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


Water

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

mmHg.


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

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