chem 31.1 experiment 4: distillation

DISTILLATIONDISTILLATION

A laboratory technique used in separation and/or purification of components in a

mixture

The process is mainly based upon the boiling point of liquid substances

Experiment IV: Distillation

heating vaporizing condensing

DISTILLATIONDISTILLATION

liquid

homogeneous heterogeneous

liquid

mixtures

RAOULT’S LAWRAOULT’S LAW

It relates the vapor pressure of components to the composition of the solution

for ideal mixtures

It assumes ideal behavior, that is, the physical properties of the components

are identical

If the two components are very similar, or in the limiting case, differ only in isotopic content,

then the vapor pressure of each component will be equal to the vapor pressure of the pure

substance Po times the mole fraction in the solution

The total vapor pressure Ptot above the solution is equal to the sum of the vapor pressures of

the two [liquid] components, PA and PB

Vapor Pressure

It is the pressure exerted by a vapor in equilibrium with its non-vapor phases

Boiling Point

The temperature at which the vapor pressure equals the atmospheric pressure

Boiling PointVapor Pressure 1/1/αα

AZEOTROPE MIXTUREAZEOTROPE MIXTURE

A mixture of liquids that has a constant boiling point because the vapour has the same

composition as the liquid mixture

The components of the solution cannot be separated by simple distillation

POSITIVE AZEOTROPEMinimum Temperature

Maximum Pressure

NEGATIVE AZEOTROPEMaximum Temperature

Minimum Pressure

POSITIVE AZEOTROPE MIXTUREPOSITIVE AZEOTROPE MIXTURE

NEGATIVE AZEOTROPE MIXTURENEGATIVE AZEOTROPE MIXTURE

KINDS OF DISTILLATIONKINDS OF DISTILLATION

Simple Distillation

Fractional Distillation

Steam Distillation

for homogeneous mixtures

for heterogeneous mixtures

Ethanol

95.5% 4.5%

78.1°C100°C78.3°C

positive azeotrope mixture

SIMPLE DISTILLATIONSIMPLE DISTILLATION

It is usually used only to separate liquids whose boiling points differ greatly (>70°C) or to separate liquids from involatile solids or oils. In

the process, all the hot vapors produced are immediately channelled into a condenser

which cools and condenses the vapors

Therefore, the distillate will not be as pure

A simple distillation set-up consists of a boiling flask (round-bottom flask) attached to an

adapter holding a thermometer (to determine the boiling temperature of the liquid). The

adapter connects to a condenser into which cold water is constantly passed through. The condenser leads into a collection flask for the

purified liquid.

FRACTIONAL DISTILLATIONFRACTIONAL DISTILLATION

It is usually employed with separation of complex mixtures at small boiling points

difference (about 25°C). It can separate the mixture into its component parts or fractions

It is essentially the same as simple distillation except that a fractionating column is placed between the boiling flask and the condenser. The glass beads

found in the fractionating column provide "theoretical plates" on which the refluxing liquid can condense, re-

evaporate, and condense again, essentially distilling the compound over and over. The more volatile

liquids will tend to push towards the top of the fractionating column, while less volatile liquid will stay

towards the bottom, giving a better separation between the liquids

SIMPLE vs FRACTIONAL DISTILLATION SIMPLE vs FRACTIONAL DISTILLATION

Simple distillation Fractional distillation

Advantages • simpler setup than fractional• faster distillation times• consumes less energy than fractional distillation

•much better separation between liquids than simple distillation•can more readily purify complex mixtures than simple distillation

Disadvantages • requires the liquids to have large boiling point differences (>70oC)• gives poorer separation than fractional distillation• only works well with relatively pure liquids

• more complicated setup than simple distillation• takes longer for liquids to distil• consumes more energy than simple distillation

Best used for: separating relatively pure liquids with large boiling differences or liquids with solid impurities

separating complex mixtures of liquids with smaller boiling point separations.

STEAM DISTILLATIONSTEAM DISTILLATION

It is the process of purifying a substance through application of steam. It deals with compounds that are

heat sensitive (e.g. natural aromatic compounds). Steam distillation works on the principle that

immiscible substance when mixed together can lower the boiling point of each other.

0.864g/mL Xylene

Water 0.988g/mL

60% 40%

94.5°C100°C139.1°C

positive azeotrope mixture

Many organic compounds tend to decompose at high sustained temperatures. Separation by normal

distillation would then not be an option, so water or steam is introduced into the distillation apparatus. By

adding water or steam, the boiling points of the compounds are depressed, allowing them to

evaporate at lower temperatures, preferably below the temperatures at which the deterioration of the

material becomes appreciable

Volume Xylene (mL)

Volume Water (mL)

Weight Ratio

First Fraction

0.5 1.3 0.33 : 1

Second Fraction*

-no data- -no data- -no data-

*the experiment was unsuccessful

σ of xylene = 0.87 g/mLσ of water = 1 g/mL

GUIDE QUESTIONSGUIDE QUESTIONS

Explain the differences of the distillation curves between simple distillation and fractional

distillation.

In the separation of the ethanol from water using fractional distillation, the distillate always

contains about 5% water. Explain.

t vs. % concentration of ethanol-water mixture

p vs. % concentration of ethanol-water mixture

t vs. % concentration of liquid mixture, which does not form azeotrope

Based on the phase diagram of 4(a), what is the approximate composition of the ethanol-

water mixture, which begins to distill at 80°C?

At 95°C the vapor pressure of n-heptane is 684 mmHg and of n-octane is 303 mmHg.

Calculate the mole ratio and weight ratio of the two components in a mixture of n-heptane and n-octane, which begins to distill at 95°C at 650

P sol = 650 mmHg684 x + 303 (1-x) = 650

684x + 303 – 303x = 650x= 0.9107

1-x = 0.089Mole ratio: 0.9107/0.089 = 10.23 (10.23 mols

of n-heptane for every 1 mol of n-octane)Weight ratio: 91.25/10.17 = 8.98 (8.98 g of n-

heptane for every 1 g of n-octane)

Explain the difference in the values of toluene/water weight ratios between the first

and second fractions.

Cite some important application of steam distillation and vacuum distillation

chem 31.1 experiment 4: distillation

distillation raoults

solution experiment

fractions experiment

pb experiment iv

identicalexperiment

liquidsexperiment iv

pureexperiment iv

boiling pointexperiment

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