Fractional Distillation
Boiling point of mixtures
Separation of mixtures by distillation.
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.
Boiling Points - Distillations
What happens to the boiling point of water if you add antifreeze?
The boiling point goes up.
Add antifreeze
Solids Usually melt low
LiquidsUsually boil between the two components
Mixtures
Solution boils when the vapor pressure = applied pressure
Vapor pressure of water vs. temperature
If you add salt, NaCl, to water what happens to the melting point?
Impurities depress the melting point so it goes down.
Add Salt – Melting Point
If you add salt, NaCl, to water what happens to the boiling point?
The boiling point goes up.
Add Salt – Boiling Point
P = Po*NObserved Pressure
Pure Substance
Mole Fraction
Raoult’s Law
C
O
CH3CH3
Acetoneb.p = 56o Benzene
b.p. = 80o
Two Volatile Liquids
Pa = Poa * Naa
Pb = Pob * Nbb
Mixture boils when
Pa + Pb = Papplied
Boiling Point Two Volatile Liquids
Mixture of acetone and benzene.
Plot of boiling point vs. mole fraction
Mole fraction benzene = 0.50
What is the composition of the vapor?
Boiling point of an acetone-benzene mixture
Vapor richer in the lower boiling component
Vapor Composition
Fractional Distillation
Temperature vs. Volume
HETP = 1.5 cm
Packed Column
HETP = 10 cm
Vigreux Column
Which column has the greater holdup?
Packed because it has the larger surface area.
Separate Pentane and Octane.
Which column would you use to separate a mixture of pentane, b.p. 36o and octane, b.p. 99o?
Vigreux
Separate Methanol and Water
Which column would you use to separate a mixture of methyl alcohol, b.p. 65o and water, b.p. 100o ?
Packed
CH3OHmethyl alcoholmethanolb.p. = 64.7o
H2Owater
b.p. = 100o
Separate mixture of methanol and water. Plot volume distilled vs. temperature.
Experiment
Equipment
CH3OH
Methyl alcohol is toxic!
14/20
20 mm
14 mm
Standard Taper Joints
Grease joints to prevent sticking.
Thermowell
Plug the thermowell into the variable transformer
Use Variable Transformer
Boiling Chips
Put boiling chips in bottom of flask
Equipment
Thermometer Adapter
Water out
Water in
Joint – Hose Clamps
Same interactions between like and unlike molecules
NONIDEAL MIXTURES:
Strong attractions between like molecules
Strong attractions between unlike molecules
A.
B.
IDEAL MIXTURES:
Minimum - boiling azeotrope
Ethanol - Benzene
Azeotrope
Maximum-boiling azeotrope
Acetone - Chloroform
Maximum Azeotrope
Simple Distillation
Sample Equipment
Distillation Equipment
Solvent Purification
