Boiling Points - Distillations

Ethylene glycol,HOCH2CH2OH, boils at198oC and melts at -13o C.

What happens to the meltingpoint of water if you addantifreeze?

Melting point goes down.

What happens to theboiling point of water ifyou add antifreeze?

The boiling point goes up.

Mixtures

Solids Usually melt low

LiquidsUsually boilbetween the twocomponents

Vapor pressure of water vs. temperature

Solution boils when the vapor pressure =applied pressure

If you add salt, NaCl,to water what happensto the melting point?

Impurities depressthe melting point soit goes down.

If you add salt, NaCl,to water what happensto the boiling point?

The boiling pointgoes up.

Raoult’s Law

ObservedPressure

Pure Substance

Mole Fraction

C

O

CH3CH3

Acetoneb.p = 56o Benzene

b.p. = 80o

Two Volatile Liquids

Two Volatile LiquidsPa = Po

a * Naa

Pb = Pob * Nbb

Mixture boils when

Pa + Pb = Papplied

Plot of boiling point vs. mole fractionfor a mixture of acetone and benzene.

Boiling point of an acetone-benzene mixturewith 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 hasthe greater holdup?

Packed because it has thelarger surface area.

Which column would youuse to separate a mixtureof pentane, b.p. 36o andoctane, b.p. 99o?

Vigreux

Which column would youuse to separate a mixture ofmethyl alcohol, b.p. 65o andwater, b.p. 100o ?

Packed

CH3OHmethyl alcoholmethanolb.p. = 64.7o

H2Owaterb.p. = 100o

ExperimentSeparate mixture of methanol and water.Plot volume distilled vs. temperature.

CH3OH

Methyl alcohol is toxic!

Standard Taper Joints

14/20

20 mm

14 mm

Grease joints toprevent sticking.

Plug the thermowell into thevariable transformer

Put boiling chips in bottom of flask

Boiling Chips

ThermometerAdapter

Digital Thermometer

Probe fordigitalthermometer

Water in

Water out

IDEAL MIXTURES:

Same interactions between likeand unlike molecules

NONIDEAL MIXTURES:

Strong attractions between likemoleculesStrong attractions between unlikemolecules

A.

B.

Ethanol - Benzene

Minimum - boiling azeotrope

Acetone - Chloroform

Maximum-boiling azeotrope

Simple Distillation