Boiling Points - Distillations
Ethylene glycol,HOCH2CH2OH, boils at198oC and melts at -13o C.
What happens to the meltingpoint of water if you addantifreeze?
Melting point goes down.
What happens to theboiling point of water ifyou add antifreeze?
The boiling point goes up.
Mixtures
Solids Usually melt low
LiquidsUsually boilbetween the twocomponents
Vapor pressure of water vs. temperature
Solution boils when the vapor pressure =applied pressure
If you add salt, NaCl,to water what happensto the melting point?
Impurities depressthe melting point soit goes down.
If you add salt, NaCl,to water what happensto the boiling point?
The boiling pointgoes up.
Raoult’s Law
ObservedPressure
Pure Substance
Mole Fraction
C
O
CH3CH3
Acetoneb.p = 56o Benzene
b.p. = 80o
Two Volatile Liquids
Two Volatile LiquidsPa = Po
a * Naa
Pb = Pob * Nbb
Mixture boils when
Pa + Pb = Papplied
Plot of boiling point vs. mole fractionfor a mixture of acetone and benzene.
Boiling point of an acetone-benzene mixturewith a mole fraction benzene of 0.50
What is the composition of the vapor?
Vapor richer in the lower boiling component
Fractional Distillation
Packed Column
HETP = 1.5 cm
Vigreux Column
HETP = 10 cm
Which column hasthe greater holdup?
Packed because it has thelarger surface area.
Which column would youuse to separate a mixtureof pentane, b.p. 36o andoctane, b.p. 99o?
Vigreux
Which column would youuse to separate a mixture ofmethyl alcohol, b.p. 65o andwater, b.p. 100o ?
Packed
CH3OHmethyl alcoholmethanolb.p. = 64.7o
H2Owaterb.p. = 100o
ExperimentSeparate mixture of methanol and water.Plot volume distilled vs. temperature.
CH3OH
Methyl alcohol is toxic!
Standard Taper Joints
14/20
20 mm
14 mm
Grease joints toprevent sticking.
Plug the thermowell into thevariable transformer
Put boiling chips in bottom of flask
Boiling Chips
ThermometerAdapter
Digital Thermometer
Probe fordigitalthermometer
Water in
Water out
IDEAL MIXTURES:
Same interactions between likeand unlike molecules
NONIDEAL MIXTURES:
Strong attractions between likemoleculesStrong attractions between unlikemolecules
A.
B.
Ethanol - Benzene
Minimum - boiling azeotrope
Acetone - Chloroform
Maximum-boiling azeotrope
Simple Distillation