Chapter 13 Properties of Solutions

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Chapter 13Properties of Solutions
Jozsef DevenyiDepartment of Chemistry, UTM
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The Solution Process

• A solution is a homogeneous mixture of solute
• (present in smallest amount) and solvent
  (present
• in largest amount).
• Solutes and solvent are components of the
  solution.
• In the process of making solutions with condensed
  phases, intermolecular forces become rearranged.

• Consider NaCl (solute) dissolving in water
  (solvent)
• the water H-bonds have to be interrupted,
• NaCl dissociates into Na and Cl-,
• ion-dipole forces form Na ?-OH2 and Cl-
  ?H2O.
• We say the ions are solvated by water.
• If water is the solvent, we say the ions are
  hydrated.

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The Solution Process
Recall cation-dipole interaction
anion-dipole interaction
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The Solution Process
Energy Changes and Solution Formation
There are three energy steps in forming a
solution
- separation of solute molecules (?H1)

• separation of solvent
• molecules (?H2)

- formation of solute-solvent interactions
(?H3)
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The Solution Process
Energy Changes and Solution Formation

• We define the enthalpy change in the solution
  process as
• ?Hsoln ?H1 ?H2 ?H3
• ?Hsoln can either be positive or negative
  depending on the intermolecular forces.

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The Solution Process
Energy Changes and Solution Formation

• Breaking attractive intermolecular forces is
  always endothermic.
• Forming attractive intermolecular forces is
  always exothermic.

• To determine whether ?Hsoln is positive or
  negative, we
• consider the strengths of all solute-solute
  and
• solute-solvent interactions

• ?H1 and ?H2 are both positive
• ?H3 is always negative
• It is possible to have either ?H3 gt (?H1
  ?H2) or
• 
  ?H3 lt (?H1 ?H2).

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The Solution Process
Energy Changes and Solution Formation
Examples

• NaOH added to water has ?Hsoln - 44.48 kJ/mol.
• NH4NO3 added to water has ?Hsoln 26.4 kJ/mol.

• General rule similar dissolves in similar,
  i.e., polar
• solvents dissolve polar solutes. Non-polar
  solvents
• dissolve non-polar solutes.

• - If ?Hsoln is too endothermic a solution will
  not form.
• NaCl in gasoline the ion-dipole forces are
  weak because
• gasoline is non-polar. Therefore, the
  ion-dipole forces do
• not compensate for the separation of ions.

- Water in octane water has strong H-bonds.
There are no attractive forces between
water and octane to compensate for the
H-bonds.
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Ways of Expressing Concentration

• All methods involve quantifying amount of
• solute per amount of solvent (or solution).
• Generally amounts or measures are masses, moles
  or liters.
• Qualitatively solutions are dilute or
  concentrated.
• Definitions

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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Recall
mass can be converted to moles using the molar
mass
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality

• We define

Converting between molarity (M) and molality (m)
requires density.
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Examples

• Calculate the percent by mass concentration of
  sugar when 6.28g of sugar is dissolved in 173g of
  water.

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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Examples
B)
Calculate the molality of the above solution if
the molar mass of sugar is 180.16g/mol.
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Examples
C)
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Examples
D)
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Ways of Expressing Concentration
Mole Fraction, Molarity, and Molality
Examples
E)
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Factors Affecting Solubility
Solute-Solvent Interaction

• Polar liquids tend to dissolve in polar solvents.
• Miscible liquids mix in any proportions.
• Immiscible liquids do not mix.
• Intermolecular forces are important water and
  ethanol are miscible because the broken hydrogen
  bonds in both pure liquids are re-established in
  the mixture.
• The number of carbon atoms in a chain affect
  solubility the more C atoms the less soluble in
  water
• (see Table 13.3)

• The number of -OH groups within a molecule
  increases
• solubility in water.

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Factors Affecting Solubility
Solute-Solvent Interaction

• Generalization like dissolves like.
• The more polar bonds in the molecule, the better
  it dissolves in a polar solvent.
• The less polar the molecule the less it dissolves
  in a polar solvent and the better is dissolves in
  a non-polar solvent.

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Factors Affecting Solubility
Solute-Solvent Interaction
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Factors Affecting Solubility
Temperature Effects

• Experience tells us that sugar dissolves better
  in warm water than cold.
• As temperature increases, solubility of solids
  generally increases.
• Sometimes, solubility decreases as temperature
  increases (e.g. Ce2(SO4)3).

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Factors Affecting Solubility
Temperature Effects
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Factors Affecting Solubility
Temperature Effects

• Experience tells us that carbonated beverages go
  flat as they get warm.
• Therefore, gases get less soluble as temperature
  increases.
• Why?
• Thermal pollution if lakes get too warm, CO2 and
  O2 become less soluble and are not available for
  plants or animals.

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Factors Affecting Solubility
Temperature Effects
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Colligative Properties

• Observations

• freezing point of solution is lower than that of
  pure
• solvent e.g., salt added to road in cold to
  lower melting
• point (prevent freezing) of precipitation
  (water)
• - boiling point of solution is higher than that
  of pure solvent

• These effects of a solute on the solvent are
  called colligative properties. These properties
  depend only on the quantity of solute dissolved
  and independent of the type of solute that is
  dissolved.

• Colligative properties
• freezing point depression
• melting point elevation
• vapor pressure lowering
• osmosis

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Colligative Properties
Lowering Vapor Pressure

• Non-volatile solutes reduce the ability of the
  surface solvent molecules to escape the liquid.
• Therefore, vapor pressure above solution is
  lowered.
• The amount of vapor pressure lowering depends on
  the amount of solute dissolved.

• Raoults Law
• PA is the vapor pressure with solute, PA? is
  the vapor pressure without solute, and ?A is the
  mole fraction of A, then

Recall Daltons Law
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Colligative Properties
Lowering Vapor Pressure

• ideal solution one that obeys Raoults law
• Raoults law breaks down when the solvent-solvent
  and solute-solute intermolecular forces are
  greater than solute-solvent intermolecular forces.

Boiling-Point Elevation

• - How does vapor pressure lowering effect the
  phase diagram?
• Non-volatile solute lowers the vapor pressure.
• - Therefore the triple point - critical point
  curve is lowered.

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Colligative Properties
Boiling-Point Elevation
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Colligative Properties
Boiling-Point Elevation

• At 1 atm (normal boiling point of pure liquid)
  there is a lower vapor pressure of the solution.
  Therefore, a higher temperature is required to
  teach a vapor pressure of 1 atm for the solution.

The increase in the boiling point (?Tb) compared
to the boiling point of the pure solvent is
directly proportional to the molal concentration
of the solute (m).
Mathematically
where Kb is the molal boiling-point-elevation
constant.
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Colligative Properties
Freezing-Point Depression

• The melting-point (freezing-point) curve is a
  vertical line from the triple point.
• The solution freezes at a lower temperature (?Tf)
  than the pure solvent.
• Decrease in freezing point (?Tf) is directly
  proportional to molal cncentration of the
  solution, m.

Mathematically
where Kf is the molal freezing-point-depression
constant)
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Colligative Properties
Freezing-Point Depression
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Colligative Properties
Examples
A solution was prepared by dissolving 34.6g of
NaCl in 827g of water. The molar freezing point
and boiling point constants of water are Kf
1.86 oC/m and Kb 0.52 oC/m, respectively.
F)
Calculate the freezing point of the solution.
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Colligative Properties
Examples
A solution was prepared by dissolving 34.6g of
NaCl in 827g of water. The molar freezing point
and boiling point constants of water are Kf
1.86 oC/m and Kb 0.52 oC/m, respectively.
F)
Calculate the boiling point of the solution.
G)
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Colligative Properties
Examples
H)
I)
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Colligative Properties
Examples
J)
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Colligative Properties
Examples
K)
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End of Chapter 13Properties of Solutions