Chapter 13: Solutions

Lecture Notes for Monday, Sepatember 10, 2001

We viewed the particulate level animation depicting the mixing of a solid with a liquid.

When we discuss the formation of a solution when viewed at the atomic level we need to consider three steps;

The following interaction must be considered.

1) solvent-solvent intermolecular attractions

2) solute-solute intermolecular attractions

3) solute-solvent intermolecular attractions

When solute dissolve in the solvent, the particles of the solute must distribute themselves throughout the solvent. That is solute particles must occupy positions normally taken by solvent particles. Because molecules (particles) are packed close together in a liquid solvent the ease with which a solute particle displaces a solvent molecule depends on the relative forces of attraction of the solvent molcules for each other the solute particles for each other and the strength of the soluteÐsolvent particles.

Recall that I recommended a simple experiment to demonstrate the energy changes associated with the solution process. We can better understand the energy changes by careful consideration of these three steps. If we begin with the separated solute and solvent, the first two steps, expanding the solute particles and solvent particles are both endothemic processes (See Figure above). If the energy released from the soluteÐsolvent interactions is greater than the energy required to expand the solute ands solvent particles energy is released in the solution process, i.e. the solution warms up. If, on the other hand the energy released is less than the energy absorbed to expand the solute and solvent particles the solution process is endothermic, i.e. the solution cools off.

However, if the energy liberated from the solute-solvent interactions is too small compared to the energy required to separate the solute particles and the energy required to separate the solvent particles no solution results. If the calculated heat of solution is exothermic we can expect the homogeneous solution to be formed. However, if the calculated heat of solution is endothermic we can not know for sure whether a homogeneous solution will form. We must consider one other factor in the endothermic case. The other factor is related to the natural tendency towards disorder when mixing two pure substances. This natural tendency towards disorder must be considered when discussing the solution process. In every case this factor favors the formation of the solution. However, if the energy required for the solution process to occur is large, it is unlikely the solution will be formed.