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Colligative Properties of Solutions

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Title: Colligative Properties of Solutions


1
Colligative Properties of Solutions
2
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

3
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?

4
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)

5
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

6
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 !
7
Temperature and Solubility
  • In general, the warmer the solvent the greater
    amount of solvent can be dissolved.
  • BUT, not always. . .

8
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

9
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

10
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

11
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

12
  • 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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14
  • 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
15
  • 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?

16
  • 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.

17
  • 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
18
  • 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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21
  • 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?

22
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.

23
BloodA Colloid
24
  • 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

-

25
  • 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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27
Cottrell precipitatorCharged plates attract
collodial particles of smoke
28
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