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Chapter 12

CHMA11H3 Chapter Notes - Chapter 12: Osmosis, Semipermeable Membrane, Yater


Department
Chemistry
Course Code
CHMA11H3
Professor
Jamie Donaldson
Chapter
12

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Chapter 12
12.1 Thirsty Solutions: Why You Should Not Drink Water
Seawater actually draws water out of the body as it passes through the stomach and
intestines, resulting in diarrhoea and further dehydration
Thirsty Solution, is a solution that draws water to itself it has a tendency to mix
Solution is a homogenous mixture of two or more substances or components. The majority
component is usually called the solvent and the minority component is called the solute
The reason water draws water to itself is related to natures tendency toward spontaneous
mixing
Unless it is highly unfavourable energetically, substance tend to combine into uniform
mixtures, not separate into pure substances
The tendency toward mixing results in a uniform concentration of the final solution
12.2 Types of Solution and Solubility
A solution may be composed of a solid and a liquid but may also be composed of a gas and a
liquid
Aqueous Solutions is a solution in which water is the solvent and a solid, liquid, or gas is
the solute
Solubility is the amount of the substance that will dissolve in a given amount of solvent
Many physical system tend to lower the potential energy
The formation of a solution does not necessarily lower the potential energy of its constituent
particles
We cannot think of the mixing of two idea gases as lowering their potential energy, the
tendency to mix is related to a concept called entropy
Entropy is a measure of energy randomization or energy dispersal in a system
The pervasive tendency for energy to spread out, or disperse, whenever it is not restrained
from doing so is the reason that two ideal gases mix
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The tendency for energy to disperse is why thermal energy flows from the hot end of the rod
to the cold one, and not the other way around
In the absence of intermolecular forces, two substances spontaneously mix to form a
homogenous solution
-Solvent-solute interactions: the interactions between a solvent particle and a solute
particle
-Solvent-solvent interactions: the interactions between a solvent particle and another
solvent particle
-Solute-solute interactions: the interactions between a solute particle and another
solute particle
All three interactions are of similar magnitude so the two substances are soluble in each
other in all proportions they are said to be miscible
The formation of the solution is driven by the tendency toward mixing, or toward greater
entropy
If solvent molecules and solute molecules each interact more strongly with molecules of their
own kind than with molecules of the other kind then a solution may still form, depending
on the relative disparities between the interactions
If the disparity is small, the tendency to mix results in the formation of a solution even
though the process is energetically uphill
If the disparity is large, however, a solution will not form
Although the tendency to mix is strong it cannot overcome the large energy disparity between
the powerful solvent-solvent interactions and the weak solvent-solute interactions
12.3 Energetics of Solution Formation
The separation of the solute into its constituent particles is always endothermic because
energy is required to over-come the forces that hold the solute together
Separating the solvent particles from each other to make room for the solute particles, is also
endothermic because energy is required to overcome the intermolecular forces among the
solvent particles
Mixing the solute particles with the solvent particles, is exothermic because energy is
released as the solute particles interact with the solvent particles
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According to Hess`s law, the overall enthalpy change upon solution formation, enthalpy of
solution, is the sum of the changes in the enthalpy for each step

The overall sign of Hsoln depends on the magnitudes of the individual terms
1.If the sum of the endothermic terms is about equal in magnitude than the exothermic
term, then Hsoln is about zero
2.If the sum of the endothermic terms is smaller in magnitude than the exothermic
term, then Hsol is negative and the solution process is exothermic
3.If the sum of the endothermic terms is greater in magnitude than the exothermic
term, then Hsoln is positive and the solution process is endothermic
In these aqueous solution Hsol and Hmix can be combined into a single term called the heat
of hydration
The heat of hydration is the enthalpy change that occurs when 1mol of the gaseous solute
ions are dissolved in water
When initially dissolved sodium chloride can then begin to re-crystallizes as solid sodium
chloride
Initially the rate of dissolution far exceeds the rate of re-crystallization, but as the
concentration of dissolved sodium chloride increases the rate of re-crystallization also
increases.
Eventually the rates of dissolution and re-crystallization become equal dynamic
equilibrium has been reached
A solution in which the dissolved solute is in dynamic equilibrium with the solid solute is a
saturated solution, if you add additional solute to a saturated solution it will not dissolve
A solution containing less than the equilibrium amount of solute is an unsaturated solution,
if you add additional solute to an unsaturated solution, it will dissolve
Under certain circumstances, a supersaturated solution one containing more than the
equilibrium amount of solute may form.
The solubility of solids in water can be highly dependent on temperature
The solubility of most solids in water increases with increasing temperature (Na2SO4 is an
exception)
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