Colligative Properties of Solutions

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Colligative Properties of Solutions
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Forming Solutions

• Step 1
• Breaking up the solute into individual components
  (expanding the solute)
• Step 2
• Overcoming the intermolecular forces in the
  solvent to make room for the solute (expanding
  the solvent)
• Step 3
• Allowing the solute and solvent to interact to
  form the solution

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Add in Enthalpy

• Step 1 and 2 are endothermic
• Step 3 often is exothermic
• Delta Hsoln deltaH1 deltaH2 deltaH3
• delta H means?
• - delta H means?

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Enthalpy of hydration (heat of hydration)

• DeltaHhyd combines deltaH2 and delta H3
• Example For NaCl
• deltaHsoln 786 kJ/mol 783 kJ/mol
• 3 kJ/mol
• we must supply a small amount of energy
• Why does it happen so readily?
• Processes run to disorder (ping pong demo)

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Solubility

• Structural Effects
• Polar vs Nonpolar
• Pressure Effects
• Henrys Law
• P kC
• P pressure of gaseous solute
• C solution
• K constant
• The amount of a gas dissolved in a solution is
  directly proportional to the pressure of the gas
  above the solution

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Lake Nyos Tragedy
Lake had thermal layers With excess CO2 on
bottom.
August 21, 1986 A cloud of gas Boiled from Lake
Nyos, killing 2000 people and many animals. It
was CO2 !
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Temperature and Solubility

• In general, the warmer the solvent the greater
  amount of solvent can be dissolved.
• BUT, not always. . .

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Vapor Pressure

• A nonvolatile solute lowers the vapor pressure of
  a solvent
• RAOULTS LAW
• Psoln XsolventPosolvent
• P vapor pressure of solution
• X mole fraction of solvent
• Po vapor pressure of pure solvent

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So, what is a colligative property anyway?

• Because changes of state depend on vapor
  pressure, the presence of a solute affects the
• BOILING POINT
• FREEZING POINT
• OSMOTIC PRESSURE

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Colligative Properties Continued

• Colligative properties are grouped together
  because they depend only on the number and not on
  the identity of particles in an ideal solution

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Why Should We Care?

• Used to characterize the nature of a solute after
  it is dissolved in a solvent
• Used to determine the molar masses of substances

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• Properties of Solutions
• Colligative Properties
• Boiling point elevation occurs as a result of
  lowering the vapor pressure of a liquid
  containing a nonvolatile solute.
• In order to reach any vapor pressure the
  temperature of the solution must be higher than
  the temperature of the pure liquid.
• In this Figure, the red line corresponds to the
  vapor pressure of pure benzene vs. temperature
  whereas the blue line is the vapor pressure of a
  2.0 m solution of a nonvolatile solute in
  benzene.
• The increase in boiling temperature is
  proportional to the concentration of the solute
• DtbpKbpm where Dtbp is the increase in the
  boiling temperature of the
  solvent
• Kbp is the boiling point elevation
  constant for the solvent.
• m is the molality of solute in the
  solution.

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• Properties of Solutions
• Freezing point depression also results from a
  lowering of the vapor pressure of a liquid
  containing a nonvolatile solute.
• The decrease in freezing temperature is
  proportional to the concentration of solute
• DtfpKfpm where Dtfp is the decrease in the
  boiling temperature of solvent
• Kfp is the freezing point depression constant
  for the solvent.
• m is the molality of the solute in the
  solution.
• Calculating molar masses from colligative
  properties

Change in vapor pressure, boiling point
elevation, or freezing point depression or
osmotic pressure
Solution concentration
Use mass of solvent
Molar mass
Moles of solute
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• Properties of Solutions
• Calculating molar masses from colligative
  properties
• Example 0.640 g azulene (empirical formula C5H4)
  is dissolved in 99.0 g benzene. The boiling
  point of the solution is 80.23 oC. What is the
  molecular formula of azulene?

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• Properties of Solutions
• Colligative Properties
• Freezing point depression
• For solutions of ionic compounds, the molality
  will be an integer multiple of the compounds
  molality.
• For NaCl, there are 2 moles of particles per mole
  NaCl, so a 1 molal solution of NaCl is 2 molal
  in particles
• For CaCl2, there are 3 moles of particles per
  mole CaCl2, so a 1 molal solution of CaCl2 is 3
  molal in particles
• The expected effect on freezing temperature and
  boiling temperature for a 1 molal solution of
  NaCl or CaCl2will be 2 or 3 times that of a 1
  molal solution of a nondissociating solid.
• A 1.00 m solution of NaCl should freeze at
  2x(-1.86) oC.
• Experimentally it is found that the freezing
  point of solutions of ionic solids is not quite
  as low as predicted.
• This results because ion pairs are formed in
  solution because of ion-ion interactions,
  reducing the expected molality of the solute.

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• Properties of Solutions
• Colligative Properties
• Freezing point depression
• The vant Hoff factor, i, is a measure of the
  dissociation of an ionic compound

Vant Hoff factors for several substances at 25
oC
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• Properties of Solutions
• Colligative Properties
• Osmosis is the transport of solvent molecules
  through a semipermiable membrane from a
  solution of high solvent concentration to a
  solution of lower solvent concentration.
• Solvent molecules move from the solution of lower
  solute concentration to the solution of higher
  solute concentration in an attempt to dilute the
  solution with higher solute concentration.
• Osmosis will cease when the pressure resulting
  from transport of solvent to the solution of
  higher solute concentration prevents further
  transport of solvent through the membrane.
• The osmotic pressure, ?, is related to the
  concentration of the solution, and the absolute
  temperature
• This is analogous to the ideal gas law, where P?
  and n/Vc

where c is the molarity of the solution

• Isotonic solutions have the same osmotic
  pressure.
• A hypotonic solution has lower osmotic pressure
  than another solution.
• A hypertonic solution has higher osmotic pressure
  than another solution.

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• Properties of Solutions
• Colligative Properties
• Osmosis
• Example Lysozyme is an enyme tha breaks
  bacterial cell walls. A solution containing
  0.150 g in 210 mL of solution has an osmotic
  pressure of 0.953 torr at 25 oC. What is the
  molecular weight of of lysozyme?

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Properties of Solutions Colloids are suspensions
of particles that have a size between
approximately 10 Å and 2,000Å dispersed in
another medium.

• Colloidal particles can be made up of an
  agglomeration of many small particles or
  molecules or could be a single, large molecule.

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BloodA Colloid
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• Properties of Solutions
• Colloids
• Hydrophilic colloids are stabilized as a result
  of having polar or ionic groups on their
  surfaces which are water loving.
• The strong attractive interactions with water
  keeps the large particles in suspension.
• Hydrophobic colloids are stabilized by adsorption
  of ions on the particle surfaces.
• Adsorption is the binding of a substance to the
  surface of bulk matter.
• The adsorbed ions interact with water to keep the
  particles suspended.
• The charges on the surfaces of different
  particles are of the same sign which keeps them
  apart and prevents agglomerating and
  precipitating.
• Soaps and detergents consist of a long
  hydrophobic tail and a hydrophilic end.
• The hydrophobic tail inserts into oil or other
  hydrophobic materials and causes the particles
  to be suspended in water as the hydrophilic end
  will be on the outside of the particle

-

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• Properties of Solutions
• Colloids
• Soaps and detergents behave as emulsifying agents
  causing oils to be suspended in aqueous
  dispersing medium.
• Removal of colloidal particles from suspension
• Colloidal particles in dispersed in liquids
  stabilized by adsorption of ions can be
  coagulated by adding an inert electrolyte - HNO3
  - and heating the suspension.
• Solid particles suspended in air can be
  precipitated by producing a surface charge on
  the particles and attracting them to an
  oppositely charged electrode.

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Cottrell precipitatorCharged plates attract
collodial particles of smoke
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