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Chapter 12

CHY 103 Chapter 12: Chemistry Study Notes


Department
Chemistry
Course Code
CHY 103
Professor
Sharonna Greenberg
Chapter
12

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Mareck Tam CHY 103
Chemistry Chapter 12 Study Notes
12.2 Types of Solutions and Solubility
Aqueous solution is a solution where water is the solvent and a solid, liquid or gas is the
solute.
The solubility of a substance is the amount of the substance that will dissolve in a given
amount of solvent.
The soluilit of oe sustae i aothe depeds oth o atue’s tede toad
mixing.
Natures Tendency toward Mixing: Entropy
The formation of a solution does not necessarily lower the potential energy of its
constituent particles.
For example: Argon and Neon both act as ideal gases at low pressures and moderate
temperatures, If the two gases were mixed there would be no change in potential
energy.
We cannot assume/think that of the mixing of two ideal gases as lowering the potential
energy. Instead, the tendency to mix is related to a concept called entropy.
Entropy is a measure of energy randomization or energy dispersal in a system.
Any gas above 0K has kinetic energy due to the motion of the atoms.
Therefore, back to the neon and argon example; when two gases are held in their
confined compartments, their kinetic energies are also confined to those
compartments. When the barrier between the two gases is removed, each gas along
with its kinetic energy - becomes spread out or dispersed over large volume. Thus the
mixture of the two gases has greater energy dispersal, or greater entropy, than the
separated compartments.
The reason two ideal gases mix is due to the high tendency for energy to spread out or
disperse whenever not restrained.
Another example of the tendency of energy to spread out is the transfer thermal energy
from hot to cold.
The Effect of Intermolecular Forces
Absence of intermolecular forces allows two substances to spontaneously mix to form a
homogenous solution.
Intermolecular forces are present between:
1. The solvent and Solute Particles
2. The solvent particles themselves
3. The solute particles themselves
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Mareck Tam CHY 103
A solution will always form if the solvent solute interactions are comparable or
stronger than the solvent solvent interactions and the solute solute
interactions.
Miscible is the ability of two or more substances to be soluble in each other in all
proportions.
The formation of the solution is driven by the tendency toward mixing, or toward
greater entropy.
If solvent solute interactions are weaker than solute solute interactions and
solvent solvent interactions, a solution may still form. (Depending on the
relative disparities between the interactions.
If the disparity is small, then the tendency to mix results in the formation of a
solution. But if the disparity is large, a solution will not form.
The rule of thumb applies: Like dissolves like
Polar solvents (like water) tend to dissolve many polar ionic solutes, and non
polar solvents (like hexane) tend to dissolve many non polar solutes.
12.3 Energetics of a Solution Formation
We can understand energy changes associated with solution formation by envisioning
the process as occurring in the following three steps, each eith an associated change in
enthalpy:
1. Separating the solute into its constituent particles:
This step is always endothermic because to break the forces that hold the solute
together it requires energy (positive )
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Mareck Tam CHY 103
2. Separating the solvent particles from each other to make room for the solute
particles.
This step is also endothermic because energy is required to overcome the
intermolecular forces among the the solvent particles.
3. Mixing the solute particles with the solvent particles.
This step is exothermic, because energy is released as the solute particles interact
(through intermolecular forces) with the solvent particles.
The enthalpy of a solution () is the sum of the changes in enthalpy for each step:
     
    
1. If the sum of endothermic terms is the about equal to the exothermic then the enthalpy ()will be
about zero. The increasing entropy will still drive the formation of a solution but the energy remains
constant.
2. If the sum of the endothermic terms is smaller in magnitude than the exothermic term, the enthalpy of
the solution will be negative and the solution process is exothermic. Both the tendency toward lower
energy and the tendency toward greater energy drive the formation of a solution
3. If the sum of the endothermic terms is greater in magnitude than the exothermic term, the enthalpy is
positive and the solution is endothermic. When enthalpy is not too large, the tendency toward greater
entropy can still drive the formation of a solution. But if too large, a solution will not form.
Aqueous Solutions and Heats of Hydration
Heat of hydration is the enthalpy change that cocurs when 1 mol ofgaseous solute ions
are dissolved in water. ()
Since the ion dipole interactions are much stronger between a dissolved ion and the
surrounding water molecules than the hydrogen bonds in water, the  is always
largely negative for ionic compounds(exothermic).
One endothermic term and one exothermic term can be added together to write the
enthalpy of a solution as just two terms:
     
    
    
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