Colligative Properties

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• Colligative Properties

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Colligative Properties

• The wood frog is a remarkable creature because it
  can survive being frozen.
• Scientists believe that a substance in the cells
  of the frog acts as a natural antifreeze.
• prevents the cells from freezing
• Although fluids surroun- ding the
  frogs cells
  may freeze, the cells
  themselves do not.

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Colligative Properties

• How can a dissolving particles in a soln change
  the freezing point of the soln?
• The properties of a soln are different then the
  properties of the pure solvent.
• Some of the differences are due to the mere
  presence of solute part-icles in the soln
• The nature of the solute matters less than its
  conc

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Colligative Properties

• The properties, which depend only on the of
  particles dissolved in a given mass of solvent,
  are called colligative properties
• vapor pressure depression
• boiling-point elevation
• freezing-point depression
• osmotic pressure.
• The crux of it is the particles of the solute
  just kind of get in the way

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Vapor Pressure Reduction

• Recall the vapor pressure is exerted by a vapor
  overtop of a liquid exerted in a closed system
• It establishes an equilibrium.
• When solute (sugar or salt) that doesnt vaporize
  easily is dissolv-ed, it always lowers the vapor
  pressure of the pure solvent
• Fewer particles escape into the vapor phase
  therefore, lowering the pressure exerted by the
  vapor

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Vapor Pressure Reduction

• How do dissolved particles lower the vapor
  pressure?
• Consider an aqueous sodium chloride soln, Na1
  and Cl-1 ions are dispersed throughout the water.
• Both within the liquid and
  at the surface, the
  ions are sur-
  rounded by layers of
  water molecules.

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Vapor Pressure Reduction

• The VP is dependent on the ability of the solvent
  particles that have enough energy to leap out of
  the surface of the liquid.
• If there are solute particles in the way, the H2O
  cant escape from the surface as easily, which
  reduces the Vapor Pressure
• Recall that Vapor Pressure is an integral part of
  the point or temp at which a liquid will boil.

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Boiling-Point Elevation

• the boiling point of a substance is the temp at
  which the vapor pressure of the liquid equals the
  atmospheric pressure.
• If the vapor pressure is lowered, it takes more
  energy to reach equil
• Therefore, the boiling point of the solution is
  higher than the BP of the pure solvent.

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Boiling-Point Elevation

• The BP elevation is the change in temp between
  the BP of the soln and that of the pure solvent.
• You can also think about the BP elevation
  in terms of particles.
• Intermolecular forces of attraction exist between
  the solvent and solute particles.

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Boiling-Point Elevation

• It takes additional Kinetic Energy for the
  solvent particles to over-come the IM forces.
• Thus, the presence of the solute elevates the
  boiling temperature of the solvent.
• The magnitude of the BP elevn is proportional
  to the of solute particles
• BP elevation depends on conc not the type of
  solute

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Freezing-Point Depression

• When a substance freezes, the particles of the
  solid take on a highly structured, ordered
  pattern.
• the solute dissolved in H2O gets in the way of
  this arrangement
• Therefore, more K.E. must be
  removed from a
  solution than from the
  pure solvent
  for it to solidify

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Freezing-Point Depression

• Freezing Point depression is the difference in
  the temperature between the F.P. of a soln and
  that of the pure solvent.
• The magnitude of the depression is dependent on
  the conc of the solute

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Colligative Properties
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Effects on B.P. and F.P.

• The amount the B.P. of the soln is elevated or
  F.P. is depressed is directly proportional to the
  molal conc of solute dissolved (mol/kg)
• The actual mathematical calcs are DTB.P. mKb or
  DTF.P. mKf
• Where m is the molal conc and Kb and Kf are the
  B.P. elevation constant and F.P. depression
  constants respectively
• The constants only change when the type of
  dissolved substance changes
• Obtained from data tables or given

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Table of Kb and Kf Values
Solvent Norm F.P. (C) Kf (C/m) Norm B.P. (C) Kb (C/m)
Acetic Acid 16.6 -3.90 117.9C 3.07
Camphor 178.8 -39.7 207.4 5.61
Ether -116.3 -1.79 34.6 2.02
Napth-alene 80.2 -6.94 217.7 5.80
Phenol 40.9 -7.40 181.8 3.60
Water 0.00 -1.86 100.0 .51
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Calcs of B.P. and F.P.

• What is the freezing-point depression of water in
  a soln of 17.1g of sucrose, C12H22O11, and 200g
  of water? What is the actual freezing point of
  the solution?

1. We need to convert 17.1g of sucrose to mols of
   sucrose
2. We need to find the molal conc of sucrose by
   dividing the mols by .200 kg of water
3. We need to look up the Kf for water and use
   DTF.P. m Kf

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Calcs of B.P. and F.P.

• 17.1 g sucrose

.0500 mol
.25m
Kf -1.86C/m
DTF.P. m Kf
DTF.P. (.25m)(-1.86C/m)
DTF.P. -.465C
DT Tf Ti so, -.465C X 0.00C
So the freezing pt of the soln -.465C
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Calcs of B.P. and F.P.

• A soln contains 50.0g of sucrose, C12H22O11,
  dissolved in 500.0 g of H2O. What is the boiling
  pt elevation?

1. We need to convert 50g of sucrose to mols of
   sucrose
2. We need to find the molal conc of sucrose by
   dividing the mols by .500 kg of water
3. We need to look up the Kb for water and use
   DTB.P. m Kb

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Calcs of B.P. and F.P.

• 50 g sucrose

.1462 mol
.292m
Kb .51C/m
DTB.P. m Kb
DTB.P. (.292m)(.51C/m)
DTB.P. .149C
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Practice Problems

1. Which soln will have a higher boiling point
   A soln containing 105 grams of sucrose
   (C12H22O11) in 500 grams of water or a soln
   containing 35 grams of NaCl in 500 grams of
   water? Which soln will have a lower freezing
   point? Justify your answers.
2. A soln that contains 12.6 g of a nonvolatile
   nondissociating solute in 400. g of benzene,
   C6H6, freezes _at_ 3.6C . The normal freezing point
   of benzene is 5.5C. What is the molar mass of
   the solute? ( KF for benzene 4.96C?kg/mole)

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Osmotic Pressure

• Certain materials are classified as
  semipermeable.
• Selective about the size of the particles it
  allows to pass
• A semipermeable membrane
  permits the passage
  of small solvent molecules,
  but not larger solvat-
  ed particles
  or ions

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Osmotic Pressure

• If a semipermeable membrane is placed between 2
  solns with differ-ent concs
• solvent particles may pass through the membrane
  either direction
• solute particles must remain on 1 side of the
  membrane or the other.
• The net flow of solvent molecules from the lower
  conc soln to the higher conc soln is defined as
  osmosis

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Osmotic Pressure
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Osmotic Pressure

• In this example of osmosis, the solv particles
  move from the left to the right to try to
  establish an equilibrium in which the concs are
  as close to even as possible.
• The only particles that can move across the
  membrane are solvent particles
• As the solvent travels across the membrane the
  liquid levels become uneven.

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Osmotic Pressure

• The flow of solvent will continue until the
  pressure difference resulting from the uneven
  heights becomes so large that the net flow of
  solvent ceases.
• The solvent cant make the conc even, it must
  attempt to dilute the higher conc soln
• We can apply a pressure on the liquid to prevent
  the osmosis from occurring

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Osmotic Pressure

• The pressure required to prevent osmosis is known
  as the osmotic press. of the soln.
• Osmotic pressure is another colli-gative property
  of solns.
• The greater the difference in conc the greater
  the applied pressure must be to prevent osmosis

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Reverse Osmosis
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Osmotic Pressure

• Osmotic press is enormously important for life.
• E.g. cell wall membranes, in many cases, are
  semipermeable memb-ranes which will pass
  molecules of water but not solute
• So the concs of material in the fluids inside and
  outside the cell must be carefully balanced so
  that they generate the same osmotic pressure.

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Osmotic Pressure

• Otherwise, the difference in solute concs would
  generate a pressure differential across the cell
  wall and either collapse the cell or burst it.
• Fluids used in intravenous feed-ing, drug
  delivery, or just plain fluid replacement are
  called iso-tonic solutions
• they are designed to have the same osmotic
  pressure as normal blood cells.

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Osmotic Pressure

• If a blood cell is placed in a soln with a lower
  conc than this (hypo-tonic soln)
• fluid moves from the soln into the cell and the
  cell bursts (hemolysis)
• If a blood cell is placed in a soln with a higher
  conc (hypertonic soln)
• fluid moves from the cell to the surrounding soln
  and the cell shrinks and dies (crenation).

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Osmotic Pressure
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Reverse Osmosis
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Electrolytes Colligative Props

• I said earlier that colligative prop-erties are
  conc of solute depend-ent not type of solute
  dependent
• But thats not entirely true, some substances
  depress freezing pt or elevate boiling pt of a
  solvent more than expected.
• For example .1 m CaCl2 soln lowers the freezing
  pt nearly 3 times as much as a .1 m soln of
  sucrose

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Electrolytes Colligative Props

• The difference really does become conc dependent,
  think about it
• For every mol of CaCl2 dissolved in soln you get
  3 mols of particles
• For every mol of sucrose dissolved in soln you
  get 1 mol of particles
• With a higher conc of particles with the CaCl2,
  you get a more signifi-cant change in the
  colligative properties.

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Electrolytes Colligative Props

• Electrolytes tend to have a more dramatic effect
  on colligative properties than nonelectrolytes.
• Electrolytes tend to be ionic in nature, so ionic
  cmpnds have a more dramatic effect on the changes
  in a solvents properties.
• Nonelectrolytes are generally covalent in nature,
  and have less of a dramatic effect on the changes
  in a solvents properties.