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.
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
If you add salt, NaCl,
to water what happens
to the melting point?
Impurities depress
the melting point so
it goes down.
If you add salt, NaCl,
to water what happens
to the boiling point?
The boiling point
goes up.
Raoult’s Law
Observed
Pressure
Mole Fraction
Pure Substance
Two Volatile Liquids
O
CH3 C
Acetone
b.p = 56o
CH3
Benzene
b.p. = 80o
Two Volatile Liquids
Pa = Poa * Na
Pb =
o
P
b
* Nb
Mixture boils when
Pa + Pb = Papplied
Plot of boiling point vs. mole fraction
for a mixture of acetone and benzene.
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
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.
Which column would you
use to separate a mixture
of pentane, b.p. 36o and
octane, b.p. 99o?
Vigreux
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. = 64.7o
H2O
water
b.p. = 100o
CH3OH
Methyl alcohol is toxic!
Standard Taper Joints
14/20
14 mm
20 mm
Grease joints to
prevent sticking.
Plug the thermowell into the
variable transformer
Put boiling chips in bottom of flask
Boiling Chips
Thermometer
Adapter
Digital Thermometer
Probe for
digital
thermometer
Water out
Water in
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
Acetone - Chloroform
Maximum-boiling azeotrope
Simple Distillation