Boiling Points - Distillations
Ethylene glycol,
HOCH2CH2OH, boils at
198oC and melts at -13o C.
What happens to the melting
point of water if you add
antifreeze?
Melting point goes down.
Add Antifreeze
What happens to the
boiling point of water if
you add antifreeze?
The boiling point goes up.
Mixtures
Solids
Usually melt low
Liquids
Usually boil
between the two
components
Vapor pressure of water vs.
temperature
Solution boils when the vapor pressure =
applied pressure
Add Salt
If you add salt, NaCl,
to water what happens
to the melting point?
Impurities depress
the melting point so
it goes down.
Salt Boiling Point
If you add salt, NaCl,
to water what happens
to the boiling point?
The boiling point
goes up.
Raoult’s Law
P=
Observed
Pressure
o
P *N
Mole Fraction
Pure Substance
Two Volatile Liquids
O
CH3 C
Acetone
b.p = 56 o
CH3
Benzene
b.p. = 80 o
Two Volatile Liquids
Pa = Poa * Na
Pb =
o
P
b
* Nb
Mixture boils when
Pa + Pb = Papplied
Boiling point vs. mole fraction
Plot of boiling point vs. mole fraction
for a mixture of acetone and benzene.
Boiling point vs. mole fraction
Boiling point vs. mole fraction
Boiling point of an acetone-benzene mixture
with a mole fraction benzene of 0.50
What is the composition of the vapor?
Vapor richer in the lower boiling
component
Fractional Distillation
Fractional Distillation
Packed Column
HETP = 1.5 cm
Vigreux Column
HETP = 10 cm
Which column has the
greater holdup?
Packed because it has
the larger surface
area.
Separations
Which column would you
use to separate a mixture
of pentane, b.p. 36o and
octane, b.p. 99o?
Vigreux
Separations
Which column would you
use to separate a mixture of
methyl alcohol, b.p. 65o and
water, b.p. 100o ?
Packed
Experiment
Separate mixture of methanol and water.
Plot volume distilled vs. temperature.
CH3OH
methyl alcohol
methanol
b.p. = 64.7 o
H2O
water
b.p. = 100 o
Setup
CH3OH
Methyl alcohol is toxic!
Standard Taper Joints
14/20
14 mm
20 mm
Grease joints to prevent
sticking.
Thermowell
Plug the thermowell into the
variable transformer
Put boiling chips in bottom of flask
Boiling Chips
Distilling Column
Thermometer
Adapter
Digital Thermometer
Probe for digital
thermometer
Digital Thermometer
Water out
Water in
Clamps
IDEAL MIXTURES:
Same interactions between like
and unlike molecules
NONIDEAL MIXTURES:
A.
Strong attractions between like
molecules
B.
Strong attractions between unlike
molecules
Ethanol - Benzene
Minimum - boiling azeotrope
Min Azeotrope
Acetone - Chloroform
Maximum-boiling azeotrope
Max Azeotrope
Simple Distillation