Solutions
13.1 – 13.5
Solution - ______________________________________________________________________
Solvent - _______________________________________________________________________
Solutes - _______________________________________________________________________
Factors that determine whether a solution will form:
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_______________________________________________________________________
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___________ - based on the strength of IMFs – neither the solute or solvent can be more
attracted to itself than to the other substance.
The Solution Process
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IMFs between the solvent particles and IMFs between the solute particles must be equal
to or weaker than IMFs between the solute and solvent.
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Oil – nonpolar – dispersion forces & Water –Polar - hydrogen bonds
The hydrogen bonds in water are stronger than any attractive force between the two
substances
NaCl (solute) dissolving in water (solvent)
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Ion-dipole forces between the ions and the water molecules (in this case) are stronger
than the ionic force holding the ions together and stronger than the hydrogen bonds holding the
water molecules together. (hydration)
Rule of Thumb:
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Polar solvents dissolve polar solutes and nonpolar solvents dissolve nonpolar solutes
“ _____________________________”
More on Energy:
ΔHsolute = energy to separate solute particles - it’s always ______________________________
ΔHsolvent = energy to separate solvent particles- it’s always ______________________________
ΔHmix= depends on IMFs between solvent & solute - it’s always _________________________
ΔHsoln = ΔHsolute + ΔHsolvent + ΔHmix
The solution process overall may be endothermic (cold packs) or exothermic (hot packs)
depending on the magnitudes of each enthalpy value
Miscible - substances that mix in any proportions
Example: Ethanol and water are miscible liquids.
Immiscible - substances that do not mix significantly.
Example: Oil and water are immiscible.
Solids (salt, sugar, etc) dissoved in a liquid (water) are somewhere in between!
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Saturated Solutions & Solubility
As a solid dissolves, a solution forms (the opposite process is crystallization)
If crystallization and dissolution are in equilibrium with undissolved solute, the solution
is ________________ (no further solute can be dissolved)
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Solubility - The amount of solute required to form a saturated solution.
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A solution with a concentration of dissolved solute that is less than the solubility is said
to be __________________________________.
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A solution is said to be ________________________________if more solute is dissolved than
in a saturated solution (heat, dissolve more, cool)
Factors Affecting Solubility
Solubility depends on:
1. The nature of the solute & solvent
o More detail on the “like dissoves like” rule
2. The temperature.
3. The pressure (for gases).
1. Consider the solubility of alcohols in water.
 Water and ethanol are miscible because the broken hydrogen bonds in both pure
liquids are re-established in the mixture.
 However, not all alcohols are miscible with water.
 The number of carbon atoms in a chain affects solubility.
o The greater the number of carbons in the chain (the larger the nonpolar part),
the more the molecule behaves like it’s nonpolar
 This decreases its solubility in water
o The greater the number of –OH groups (the polar part), the more the molecule
behaves like it’s polar
 This increases its solubility in water
Substances with similar intermolecular attractive forces tend to be soluble in one
another. (like dissolves like)
2. Pressure Effects
 The solubility of a gas in a liquid is a function of the pressure of the gas over the
solution.
 Solubilities of solids and liquids are not greatly affected by pressure.
With higher gas pressure, ____________________________________________ of the solution and
the probability of a gas molecule striking the surface and entering the solution is increased
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The higher the pressure, the greater the solubility.
The lower the pressure, the less the solubility.
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Henry’s Law - The solubility of a gas is directly proportional to the partial pressure of
the gas above the solution.
Mathematically:
Note: The Henry's law constant (k) differs for each solute-solvent pair and differs with
temperature.
S is usually expressed in mol/L (make sure units cancel when using this equation.)
Henry’s Law Application
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Carbonated beverages are bottled where PCO2> 1 atm.
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As the bottle is opened, PCO2 decreases and the solubility of CO2 decreases (solution
fizzes)
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3. Temperature Effects
As temperature increases, solubility of solids generally __________________________
(there are some exceptions)
Sugar dissolves better in warm water than in cold water.
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As temperature increases, solubility of gases generally _________________________
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Carbonated beverages go flat as they get warm.
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Thermal pollution: If lakes get too warm, O2 become less soluble and fish
suffocate
Concentration of Solutions
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Concentration
Abbreviation
Mass Percent
%
Parts per million
ppm
Mole Fraction
X
Molarity
M
Molality
m
Definition
Converting between molarity (M) and molality (m) requires density.
Colligative Properties
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Colligative properties depend on _________________________________________
There are four colligative properties to consider:
Vapor pressure lowering (Raoult's Law).
Boiling point elevation.
Freezing point depression.
Osmotic pressure.
Van’t Hoff Factor (i) (not in textbook)
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The number of particles formed when a solute dissolves
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NaCl =
–
Al(NO3)3 =
–
C12H22O11 =
This is a maximum!
Vapor Pressure Lowering
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Nonvolatile solutes (with no measurable vapor pressure themselves) reduce the ability of
the surface solvent molecules to escape the liquid.
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Therefore, vapor pressure is lowered.
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The amount of vapor pressure lowering depends on the amount of solute.
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Raoult’s law quantifies the extent to which a nonvolatile solute lowers the vapor
pressure of the solvent
PA = Vapor Pressure of sol’n
XA = mole fraction of solvent
PAo= Vapor pressure of pure solvent
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Ideal solution: one that obeys Raoult’s law.
Real solutions show ideal behavior when:
The solute concentration is low.
The solute and solvent have similarly sized molecules.
The solute and solvent have similar types of intermolecular attractions.
Boiling-Point Elevation
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A nonvolatile solute lowers the vapor pressure of a solution.
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Higher temperature is required to reach a vapor pressure of 1 atm for the solution
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The molal boiling-point-elevation constant, Kb, expresses how much Tb changes
with molality
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The boiling-point elevation is proportional to the concentration of solute particles
i = van’t hoff factor
A 1 m solution of NaCl is 2 m in total solute particles.
Freezing-Point Depression
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When a solution freezes, crystals of almost pure solvent are formed first.
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Solute molecules are usually not soluble in the solid phase of the solvent - sol’n become
more concentrated (lower X of solvent in sol’n).
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This lowers the vapor pressure for the solution
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the triple point occurs at a lower temperature
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The melting-point (freezing-point) curve is a vertical line from the triple point.
The solution freezes at a lower temperature than the pure solvent.
The decrease in freezing point (Tf) is directly proportional to molality.
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Kf is the molal freezing-point-depression constant.
Osmosis
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Semipermeable membranes permit passage of some components of a solution.
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Often they permit passage of water but not larger molecules or ions.
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Osmosis is the net movement of a solvent from an area of ___________________________
_______________________________________________________________________________________
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Consider a U-shaped tube with a two liquids separated by a semipermeable membrane.
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The rate of movement of solvent from the less concentrated solution is faster than the
rate of movement in the opposite direction.
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As solvent moves across the membrane, the fluid levels in the arms become uneven.
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The vapor pressure of solvent is higher in the arm with more solvent.
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Eventually the pressure difference due to the difference in height of liquid in the arms
stops osmosis.
The process of reverse osmosis uses mechanical pressure to make the solvent (water) flow in
the opposite direction. This process can be used to desalinate ocean water to make drinking
water.
Osmotic pressure, π
Osmotic pressure obeys a law similar in form to the ideal-gas law
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n =moles
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V= volume
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M= molarity
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R= the ideal gas constant
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T= absolute temperature
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Solutions are said to be isotonic if they have the same osmotic pressure.
Hypotonic solutions have a lower π and are less concentrated
Hypertonic solutions have a higher π and are more concentrated
Molar mass can be determined using any colligative property
Camphor (C10H16O) melts at 179.8°C, and it has a particularly large freezing-point-depression
constant, Kf=40.0°C/m. When 0.186 g of an organic substance of unknown molar mass is
dissolved in 22.01 g of liquid camphor, the freezing point of the mixture is found to be 176.7°C.
What is the molar mass of the solute?
A sample of 2.05 g of polystyrene of uniform polymer chain length was dissolved in enough
toluene to form 0.100 L of solution. The osmotic pressure of this solution was found to be 1.21
kPa at 25°C. Calculate the molar mass of the polystyrene.
Chapter 13 Book Assignment:
13 (16, 18, 26, 34, 38, 42, 46, 52, 62, 64, 66, 70, 74)