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