms, absorbing heat from the surroundings and providing a cold compress that decreases swelling.
Endothermic dissolutions such as this one require more energy input to separate the solute species than is recovered when the solutes aresolvated, but they are still spontaneously formed due to the increase in disorder that accompanies formation of the solution.