PHASE SEPARATIONS

RAOULT’S LAW

The partial vapour pressure of a component in a mixture is equal to the vapour pressure of the pure component at that temperature multiplied by its mole fraction in the mixture.

Where PA =saturated vapour pressure of A =saturated vapour pressure of pure A =mole fraction of A in the solution

Raoult’s Law is obeyed by mixtures of similar compounds they are said to form IDEAL SOLUTIONS. The substances A and B form an ideal solution if the intermolecular forces A----A,A----B and B----B are all equal.

Examples of ideal mixtures are

1.

2.

3.

hexane and heptane

benzene and methylbenzene

propan-1-ol and propan-2-ol

VAPOUR PRESSURE / COMPOSITION DIAGRAMS FOR IDEAL MIXTURE LIQUIDS.

Ptotal=PA+PB

The partial vapour pressure of A at a particular temperature is proportional to its mole fraction. If you plot a graph of the partial vapour pressure of A against its mole fraction, you will get a straight line.

The mole fraction of B falls as A increases so the line will slope down rather than up. As the mole fraction of B falls, its vapour pressure will fall at the same rate.

BOILING POINT / COMPOSITION DIAGRAM FOR IDEAL MIXTURES

Notice again that the vapour is much richer in the more volatile component B than the original liquid mixture was.

The diagram just shows what happens if you boil a particular mixture of A and B. Notice that the vapour over the top of the boiling liquid has a composition which is much richer in B - the more volatile component.

Solutions of liquids which do not obey Raoult's Law are called non-ideal solutions.

There are two types of non ideal solutions 1.positive Deviation from Raoult's law 2. Negative Deviation From Raoult’s Law

Solutions which have a vapour pressure greater than that predicted from Raoult’s Law are said to show a positive deviation from the law.

E.g. Hexane and ethanol This is where the A--B interaction is

weaker than the A--A and the B--B interactions.

As a result the molecules escape from the mixture more easily than for an ideal solution.

VAPOUR PRESSURE COMPOSITION CURVE FOR NON IDEAL SOLUTIONS

1) POSITIVE DEVIATION

Maximum vapour pressure

This vapour pressure is greater than any other composition and either of the pure liquids

BOILING TEMPERATURE-COMPOSITION CURVES FOR NON IDEAL SOLUTIONS

1) POSITIVE DEVIATION

Minimum boiling point azeotrope

The same mixture will have a minimum boiling point lower than any other composition and either of the pure liquids

Solution with a vapour pressure lower than the calculated values are said to show a negative deviation.

E.g. Nitric acid and water This is where the A--B interaction is greater

than the A--A and the B--B interactions. It is more difficult for the molecules to

escape from the mixture than for an ideal mixture.

VAPOUR PRESSURE-COMPOSITION CURVE FOR NON IDEAL SOLUTIONS

2) NEGATIVE DEVIATION

Minimum vapour pressure

Which is less than any other composition and either of the pure liquids.

BOILING TEMPERATURE-COMPOSITION CURVE FOR NON IDEAL SOLUTIONS

2) NEGATIVE DEVIATION

Maximum boiling point azeotrope

This means there is a maximum boiling point which is higher than any other composition and either of the pure liquids

NOTE THE TERMS USED Minimum boiling point azeotrope

Maximum boiling point azeotrope

SIMPLE DISTILLATION Simple distillation is designed to evaporate a

volatile liquid from a solution of non-volatile substances; the vapour is then condensed in the water condenser and collected in the receiver.

FRACTIONAL DISTILLATION Fractional distillation is used to separate the

components of a mixture(miscible) of liquids by means of the difference in their boiling temperatures.

A mixture rich in the most volatile component distils over at the top of the column, where the thermometer registers its boiling temperature.

As distillation continues the temperature rises towards the boiling temperature of the next most volatile component.

The receiver is changed to collect the second component.

FUELS ARE OBTAINED FROM CRUDE OIL BY FRACTIONAL DISTILLATION

DISTILLATION AT REDUCED PRESSURE

High boiling liquids and many liquids which have a tendency to decompose near their boiling temperatures are often purified by distillation under reduced pressure, since lowering the pressure dramatically reduces the temperature at which a liquid will distil.

Distillation under reduced pressure always carries a slight risk of the apparatus *imploding.

*imploding -to collapse inwardly with force as a result of the external pressure being greater than the internal pressure, or cause something to collapse inwardly

VACUUM DISTILLATION Vacuum distillation is distillation at a reduced pressure.

Since the boiling point of a compound is lower at a lower external pressure, the compound will not have to be heated to as high a temperature in order for it to boil

Vacuum Distillation

Used to distill compounds that

have

High boiling point

undergoes decomposition on heating

at atm pressures

Or

STEAM DISTILLATION Steam distillation operates on the principle that

immiscible liquids exert their own vapour pressure so that when the mixture boils the sum of the vapour pressure equals one

Steam distillation is a method of distilling a compound at a temperature below its normal boiling point.

STEAM DISTILLATION

Ideal for separation of organic compound

s

Simple calculation Total vapour pressure=

P0A+P0

B

e.g. extraction

of eucalyptus

oil from eucalyptus (oils from plant materials)

Purification of phenylamine

and nitrobenzene

SOLVENT EXTRACTION Liquids that form two layers when mixed

provide an opportunity for purification of materials that prefer one layer more than the other.

For example, many organic chemicals are liquids that are very non-polar and separate from water because it is quite polar.

Partition-when the solute distributes itself between he two immiscible liquids.

Partition coefficient(k)-the concentration of the solute in each solvent at equilibrium is a constant ratio and the equilibrium constant for the system.

If cU and cL are the concentrations in the upper and lower layers then

cU/cL=k

k - Only applicable in dilute solutions and it varies with temperature

THE PARTITION COEFFICIENT WILL REMAIN CONSTANT UNDER THESE CONDITIONS:

1. the temperature is constant 2. the solvents are immiscible and do not react with each other 3. The solute does not react associate or dissociate in solvents.

SOLVENT EXTRACTION

Separated using a separating funnel

Partition coefficien

tSolute in

upper layer

Solute in

lower layer

Pdts of organic

preparations are often

dissolved in water

k - Only applicabl

e in dilute

solutions and it

varies with

temperature

SIMPLE QUESTION The mass of iodine used is 0.9656g and

25.0cm3 of the aqueous layer require 4.40cm3 of 0.01000 moldm-3 thiosulphate Ar(I)=127.

TOUGHER QUESTION The product of an organic synthesis, 5.00g of

X, is obtained in a solution in 1.00dm3 of water. Calculate the mass of X that can be extracted from the aqueous solution by

1. 50.0cm3 of ethoxyethane2. Two successive portions of 25.0cm3 of

ethoxyethane.The k of X between ethoxyethane and water is

40.0 at room temperature

INDUSTRIAL APPLICATIONS OF DISTILLATION

Petroleum

Rum

Fragrance

PETROLEUM

•BEER•VODKA

•RUM

Perfumes and Fragrances