Solutions
• homogeneous mixtures
9 composition may vary from one sample to another
9 appears to be one substance, though really contains
multiple materials
• most homogeneous materials we encounter are
actually solutions
• nature has a tendency toward spontaneous mixing
9 generally, uniform mixing is more energetically
favorable
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Solutions
• solute is the dissolved substance
•
•
•
9seems to “disappear”
9“takes on the state” of the solvent
solvent is the substance solute
dissolves in
9does not appear to change state
when both solute and solvent have
the same state, the solvent is the
component present in the highest
percentage
solutions in which the solvent is
water are called aqueous solutions
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Seawater
• drinking seawater will dehydrate you and give
you diarrhea
• the cell wall acts as a barrier to solute moving
• the only way for the seawater and the cell
solution to have uniform mixing is for water to
flow out of the cells of your intestine and into
your digestive tract
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1
Common Types of Solution
Solution Phase
gaseous solutions
liquid solutions
solid solutions
Solute
Phase
Solvent
Phase
gas
gas
gas
liquid
solid
solid
liquid
liquid
liquid
solid
Example
air (mostly N2 & O2)
soda (CO2 in H2O)
vodka (C2H5OH in H2O)
seawater (NaCl in H2O)
brass (Zn in Cu)
• solutions that contain Hg and some other metal are
called amalgams
• solutions that contain metal solutes and a metal solvent
are called alloys
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Solubility
• when one substance (solute) dissolves in another
•
(solvent) it is said to be soluble
9salt is soluble in water
9bromine is soluble in methylene chloride
when one substance does not dissolve in another it is
said to be insoluble
9oil is insoluble in water
• the solubility of one substance in another
depends on two factors – nature’s tendency
towards mixing, and the types of
intermolecular attractive forces
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Spontaneous Mixing
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2
Solubility
• there is usually a limit to the solubility of one
substance in another
9gases are always soluble in each other
9two liquids that are mutually soluble are said to be
miscible
¾alcohol and water are miscible
¾oil and water are immiscible
• the maximum amount of solute that can be dissolved
in a given amount of solvent is called the solubility
• the solubility of one substance in another varies with
temperature and pressure
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Mixing and the Solution Process
Entropy
• formation of a solution does not necessarily
lower the potential energy of the system
9 the difference in attractive forces between atoms of
two separate ideal gases vs. two mixed ideal gases
is negligible
9 yet the gases mix spontaneously
• the gases mix because the energy of the
•
•
system is lowered through the release of
entropy
entropy is the measure of energy dispersal
throughout the system
energy has a spontaneous drive to spread out
over as large a volume as it is allowed
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Intermolecular Forces and the Solution Process
Enthalpy of Solution
• energy changes in the formation of most solutions also
•
involve differences in attractive forces between
particles
must overcome solute-solute attractive forces
9 endothermic
• must overcome some of the solvent-solvent attractive
forces
9 endothermic
• at least some of the energy to do this comes from
making new solute-solvent attractions
9 exothermic
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3
Intermolecular Attractions
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Relative Interactions and Solution Formation
Solute-to-Solvent
Solute-to-Solvent
Solute-to-Solvent
Solute-to-Solute +
Solution Forms
Solvent-to-Solvent
Solute-to-Solute +
=
Solution Forms
Solvent-to-Solvent
Solute-to-Solute + Solution May or
<
Solvent-to-Solvent May Not Form
>
• when the solute-to-solvent attractions are weaker than
the sum of the solute-to-solute and solvent-to-solvent
attractions, the solution will only form if the energy
difference is small enough to be overcome by the
entropy
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Solution Interactions
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4
Will It Dissolve?
• Chemist’s Rule of Thumb –
Like Dissolves Like
• a chemical will dissolve in a solvent if it has a similar
structure to the solvent
• when the solvent and solute structures are similar, the
solvent molecules will attract the solute particles at
least as well as the solute particles to each other
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Classifying Solvents
Solvent
Water, H2O
Class
polar
Structural
Feature
O-H
Methyl Alcohol, CH3OH
polar
O-H
Ethyl Alcohol, C2H5OH
polar
O-H
Acetone, C3H6O
polar
C=O
Toluene, C7H8
nonpolar
C-C & C-H
Hexane, C6H14
Diethyl Ether, C4H10O
nonpolar
nonpolar
Carbon Tetrachloride
nonpolar
C-C & C-H
C-C, C-H & C-O,
(nonpolar > polar)
C-Cl, but symmetrical
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Example 12.1a − predict whether the following
vitamin is soluble in fat or water
OH
The 4 OH groups make
the molecule highly
polar and it will also
H-bond to water.
Vitamin C is water
soluble
OH
H2C
C
H
H
C
O
C
C
O
C
HO
OH
Vitamin C
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5
Energetics of Solution Formation
• overcome attractions between the solute particles –
endothermic
• overcome some attractions between solvent molecules
– endothermic
• form new attractions between solute particles and
solvent molecules – exothermic
• the overall ΔH depends on the relative sizes of the ΔH
for these 3 processes
ΔHsol’n = ΔHsolute ΔHsolvent + ΔHmix
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Solution Process
1. add energy in to overcome solute-solute attractions
3. form new solute-solvent attractions, releasing energy
2. add energy in to overcome some solvent-solvent attractions
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Energetics of Solution Formation
if the total energy cost for
breaking attractions between
particles in the pure solute and
pure solvent is greater
less thanthan
the the
energy released in making the
new attractions between the
solute and solvent, the overall
process will be endothermic
exothermic
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6
Heats of Hydration
• for aqueous ionic solutions, the energy added to
overcome the attractions between water molecules and
the energy released in forming attractions between the
water molecules and ions is combined into a term
called the heat of hydration
9 attractive forces in water = H-bonds
9 attractive forces between ion and water = ion-dipole
9 ΔHhydration = heat released when 1 mole of gaseous ions
dissolves in water
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Heat of Hydration
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Ion-Dipole Interactions
• when ions dissolve in water they become
hydrated
• each ion is surrounded by water molecules
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7
Solution Equilibrium
• the dissolution of a solute in a solvent is an equilibrium
process
• initially, when there is no dissolved solute, the only
process possible is dissolution
• shortly, solute particles can start to recombine to
•
reform solute molecules – but the rate of dissolution >>
rate of deposition and the solute continues to dissolve
eventually, the rate of dissolution = the rate of
deposition – the solution is saturated with solute and no
more solute will dissolve
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Solution Equilibrium
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Solubility Limit
• a solution that has the maximum amount of solute
dissolved in it is said to be saturated
9 depends on the amount of solvent
9 depends on the temperature
¾ and pressure of gases
• a solution that has less solute than saturation is said to
be unsaturated
• a solution that has more solute than saturation is said to
be supersaturated
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8
How Can You Make a Solvent Hold
More Solute Than It Is Able To?
• solutions can be made saturated at non-room conditions
– then allowed to come to room conditions slowly
• for some solutes, instead of coming out of solution
•
when the conditions change, they get stuck in-between
the solvent molecules and the solution becomes
supersaturated
supersaturated solutions are unstable and lose all the
solute above saturation when disturbed
9 e.g., shaking a carbonated beverage
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Adding Solute to a Supersaturated
Solution of NaC2H3O2
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Temperature Dependence of Solubility
of Solids in Water
• solubility is generally given in grams of solute that will
•
dissolve in 100 g of water
for most solids, the solubility of the solid increases as
the temperature increases
9 when ΔHsolution is endothermic
• solubility curves can be used to predict whether a
solution with a particular amount of solute dissolved in
water is saturated (on the line), unsaturated (below the
line), or supersaturated (above the line)
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9