Colligative Properties

1
Colligative Properties

• With your host
• Dr. Mike Daniel

2
Acetone and H2O Comparison

• CH3COCH3
• Eatt
• Dipole-Dipole
• London Forces

• H2O
• Eatt
• H Bonding
• Dipole-Dipole
• London Forces

3
Eatt between Acetone and H2O

• CH3COCH3.H2O
• Eatt
• H bonding
• Dipole-Dipole
• London Forces

4
?Hsoln of Acetone and H2O
Acetone (g)
Acetone(L)
5
?Hsoln of Acetone and H2O
H2O (g)
H2O (L)
6
?Hsoln of Acetone and H2O
H2O (g) Acetone (g)
Acetone (aq)
?Hsoln
7
Molecules that can H bond to H2O but not to
themselves

• Formaldehyde
• Ethyl Acetate

8
Colligative Properties

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

9
Vapor Pressure

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

10
Raoults Law

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

11
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

12
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)

13
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?

14
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

15
Raoults Law
16
Purification using Raoults Law

• Say I have EtOH(aq) with XEtOH .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 76 / (76 294) .2

17
Purification using Raoults Law

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

18
Phase Diagram of H2O Solute
19
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 vaporize, raising the b.p.

20
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

21
M.P. Depression B. P. Elevation Constants
22
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

23
Applications of Freezing Point Depression

• Ethylene glycol (CH2OHCH2OH) miscible with water,
  
• Used to de-ice airplanes by lowering f.p.
• Car anti-freeze, provides high and low T
  protection
• Glycerol (CH2OHCHOHCH2OH) used by fish and
  insects to lower f.p. of blood

24
Applications of Freezing Point Depression

• NaCl lowers f.p. of water, used to make
  homemade ice cream by solidifing milk/sugar
  solution
• NaCl CaCl2 Street deicing
• Dissolves, lowers ice f.p., ice melts, more
  dissolves, etc
• CaCl2 ?Hsoln lt0, heat of solution helps melt ice

25
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

26
Osmosis
Net flow of water to left
27
Osmotic Pressure

• For dilute solutions
• ? V nRT
• ? nRT/V MRT
• Isotonic Soln. Soln has same solute
  concentration as cell fluid
• Hypotonic Soln has lower solute conc., water
  enters cell causing cell to burst
• Hypertonic Soln has higher solute conc., water
  leaves cell, causing cell to shrink

28
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