Transcript General Chemistry - Valdosta State University

Solutions and Their
Behavior
Chapter 14
Chapter 14
1
The Solution Process
Solution - A homogeneous mixture composed of a solute
and a solvent.
Solute – The substance which is dissolved.
Solvent – The substance which acts as the dissolving
medium.
Chapter 14
2
Saturated Solutions and Solubility
Mole Fraction, Molarity, and Molality
Qualitative Terms:
Dilute Solution – A solution in which additional solute
may be dissolved.
Saturated Solution – A solution in which no more
solute may be dissolved.
Supersaturated Solution – A solution in which more
solute is dissolved than in a saturated solution.
Solubility - amount of solute required to form a
saturated solution.
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3
Ways of Expressing Concentration
• Definitions:
mass of component in soln
Mass % 
 100
total mass of soln
mass of component in soln
ppm 
 10 6
total mass of soln
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Mole Fraction
molesof component
Mole fraction
totalmolesof all components
Molarity
moles of solute
Molarity 
liters of solution
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Molality (m) – moles of solute per kilogram of solution
molesof solute
Molality
kilogramsof solvent
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The Solution Process
Liquids Dissolving in Liquids
Miscible liquids - Mix in any proportions.
Immiscible liquids - Do not mix.
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The Solution Process
Liquids Dissolving in Liquids
“Like Dissolves Like”
• Polar molecules will dissolve in polar solvents.
• Non-polar molecules will dissolve in non-polar
solvents.
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The Solution Process
Solids Dissolving in Liquids
“Like Dissolves Like”
• Still true but less effective (i.e. there are exceptions)
• Also, many dissolution processes are endothermic.
• A classic example is Ammonium nitrate in water.
• A classic exception is Sulfuric acid in water.
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9
The Solution Process
Heat of Solution
For a solid to dissolve:
• Energy must be supplied to separate the ions in the
crystal lattice: -DHlattice
• Energy is evolved individual items are surrounded by
the solvent: DHhydration
DHsolution = (-DHlattice + DHhydration)
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10
Factors Affecting Solubility
Temperature Effects
Chapter 14
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Factors Affecting Solubility
Pressure Effects
• Solubility of a gas in a liquid is a function of the
pressure of the gas.
• The higher the pressure, the greater the solubility.
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Factors Affecting Solubility
Pressure Effects
Henry’s Law – The solubility of a gas increases in direct
proportion to its partial pressure above the solution.
S g  kH Pg
Sg - solubility of gas
Pg - the partial pressure of the gas
kH - Henry’s law constant.
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Factors Affecting Solubility
Temperature Effects
Chapter 14
14
Factors Affecting Solubility
Temperature Effects
• The solubility of a gas is greater in a cold solvent.
• The dissolution of a gas in water is an exothermic
process.
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Colligative Properties
• Properties of a solution which depend on quantity of
solute molecules.
• Solutions formed with a nonvolatile solute will:
• have lower vapor pressure
• lower freezing point
• higher boiling point
Chapter 14
16
Colligative Properties
Boiling-Point Elevation
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Colligative Properties
Raoult’s Law
Raoult’s Law – The equilibrium vapor pressure of the
solvent over the solution is directly proportional to the
mole fraction of the solvent in the solution
Psolvent  
o
solvent solvent
P
Psolvent - vapor pressure of the solvent in the solution
Psolvent - vapor pressure of the pure solvent
solvent - the mole fraction of solvent
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Colligative Properties
Change in Vapor Pressure
DPsolvent  
o
solute solvent
P
DPsolvent - vapor pressure of the solvent in the solution
Psolvent - vapor pressure of the pure solvent
solute - the mole fraction of solute
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Colligative Properties
Boiling-Point Elevation
• As the vapor pressure of a solution decreases, the
boiling point increases.
• Recall, a solvent boils when its’ vapor pressure equals
the atmospheric pressure.
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Colligative Properties
Boiling-Point Elevation
ΔTbp  Kbp m
Kbp - Molal boiling-point elevation constant
m – solution molality
DTbp – boiling point change
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Colligative Properties
Freezing-Point Depression
• Just as the boiling point is raised, the freezing point is
lowered.
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Colligative Properties
Freezing-Point Depression
DT fp  K fpm
Kfp - Molal freezing-point depression constant
m – solution molality
DTfp – freezing point change
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Colligative Properties
F.Pt. and B.Pt. Change with Ionic Solutes
• The previous equations assumed molecular solids
– example, sugar (C6H12O6)
C6 H12O6 (s) 
 C6 H12O6 (aq)
H 2O
• Recall that colligative properties are a function of the
number of bodies in the solution.


NaCl(s) 
 Na (aq)  Cl (aq)
H 2O
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Colligative Properties
F.Pt. and B.Pt. Change with Ionic Solutes
• F. Pt. depression and B. Pt. equations must be
modified to account for this.
ΔTbp  Kbp m i
Kbp - Molal boiling-point elevation constant
m – solution molality
DTbp – boiling point change
i – van Hoff factor
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Colligative Properties
F.Pt. and B.Pt. Change with Ionic Solutes
ΔT fp  K fpm i
Kfp - Molal freezing-point depression constant
m – solution molality
DTfp – freezing point change
i – van Hoff factor
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Colligative Properties
Molar Mass Determination
The molar mass (molecular weight) can be determined
by observing the freezing point change to a solution in
which a know quantity of non-volatile solute has been
added.
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Colligative Properties
Osmosis
The net movement of solvent through a semi-permeable
membrane from a dilute to a concentrated solution.
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Colligative Properties
Osmosis
Eventually the pressure difference between the arms
stops osmosis.
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Colligative Properties
Osmosis
Osmotic pressure() - The pressure required to stop
osmosis:
  cRT
c - molarity of the solution
R - gas constant (0.08206 L(atm)/mol(K))
T - temperature in Kelvin
Chapter 14
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Colloids
• Colloids are suspensions in which the suspended
particles are larger than molecules but too small to
drop out of the suspension due to gravity.
• Particle size: 10 to 2000 Å.
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31
Colloids
• Tyndall effect: ability of a Colloid to scatter light. The
beam of light can be seen through the colloid.
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Homework
4, 8, 23, 28, 38,42, 52
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