R AOULT ’ S L AW 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 P A =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 hexane and heptane benzene and methylbenzene propan-1-ol and propan-2-ol

V APOUR P RESSURE / C OMPOSITION D IAGRAMS FOR I DEAL M IXTURE L IQUIDS. P total =P A +P B

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.

B OILING P OINT / C OMPOSITION D IAGRAM FOR I DEAL M IXTURES 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. Raoult's Law  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.

V APOUR P RESSURE C OMPOSITION C URVE F OR N ON I DEAL SOLUTIONS 1) POSITIVE DEVIATION Maximum vapour pressure This vapour pressure is greater than any other composition and either of the pure liquids

B OILING T EMPERATURE -C OMPOSITION C URVES F OR N ON I DEAL S OLUTIONS 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.

V APOUR P RESSURE -C OMPOSITION C URVE F OR N ON I DEAL S OLUTIONS 2) NEGATIVE DEVIATION Minimum vapour pressure Which is less than any other composition and either of the pure liquids.

B OILING T EMPERATURE -C OMPOSITION C URVE F OR N ON I DEAL S OLUTIONS 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

N OTE THE TERMS USED  Minimum boiling point azeotrope  Maximum boiling point azeotrope

S IMPLE D ISTILLATION  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.

F RACTIONAL D ISTILLATION  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.

F UELS ARE OBTAINED FROM CRUDE OIL BY FRACTIONAL DISTILLATION

D ISTILLATION A T R EDUCED P RESSURE  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

V ACUUM D ISTILLATION  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

S TEAM D ISTILLATION  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.

S TEAM D ISTILLATION Ideal for separation of organic compounds Simple calculation Total vapour pressure= P 0 A+P0 B e.g. extraction of eucalyptus oil from eucalyptus (oils from plant materials) Purification of phenylamine and nitrobenzene

S OLVENT E XTRACTION  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 c U and c L are the concentrations in the upper and lower layers then  c U /c L =k k - Only applicable in dilute solutions and it varies with temperature

T HE 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.

S OLVENT E XTRACTION Separated using a separating funnel Partition coefficient Solute in upper layer Solute in lower layer Pdts of organic preparations are often dissolved in water k - Only applicable in dilute solutions and it varies with temperature

S IMPLE Q UESTION The mass of iodine used is g and 25.0cm 3 of the aqueous layer require 4.40cm 3 of moldm -3 thiosulphate A r (I)=127.

T OUGHER QUESTION The product of an organic synthesis, 5.00g of X, is obtained in a solution in 1.00dm 3 of water. Calculate the mass of X that can be extracted from the aqueous solution by cm 3 of ethoxyethane 2. Two successive portions of 25.0cm 3 of ethoxyethane. The k of X between ethoxyethane and water is 40.0 at room temperature

I NDUSTRIAL A PPLICATIONS OF D ISTILLATION Petroleum Rum Fragrance

P ETROLEUM

BEER VODKA RUM

Perfumes and Fragrances