DISTILLATION - method for separating a mixture of two liquids having

                                        different b.p.

 

since -            cyclohexane boils at 78oC

                        toluene boils at 111oC

 

one might think that heating a

 

                        25:75 mole % mixture of cyclohexane-toluene at 78oC

 

                        would cause cyclohexane to leave as a vapor that could be

                        condensed as pure cyclohexane such is not the case

 

boiling point - is the temp. at which the vapor pressure of a liquid =

                                  external pressure (atm pressure)

 

pure cyclohexane - vapor pressure = 760 mm of Hg at 78oC

pure toluene - vapor pressure = 760 mm of Hg at 111oC

 

consider the 25:75 mole % mixture of cyclohexane-toluene

 

                        Dalton’s Law       Pc  +  Pt  =  Ptotal  =  760 mm of Hg boiling will occur

 

                        Raoult’s Law       Pc  =  PcoNc                          N  =  mole fraction of liquid

                                                         Pt  =  PtoNt

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                100                          75                            50                            25                                0  cyclohexane

                                    0                          25                            50                            75                            100  toluene

                                                                                mole percent

 

25 mole % cyclohexane & 75 mole % toluene in liquid boils at 100oC

 

To calculate mole % of cyclohexane and toluene in vapor first use Raoult’s Law

to calculate partial pressure of cyclohexane and toluene vapors from mixture.

 

vapor pressure of pure cyclohexane at 100oC  =  1732 mm of Hg

 

                        Pc  =  0.25 (1732 mm of Hg)

                        Pc  =  433 mm of Hg

 

vapor pressure of pure toluene at 100oC  =  436 mm of Hg

 

                        Pt  =  0.75 (436 mm of Hg)

                        Pt  =  327 mm of Hg

 

                Note:    Pc  +  Pt  =  327 mm Hg  +  433 mm Hg  =  760 mm Hg

 

Then use Dalton’s Law to calculate the mole % of cyclohexane and toluene in the vapor.

 

                Xc  =    Pc 

                           total vapor pressure

 

                Xc  =  433  x  100%  =  57 mole % of cyclohexane in vapor

                           760

 

                Xt  =  327  x  100%  =  43 mole % of toluene in vapor

                           760

 

for a simple distillation

 

In order to obtain pure cyclohexane one must condense the vapor & redistill it, numerous times.

 

This series of redistillations can be can “automatically” in a fractionating column.

 

The easiest way to understand how a fractionating column works is to look at a bubble cap column.

 

SEE OVERHEAD for bubble cap column.

 

On each plate a simple distillation takes place.

 

At equilibrium

                the low b. p. liquid is ascending

                the high b. p. liquid is descending

 

A theoretical plate - corresponds to one simple distillation and condensation.

 

In a fractionating column successive distillations and condensations take              place in a distilling column packed with some material on which

                heat exchange between the ascending & descending liquid can take place.

 

Large surface area is desirable  - best copper sponge (Chore Boy)

                BUT packing cannot be so dense that pressure changes take place within

                the column causing nonequilibrium conditions

 

 

                AND a large surface area will absorb (hold up) much of the material

                being distilled

 

                advantage of fractional distillation - better separation

                disadvantage of fractional distillation - more material is lost in fractionating

                                                                                                                column

 

Various packings are compared to one another using

                HETP  -  height equivalent to theoretical plate

 

 

Equililibrium must be established for good separation to take place.

                Good fractional distillation takes a long time.

                Rate of distillation is the most important factor in getting a good separation.

                Rate should be SLOW.

 

AZEOTROPES

 

Not all liquids form ideal solutions and conform to Raoult’s Law

 

                example - 95.5 % ethanol & 4.5 % water boils at 78.15oC

                                       below the b. p. of ethanol 78.3oC or water 100oC

                                    - because of H-bonding

 

No matter how efficient the distilling apparatus -  100% pure ethanol can never be

attained using distillation.

 

azeotrope - a mixture of liquids of a definite composition that distills at a constant

                           temp. without a change in composition

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                100%      95.5%                                                                                                         0%  ethanol

                    0%        4.5%                                                                                                     100%  water

 

95.5% ethanol - 4.5% water is a min. - boiling azeotrope

 

another example -

32.4% ethanol - 67.6% benzene boils at 68.2oC

            (b.p. 78.3oC)        (b.p. 80.1oC)

 

max.-boiling azeotrope -

formic acid (100.7oC) & water (100oC) boils at 107.3oC

 

 

 

Pure liquids have constant b. p.

 

A change in b. p. during distillation indicates an impurity

 

BUT a constant b. p. does not mean a liquid is pure (could be an azeotrope)

 

 

 

 

 

 

 

 

 

 

 

 

How do soluble solids affect b. p.?

 

Solution of a soluble solid + liquid

                        example - sugar water solution

 

                        temperature of vapor remains 100oC throughout the distillation

                        only pure water distills over

                        sugar remains as water leaves

                        as sugar solution becomes more conc.

                        temp. of sugar soln. increases (boiling pt. elevation)

 

How does pressure affect b. p.?

 

                        boiling pt. depends on pressure

                        may need to correct b. p. for external pressure

                        distillation can be done at lower temp. at lower pressures using

                                oil pump or aspirator

                        lower pressures are used for high boiling liquids, but then differences

                                in b. p. are decreased

 

                        best separation- boiling points of two liquids should be 20oC apart

 

 

 

 

Apparatus

            Micro

                        Simple distillation -  p. 88 fig. 5.5 (p. 87, fig. 5.5) or p.90 fig 5.7 (p. 89, fig. 5.7)

                        Fractional distillation - p. 89, fig. 5.6 (p. 88, fig 5.6)

            Macro

                        Simple distillation -  p. 94, fig 5.10 (p. 93, fig. 5.10)

                        Fractional distillation -  p. 96 fig. 5.11 (p. 95, fig. 5.11)