Solutions 2 Colligative Properties

1
Solutions 2Colligative Properties

• With your
• Chemistry 102 host
• Dr. Mike Daniel

2
Colligative Properties

• Change in solvent properties depending mainly on
  number of solute particles
• Ideally Does not depend on solute identity
• Change in
• Vapor Pressure
• melting point
• Boiling point
• Osmotic pressure

3
Vapor Pressure

• Solvent(L) ? Solvent (g)
• Solute Particles block solvent from vaporizing,
  reduce rate L? G
• Reduces Pvap

4
Raoults Law

• PA XA Pao
• Pvap, A from solution
• Mole Fraction A X Pvap Pure A
• XA moles A / (total of moles)

5
Example of Raoults Law

• Say you have a solution with 180.g of glucose
  (C6H12O6) dissolved in 100. g of H2O _at_ 100.oC.
  What is the Pvap of water above this solution?
• PA XA Pao
• XA from moles of glucose H2O
• Pao from normal b.p. definition

6
Example of Raoults Law

• Moles glucose 180 g X 1mole/180 g
• Moles water 100 g X 1mole/18.0 g
• XH2O nH2O /(nH2O nglucose)
• XH2O 5.56 /(5.56 1.00)

7
Example of Raoults Law

• PA XA Pao
• PA .848 ( 760 torr)
• PA 644 torr
• How did the addition of glucose change the normal
  b.p. of water?

8
Phase Diagram of H2O Solute
9
More Raoults Law

• Glucose example is a nonvolatile solute (solute
  with very low Pvap)
• If solute volatile, then solute lowers solvent
  Pvap , and solvent lowers solute Pvap
• Ptot PA PB
• Ptot XAPAo XBPBo

10
Raoults Law
11
Purification using Raoults Law

• Say I have EtOH(aq) with XEtOH, soln .1 _at_ its
  EtOHs normal b.p. of 76oC.
• What is XEtOH of the vapor?
• Ptot PEtOH PH2O
• Ptot .1 (760 torr) .9 (327 torr)
• Ptot (76 294 ) torr
• XEtOH, gas 76 / (76 294) .2

12
Purification using Raoults Law

• What is XEtOH of the vapor?
• XEtOH, soln .1
• XEtOH, gas .2
• The vapor has more of the lower b.p. substance.
• Repeated vaporization condensation will have a
  greater concentration of EtOH

13
Distillation Column
Fractional distillation- Vapors Rise and
Condense and are Enriched it the Low BP
substance. High BP substances drip Down.
14
Phase Diagram of H2O Solute
15
Freezing Point Depression Boiling Point
Elevation

• Solute Particles make it harder for sovlent to
  form a solid, lower the m.p.
• Solute Particles make it harder for sovlent to
  form vapor, raising the b.p.

16
Freezing Point Depression Boiling Point
Elevation

• ?Tf Kf m
• ?Tf Tf,pure Tf,soln
• Kf molal freezing pt. depression constant,
  depends on solvent only
• m molality mol. solute/kg solvent
• ?Tb Kb m
• ?Tb Tb,soln Tb,pure
• Kb molal b. p. depression constant, solvent
  dependant only

17
M.P. Depression B. P. Elevation Constants
18
Freezing Point Depression Boiling Point
Elevation

• Calculations work reasonably well for dilute
  soln.
• Since m mass solute/molar mass
  kg solvent
• Experimental method to measure molar mass

19
Applications of Freezing Point Depression

• Ethylene glycol (CH2OHCH2OH) miscible with water,
  
• Used to de-ice airplanes by lowering waters f.p.
• Car anti-freeze, provides high and low T
  protection

20
Applications of Freezing Point Depression

• Glycerol (CH2OHCHOHCH2OH) used by fish and
  insects to lower f.p. of their blood

21
Applications of Freezing Point Depression

• NaCl lowers temperature of water/ice
  equilibrium mixture.
• Used to make homemade ice cream by solidifing
  milk/sugar solution

22
Applications of Freezing Point Depression

• NaCl CaCl2 Street deicing
• Dissolves, lowers ice f.p., ice melts, more
  dissolves, etc
• CaCl2 ?Hsoln lt0, heat of solution helps melt ice

23
Freezing Point Depression Applications

• Impurities in a pure solvent lower and broaden
  f.p.

24
Osmotic Pressure

• 2 solutions of different concentration separated
  by semipermeable membrane
• Membrane allows solvent but not solute particles
  to pass through
• Solute particles restrict solvent from passing

25
Osmosis
Net flow of water to left
26
Osmotic Pressure

• For dilute solutions
• ? V nRT
• ? (n/V)RT MRT

27
Osmotic Pressure

• For dilute solutions
• Isotonic Soln. Soln has same solute
  concentration as cell fluid
• ? 0

28
Osmotic Pressure

• Hypotonic Soln has lower solute conc.,
• If Soln. outside of cell hypotonic, water enters
  cell, may burst cell
• Hypertonic Soln has higher solute conc.,
• water leaves cell, causing cell to shrink

29
Hypotonic
Outside of Cell Net flow of water
30
Hypertonic
Inside of Cell Net flow of water
31
Osmotic Pressure

• For dilute solutions
• ? V nRT mass solute RT/molar mass
• Experimental method of measuring molar mass
• More sensitive than f.p. depression, especially
  for large molar masses

32
Colloidal Dispersions (Colloids)

• Mixture containing particles larger than normal
  solutions, but small enough to remain suspended
• Tyndall Effect Particles Wavelength of light
  will scatter light Mixture appears cloudy

33
Types of Colloids