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
If you add salt, NaCl, to water what happens to the boiling point?
The boiling point goes up.
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
Two Volatile Liquids
Plot of boiling point vs. mole fraction for a mixture of acetone and benzene.
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?
Boiling point vs. mole fractionBoiling point vs. mole fraction
Vapor richer in the lower boiling component
Fractional Distillation
Fractional Distillation
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.
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
CH3OHmethyl alcoholmethanolb.p. = 64.7o
H2Owater
b.p. = 100o
Separate mixture of methanol and water. Plot volume distilled vs. temperature.
Experiment
Setup
CH3OH
Methyl alcohol is toxic!
14/20
20 mm
14 mm
Standard Taper Joints
Grease joints to prevent sticking.
Thermowell
Plug the thermowell into thevariable transformer
Boiling Chips
Put boiling chips in bottom of flask
Distilling Column
Thermometer Adapter
Digital Thermometer
Probe for digital thermometer
Digital Thermometer
Water in
Water out
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
Min Azeotrope
Maximum-boiling azeotrope
Acetone - Chloroform
Max Azeotrope
Simple Distillation