19 152-3-21-2011 colligative properties
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Transcript of 19 152-3-21-2011 colligative properties
∆H1
Solute
∆H2
solvent
∆H3
Solute-solvent
∆Hsolution
overall
outcome
Polar solute (EtOH)
Polar solvent (water)
Large + Large + Large - Small (- or +) Soln forms
(if ∆Hsolution
not too positive)
∆G =∆H + T∆S ∆G = + for spontaneous processes
∆H1Solute pull apart solute particles
∆H2 Solvent – pull apart solvent molecules
∆H3 attraction of solute and solvent
not too positive)
Nonpolar solute (oil)
Polar solvent (water)
Small + Large + Small - Large + No solution
Non-polar solute (oil)
Non-polar solvent (gasoline)
Small + Small + Small - Small (+ or -) Solution forms
(~ideal)
Polar solute (EtOH)
Non-polar solvent (oil)
Large + Small + Small - Large + No solution
forms
Ionic solute (NaCl)
Polar solvent (water)
Large + Large + Large - Small (+ or -)
(can be
relatively large)
Solution forms if
∆Hsolution not too
positive
if ∆Hsolution + solution process is “entropy driven”
Like dissolves like
� # 21 Which likely to be soluble in benzene
� CCl4 or NaCl
� Hexane or glycerol
� Acetic acid or hexanoic acid
� HCl or propyl chloride� HCl or propyl chloride
Temp effect – gases
� Gas + liquid � solution + energy
� Temp increase increases energy available
� Favors “dis” solution
� Thermal pollution – effect on fish� Thermal pollution – effect on fish
# 25 and 26
� PbCl2 NaOH
� ∆H for dissociation to aqueous ions
� Using data from Ch 8 or appendix
� Will solubility increase with increasing temp?
Pressure effect on solubility
� Henry’s law
� Increased pressure increase gas solubility
� Cg = k Pg
� Pg = partial pressure of the gas above liquid
� k – constant – characteristic of particular gas/liquid system� k – constant – characteristic of particular gas/liquid system
� Carbonated beverages
� The bends –� Difference in solubility of gases (nitrogen or helium as carrier gas) with
increasing P
� Decompressing after diving
� Hyperbaric chambers for CO poisoning, wound healing etc (direct solubility)
� Practice # 29
Typical Henry’s law problem
� He in water 3.8 X 10-4 M/atm @ 25 oC (k)
� ? M/mm Hg
� pHe =293 mm Hg @ 25 oC, what is [ ] in M
� Cg = k Pg Cg = k Pg
� HW: 27, 29 Ch 10
Colligative properties
� Due to number of particles in solution
� Be familiar with
� Electrolytes (soluble salts, SA, SB)
� Completely dissociate into ions
� Non-electrolytes
Molecular compounds (no ions)� Molecular compounds (no ions)
� Polar molecular compounds are water soluble
� Weak electrolytes
� WA, WB – partially dissociate
� Note worksheet
Colligative properties
� Due to number of particles in solution
� 1 mole sugar per liter = ? mol/L particles?
� 1 mole NaCl per liter
� 1 mole aluminum sulfate per liter� 1 mole aluminum sulfate per liter
� 1 mole acetic acid
� Electrolytes/nonelectrolytes
Vapor pressure
� Pressure of the vapor above a liquid or solution –depends on temp and identity
� Raoult’s law - mix with volatile solute
� VPsoln = XsoluteVP solute + XsolventVPsolvent
evaporation
Ideal solution of 2 volatile solvents (liquids)1. What is the VP of pure hexane ___2. What is the VP of pure pentane ___3. Which is more volatile? 4. If X = 0.5, what is VP due to pentane___ hexane ___total VP _____
Related exercise:1 mol benzene (C6H6), P
o = 75 torr 2 mol toluene (C7H8) P
o = 22 torrWhat is the mf of benzene in the mixture?What is the vp of the mixture?
Raoult’s Law VPT = XAVPA + XBVPB
What is the vp of the mixture?What is the mf of benzene in the vapor phase?
Data is for a given tempsince pentane is a liquidat RT, this must be for ahigher T
VP lowering
� Non-volatile solutes decrease vp
� Independent of nature of solute
� Dependent on number of particles in solution
� Raoult’s law -- except solute does not exert vp (non-volatile) volatile)
� VPsolution = XsolventVPsolvent
� VPlowering = XsoluteVpsolvent
� VPsolution + Vplowering = VPsolvent
� VPsolution = XsolventVPsolvent
� VPlowering = XsoluteVPsolvent
� VPsolution + VPlowering = VPsolvent
Normally used with non-electrolytes� Normally used with non-electrolytes
� # 35
Note: mole fraction concentration unit used
Boiling point elevation
� If solutes depress vp, to boil (vp = 760) temp must be elevated� Depends on concentration of particles in solution and on
solvent
� Use molality mol solute particles/kg solvent
� BP elevation = k i m� BP elevation = k i m� k= bp constant for solvent
� i= activity of solute (# particles/mol)
� m = molality of solute (mol solute/kg solvent)
� 31,32 (bp’s)
� Lab Wed benzoic acid in lauric acid
Freezing point depression
� Solutes interfere with crystal formation
� Salt added to ice – interferes with crystal structure, ice structures breaks down – endothermicprocess!!!!
� Ocean water – ice bergs are fresh water --salt is not � Ocean water – ice bergs are fresh water --salt is not included
� 1.86oC/mole of particles in 1 kg water
� Melting ice
� 31, 32
� General Chemistry Online: FAQ: Solutions: Why does salt melt ice?
Review spec lab end of class
� Assignments due
� Make sugar solns
� Spec lab:
� Calc k
� Calc energies
� Calc unk conc
� Emitted and abs colors
� Resubmit
� HW Ch 10 28-30 (Henry’s) 39 40 42-44 (fp, bp)