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Chapter 14:
Solutions and Their Properties
If you’re not part of the solution,
you’re part of the precipitate!
Solvation
Structure/Interm olecular Forces
Henry's Law
B. pt. E levation Fr. pt. Depression Osom otic Pressure
Colligative Properties
Vapor Pressures Raoult's Law
T em perature Pressure
Heats of Solution
Com positions Solubility Rules
T erm inology
Properties of SolutionsProperties of Solutions
o
AAA PP
Sfbfb mKT ,, TRM S
AS
AA nn
n
Akg
nm S
S
V
nM S
S
Physical Properties of Solutions
Have you ever wondered...•Why antifreeze keeps water from freezing?•Why salt causes ice to melt?•Why cooks add salt to boiling water?•Why root beer foams only when poured?•What force opposes gravity to allow water to climb up a tree?
Solutions
• Solutions are homogeneous mixtures of two or more pure substances.
• In a solution, the solute is dispersed uniformly throughout the solvent.
An electrolyte is a substance that, when dissolved in water, results in a solution that can conduct electricity.
A nonelectrolyte is a substance that, when dissolved, results in a solution that does not conduct electricity.
nonelectrolyte weak electrolyte strong electrolyte
“like dissolves like”
Two substances with similar intermolecular forces are likely to be soluble in each other.
• non-polar molecules are soluble in non-polar solvents
CCl4 in C6H6
• polar molecules are soluble in polar solvents
C2H5OH in H2O
• ionic compounds are more soluble in polar solvents
NaCl in H2O or NH3 (l)
Energy Changes in SolutionTo determine the enthalpy
change, we divide the process into 3 steps. 1. Separation of solute
particles.
2. Separation of solvent particles to make ‘holes’.
3. Formation of new interactions between solute and solvent.
Three types of interactions in the solution process:• solvent-solvent interaction• solute-solute interaction• solvent-solute interaction
Hsoln = H1 + H2 + H3
Enthalpy Changes in Solution
The enthalpy change of the overall process depends on H for each of these steps.
Start
End
EndStart
Calculating⁄Hsolution
(enthalpy of solution)
KF(s) -> K+(g) + F-(g) ⁄ E = +821 kJ
K+(g) + F-
(g) + H2O -> K+(aq) + F-
(aq)
⁄ E=-819 kJ
So Net -
KF(s) -> K+(aq) + F-
(aq)
⁄ Hsolution = +2kJ
Degree of saturation
• Saturated solutionSolvent holds as much
solute as is possible at that temperature.
Undissolved solid remains in flask.
Dissolved solute is in dynamic equilibrium with solid solute particles.
Degree of saturation
• SupersaturatedSolvent holds more solute than is normally
possible at that temperature.These solutions are unstable; crystallization can
often be stimulated by adding a “seed crystal” or scratching the side of the flask.
Gases in Solution
• In general, the solubility of gases in water increases with increasing mass.
Why?• Larger molecules
have stronger dispersion forces.
Gases in Solution
• The solubility of liquids and solids does not change appreciably with pressure.
• But, the solubility of a gas in a liquid is directly proportional to its pressure.
Increasing pressure above solution forces more gas to dissolve.
Henry’s Law
What happens to the solubility of carbon dioxide in a bottle of soda when the pressure is reduced?
Pressure and Solubility of Gases
The solubility of a gas in a liquid is proportional to the pressure of the gas over the solution (Henry’s law).
c = kP
c is the concentration (M) of the dissolved gas
P is the pressure of the gas over the solution
k is a constant (mol/L•atm) that depends onlyon temperature
low P
low c
high P
high c
Chemistry In Action: The Killer Lake
Lake Nyos, West Africa
8/21/86CO2 Cloud Released
1700 Casualties
Trigger?
• earthquake
• landslide
• strong Winds
Temperature
Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.
Solubility is measured as the mass of solute dissolved in 100 g of solvent at a given temperature
Temperature• The opposite is true of
gases. Higher temperature drives gases out of solution.
Carbonated soft drinks are more “bubbly” if stored in the refrigerator.
Warm lakes have less O2 dissolved in them than cool lakes.
moles of Atotal moles in solution
XA =
Mole Fraction (X)
• In some applications, one needs the mole fraction of solvent, not solute—make sure you find the quantity you need!
mol of soluteL of solution
M =
Molarity (M)
• Because volume is temperature dependent, molarity can change with temperature.
mol of solutekg of solvent
m =
Molality (m)
Because neither moles nor mass change with temperature, molality (unlike molarity) is not temperature dependent.
Changing Molarity to Molality
If we know the density of the solution, we can calculate the molality from the molarity, and vice versa.
Colligative Properties
• Colligative properties depend only on the number of solute particles present, not on the identity of the solute particles.
• Among colligative properties areVapor pressure lowering Boiling point elevationMelting point depressionOsmotic pressure
Vapor Pressures of Pure Water and a Water Solution
The vapor pressure of water over pure water is greater than the vapor pressure of water over an aqueous solution containing a nonvolatile solute.
Solute particles take up surface area and lower the vapor pressure
Vapor Pressure
As solute molecules are added to a solution, the solvent becomes less volatile (has decreased vapor pressure).
Solute-solvent interactions contribute to this effect.
Lowering Vapor Pressure• Raoult’s Law:
• Where: PA = vapor pressure with solute,
• PA = vapor pressure without solute (pure solvent), and
A = mole fraction of A (the pure solvent).
Colligative PropertiesColligative Properties
AAA PP
Boiling Point Elevation and Freezing Point Depression
Solute-solvent interactions also cause solutions to have higher boiling points and lower freezing points than the pure solvent.
Boiling Point ElevationThe change in boiling point is proportional to the molality of the solution:
Tb = Kb m
where Kb is the molal boiling point elevation constant, a property of the solvent.Tb is added to the normal
boiling point of the solvent.
Freezing Point Depression• The change in freezing
point can be found similarly:
Tf = Kf m
• Here Kf is the molal freezing point depression constant of the solvent.
Tf is subtracted from the normal freezing point of the solvent.
Boiling-Point Elevation
• Molal boiling-point-elevation constant, Kb, expresses how much Tb changes with molality, mS :
• Decrease in freezing point (Tf) is directly proportional to molality (Kf is the molal freezing-point-depression constant):
Colligative PropertiesColligative Properties
Sbb mKT
Sff mKT
In both equations, T does not depend on what the solute is, but only on how many particles are dissolved.
Colligative Properties of Electrolytes
Because these properties depend on the number of particles dissolved, solutions of electrolytes (which dissociate in solution) show greater changes than those of nonelectrolytes.
e.g. NaCl dissociates to form 2 ion particles; its limiting van’t Hoff factor is 2.
Colligative Properties of Electrolytes
However, a 1 M solution of NaCl does not show twice the change in freezing point that a 1 M solution of methanol does.
It doesn’t act like there are really 2 particles.
van’t Hoff Factor
Some Na+ and Cl− reassociate as hydrated ion pairs, so the true concentration of particles is somewhat less than two times the concentration of NaCl.
The van’t Hoff Factor
• Reassociation is more likely at higher concentration.
• Therefore, the number of particles present is concentration dependent.
The van’t Hoff Factor
We modify the previous equations by multiplying by the van’t Hoff factor, i
Tf = Kf m i
i = 1 for non-elecrtolytes
Osmosis
• Semipermeable membranes allow some particles to pass through while blocking others.
• In biological systems, most semipermeable membranes (such as cell walls) allow water to pass through, but block solutes.
OsmosisIn osmosis, there is net movement of solvent from the area of higher solvent concentration (lower solute concentration) to the are of lower solvent concentration (higher solute concentration).
Water tries to equalize the concentration on both sides until pressure is too high.
Osmosis• Osmotic pressure, , is the pressure required to stop
osmosis:
Colligative PropertiesColligative Properties
TRV
n
TRnV
TRM S TRM S
Molar Mass from Colligative Properties
We can use the effects of a colligative property such as osmotic pressure to determine the molar mass of a compound.
Solvation
Structure/Interm olecular Forces
Henry's Law
B. pt. E levation Fr. pt. Depression Osom otic Pressure
Colligative Properties
Vapor Pressures Raoult's Law
T em perature Pressure
Heats of Solution
Com positions Solubility Rules
T erm inology
Properties of SolutionsProperties of Solutions
o
AAA PP
Sfbfb mKT ,, TRM S
AS
AA nn
n
Akg
nm S
S
V
nM S
S
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