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Chapter 12Physical Properties of Solutions
General Chemistry: An Integrated Approach
Hill, Petrucci, 4th Edition
Mark P. HeitzState University of New York at Brockport
© 2005, Prentice Hall, Inc.
Chapter 12: Physical Properties of Solutions 2
Some Types of Solutions
EOS
Chapter 12: Physical Properties of Solutions 3
Solution ConcentrationMolarity is the amount of solute dissolved into a volume of solvent – the ratio is moles solute to liters solution = mol L–1
Percent by mass is the mass ratio of solute to solution multiplied by 100
Percent by volume is the volume ratio of solute to solution multiplied by 100
EOS
Mass/volume percent is the mass of solute divided by the volume of solution multiplied by 100
Chapter 12: Physical Properties of Solutions 4
Solutions by “Parts”
Parts per billion is the number of particles of solute per one billion particles of solution.
1 ppb = 1 g/L
EOS
Parts per trillion is the number of particles of solute per one trillion particles of solution.
1 ppt = 1 ng/L
Parts per million is the number of particles of solute per one million particles of solution
units are 1 ppm = 1 mg/L
Chapter 12: Physical Properties of Solutions 5
Molality
Molality (m) is the number of moles of solute per one kilogram of solvent (not solution!)
EOS
To be independent of temperature, a concentration unit must be based on mass only, not volume
Molarity (M) varies with temperature due to the expansion or contraction in the volume of the solution
Chapter 12: Physical Properties of Solutions 6
Mole Fraction and Mole PercentThe mole fraction (xi) of a solution component i is the fraction of all the molecules in the solution that are molecules of i
total
( ) ii
nmol ix
total mol n
EOS
The mole percent of a solution component is its mole fraction multiplied by 100
Sum of xi must equal 1
Chapter 12: Physical Properties of Solutions 7
Enthalpy of Solution
Solution formation can be considered to take place in three steps
Move the molecules of solvent apart to make room for the solute molecules. H1 > 0
Separate the molecules of solute to the distances found between them in the solution. H2 > 0
Allow the separated solute and solvent molecules to mix randomly. H3 > 0
EOS
Hsoln = H1 + H2 + H3
Chapter 12: Physical Properties of Solutions 8
Visualizing Enthalpy of Solution
EOS
Chapter 12: Physical Properties of Solutions 9
Intermolecular Forcesin Solution Formation
If all intermolecular forces are of comparable strength, this type of solution is called an ideal solution and Hsoln = 0
If the intermolecular forces between solute and solvent molecules are stronger than other intermolecular forces, Hsoln < 0
EOS
For solute/solvent forces that are weaker than other intermolecular forces, Hsoln > 0
Chapter 12: Physical Properties of Solutions 10
Non-ideal Solutions
Ethanol and water have strong attractions – when mixed, the volume of 50 mL H2O + 50 mL EtOH is less than 100 mL
EOS
Chapter 12: Physical Properties of Solutions 11
Aqueous Solutions of Ionic Compounds
The forces causing an ionic solid to dissolve in water are ion–dipole forces, the attraction of water dipoles for cations and anions.
EOS
The attractions of water dipoles for ions pulls the ions out of the crystalline lattice and into aqueous solution
Chapter 12: Physical Properties of Solutions 12
When there is a dynamic equilibrium between an undissolved solute and a solution, the solution is saturated
Some Solubility TermsLiquids that mix in all proportions are called miscible
EOS
A solution which contains less solute than can be held at equilibrium is unsaturated
Chapter 12: Physical Properties of Solutions 13
SolubilityThe concentration of the solute in a saturated solution is the solubility of the solute
EOS
About 95% of all ionic compounds have aqueous solubilities that increase with increasing temperature
Chapter 12: Physical Properties of Solutions 14
SupersaturationA supersaturated solution is created when a warm, saturated solution is allowed to cool without the precipitation of the excess solute
EOS
Chapter 12: Physical Properties of Solutions 15
Selective Crystallization
When KNO3(s) is crystallized from an aqueous solution of KNO3 containing CuSO4 as an impurity, CuSO4 (blue) remains in the solution
EOS
Chapter 12: Physical Properties of Solutions 16
The Solubilities of Gases
Most gases become less soluble in liquids as the temperature increases
a common example is carbonated beverages
At a constant temperature, the solubility (S) of a gas is directly proportional to the pressure of the gas (Pgas) in equilibrium with the solution.
S = k Pgas
EOS
The effect of pressure on the solubility of a gas is known as Henry’s law
Chapter 12: Physical Properties of Solutions 17
Temperature and Solubility of Gases
EOS
Chapter 12: Physical Properties of Solutions 18
Pressure and Solubility of Gases
EOS
The concentration of the solution increases
Gas compressed into a smaller volume increases the number of molecules per unit volume
Chapter 12: Physical Properties of Solutions 19
Solubility and Gas Pressure
EOS
Chapter 12: Physical Properties of Solutions 20
Vapor Pressures of Solutions
Raoult’s law states that the vapor pressure of the solvent above a solution (Psolv) is the product of the vapor pressure of the pure solvent (Po
solv) and the mole fraction of the solvent in the solution (xsolv)
EOS
Psolv = xsolv . Posolv
Chapter 12: Physical Properties of Solutions 21
Solution SeparationThe vapor in equilibrium with an ideal solution of two volatile components has a higher mole fraction of the more volatile component than is found in the liquid
EOS
Separation can be achieved by fractional distillation
Chapter 12: Physical Properties of Solutions 22
Colligative PropertiesFreezing Point Depression (FPD)
Boiling Point Elevation (BPE)
Consider situations with a volatile solvent and a solute that is nonvolatile, nonelectrolytic, and soluble in the liquid solvent, but not the solid solvent
EOS
The vapor pressure of the solution is that of the solvent in the solution, and at all temperatures this vapor pressure is lower than that of the pure solvent
Chapter 12: Physical Properties of Solutions 23
FPD and BPE
The presence of the solute lowers (depresses) the freezing point of the solvent (Tf) and increases (elevates) the boiling point of the solvent (Tb)
EOS
Example: adding salt to water allows the water temperature to exceed 100 oC, thereby cooking food faster
Chapter 12: Physical Properties of Solutions 24
Vapor Pressure Lowering
In a solution, the solvent vapor pressure is lowered and the fusion curve is displaced to lower temperatures
EOS
Chapter 12: Physical Properties of Solutions 25
FPD and BPE Constants
EOS
Chapter 12: Physical Properties of Solutions 26
Osmosis
Semipermeable membranes are usually films of a material containing a network of microscopic pores through which small solvent molecules can pass, but larger solute molecules cannot
EOS
Osmosis is the net flow of solvent molecules from pure solvent through a semipermeable membrane into a solution
Chapter 12: Physical Properties of Solutions 27
Osmotic PressureThe pressure required to stop osmosis is called the osmotic pressure of the solution = (n/V)RT = M RT
A solution (green) is separated from pure water by a membrane permeable to H2O molecules but not to solute particles
EOS
When the flow of water is at equilibrium, the hydrostatic pressure is now called the osmotic pressure, .
Chapter 12: Physical Properties of Solutions 28
Osmosis Applications
Water purification:
EOS
Reverse osmosis is the process of reversing the normal net flow of solvent molecules through a semipermeable membrane by applying to the solution a pressure exceeding the osmotic pressure
Chapter 12: Physical Properties of Solutions 29
Solutions of Electrolytes
The van’t Hoff factor (i) is used to modify the equations for colligative properties
FPD: Tf = –i Kfm BPE: Tb = i Kbm OP: = i M RT
EOS
i is dependent on solution molality
Chapter 12: Physical Properties of Solutions 30
Colloids
A colloid is a dispersion in an appropriate medium for particles ranging in size from about 1 to 1000 nm
EOS
The scattering of a light beam through a colloidal material is known as the Tyndall effect
Chapter 12: Physical Properties of Solutions 31
Common Colloids
EOS
Chapter 12: Physical Properties of Solutions 32
Formation and Coagulationof a Colloid
A high concentration of an electrolyte can cause a colloid to coagulate, or precipitate
EOS
Chapter 12: Physical Properties of Solutions 33
A Suspension and a Colloid
EOS
Chapter 12: Physical Properties of Solutions 34
Summary of Concepts
• Molarity (M) is expressed as moles per liter and molality (m) is expressed as moles of solute per kilogram of solvent
• Units used for very dilute concentrations of solutes include parts per million (ppm), parts per billion (ppb), and parts per trillion (ppt)
EOS
• The type and magnitude of intermolecular forces are important in solution composition
Chapter 12: Physical Properties of Solutions 35
Summary of Concepts
• The solubility of a solute is its concentration in a saturated solution
• The solubility of gases in solutions decreases with an increase in temperature but increases with an increase in pressure
EOS
• The presence of solutes lowers the vapor pressure of the solution and causes both a freezing point depression and a boiling point elevation of the solvent
Chapter 12: Physical Properties of Solutions 36
Summary of Concepts
• Colligative properties depend on the particular solvent and the number of solute particles present
• Colloids are dispersions in an appropriate medium of particles ranging in size from 1 nm to 1000 nm
EOS