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



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



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



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.




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







                        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)


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

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