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Experiment 2 - Distillations Penn State Erie, The Behrend College

CHEM 213. Fall 2007. Experiment 2 - Distillations Penn State Erie, The Behrend College .

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Experiment 2 - Distillations Penn State Erie, The Behrend College

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  1. CHEM 213 Fall 2007 Experiment 2 - Distillations Penn State Erie, The Behrend College Distillation techniques utilize the vaporization and re-condensation of vapors of a liquid mixture to effect purification of a liquid component. In this experiment we will compare the apparatus for simple and fractional distillation of organic liquids. This comparison will be made on the basis of scale, speed and the efficiency of the separation for a mixture of containing an unknown alcohol.

  2. Distillation - Theory • Vaporization and Condensation – One component system • For any liquid, the individual molecules within the liquid are continuously in motion • A small percentage of these molecules attain enough kinetic energy to leave the liquid phase • This exerts an opposing pressure on the atmosphere above the solution known as the vapor pressure, P Atmospheric pressure, Patm Vapor Pressure, P

  3. Distillation - Theory • Vaporization and Condensation - One component system • When enough energy, in the form of heat, is imparted to the solution the vapor pressure becomes equal to the atmospheric pressure and the liquid begins to boil P < Patm P ≥ Patm

  4. Distillation - Theory • Vaporization and Condensation – One component system • The vapor obtained from a boiling liquid, once cooled, will re-condense to a liquid known as the distillate • The complete process is called a distillation

  5. Distillation - Applications • Why perform a distillation? • Organic chemistry is driven to the preparation of organic compounds – whereas other disciplines are concerned with description of systems and analysis • For your first few experiments you are learning the means of “cleaning up” a reaction mixture to obtain a pure product • Look at a typical organic reaction: • Distillation is one of the routine techniques for the separation and purification of product – in this case distillation would be a good technique for the separation of the two liquid alkene products

  6. Distillation - Applications • Some applications you could encounter: • Separation of a reaction solvent from a non-volatile solute Mixture of solvent and solute is rotated to increase the surface area for evaporation which helps alleviate the effects of boiling point elevation Vacuum is applied to reduce the boiling point of the solvent This is actually the most common distillation performed in organic laboratories – the simple removal of solvent from a reaction mixture

  7. Distillation - Applications • Some applications you could encounter: • Separation of one liquid from another – This experiment! • This situation is a little more complex, and we will use this experiment to illustrate the separation process and the apparatus required Pure C? Pure D? Mixture of C + D? C + D

  8. Distillation – Back to Theory • Separation of Two Liquids • For each component: if vapor pressure is plotted versus temperature an exponential increase of vapor pressure is observed as the boiling point is approached • Note: even if two compounds have the same ultimate boiling point, the curvature of this line may be different!

  9. Distillation – Theory • Separation of Two Liquids • This relationship of vapor-pressure vs. temperature is given by the Clausius-Clapeyron equation: • p = po exp [ ] - DH (1/T – 1/To) R • The x,y (independent and dependent variables) for this equation are the known temperature (T) and the vapor pressure (p) calculated for that temperature • The constants for this equation: • po and To: known vapor pressure for a known temp. (°K) • DH: heat of vaporization of the liquid • R: gas constant (8.314 J . mol-1.°K)

  10. Distillation - Theory • Separation of Two Liquids • Above a mixture of two or more volatile liquids each liquid makes a partial contribution to the overall vapor pressure. • Pmixture = PA + PB + … • When the sum of these partial pressures equals the atmospheric pressure (or pressure above the mixture), the mixture boils • This Law implies that if a mixture of different volatile liquids is heated to boiling and the condensed vapors are collected they will be enriched in the component that is more volatile • more volatile = higher partial pressure, lower boiling point • This is the basis for using distillation as a technique for the separation and purification of liquids.

  11. Distillation - Theory • Separation of Two Liquids – Raoult’s Law • Raoult extended Dalton’s Law to illustrate that the contribution of each components vapor pressure is related to its mole fraction in the mixture at the interface of the liquid and vapor phases • Pmixture = XAPA + XBPB + … • Once again at the boiling point: • Patm = XAPA + XBPB (2 component system) • The enrichment of a particular component in the condensed vapors of a boiling mixture is related to both their volatility (P) and their concentration (X) in the original mixture.

  12. Distillation - Theory Where does all of this get us? Organic chemists are interested in separations and purification not necessarily physical derivations! Dalton + CC? ??? What do I get? Raoult + CC? Why am I doing this?

  13. Distillation - Theory • What we need is: • A relation of the component mole fractions within a given mixture to the observed boiling point • In English – if I have an 80 : 20 mixture of A : B at what temperature will it boil? • An estimation of the given enrichment of the condensate collected from the distilling mixture • In English – if I distill this 80 : 20 mixture of A : B will I get more of one or the other in the condensed vapor, and is it worth my time? • Which apparatus would I use?

  14. Distillation - Theory • Combining Raoult with Clausius-Clapeyron • The sum of mole fractions of all components must equal one • 1 = XA + XB • Substitute the equation for a single component into Raoult’s Law: • XB = 1 – XA so Patm = XAPA + (1-XA)PB • Expansion and rearrangement of the expression gives us the variation of mole fraction versus partial and atmospheric pressure: • XA = _______________ Patm - PB (PA - PB)

  15. Distillation - Theory • Combining Raoult and Clausius-Clapeyron: • If we substitute this expression: • XA= __________ • into the Clausius-Clapeyron Equation: • p = po exp • We obtain an expression for the mole fraction of each component in liquid that boils at a given temperature: Patm - PB (PA - PB) [ ] - DH (1/T – 1/To) R [ ] - DHB (1/T – 1/ToB) Patm - P°B exp R XA= _________________________________________________________ [ ] [ ] - DHA _ - DHB P°A exp (1/T – 1/ToA) P°B exp (1/T – 1/ToB) R R

  16. Distillation - Theory • Combining Raoult with Clausius-Clapeyron: • Combining the expressions for each component in a two component mixture we obtain the following graphical relationship: bp of pure A Vapor Temperature bp of pure B Liquid 0.0, 1.0 0.5, 0.5 1.0, 0.0 Mole Fraction, XA, XB This relationship gives us the boiling point for any mixture of A and B

  17. Distillation - Theory • Dalton and Clausius-Clapeyron: • We have just described how the liquid composition relates to the boiling temperature – what is happening in the vapor phase? • The composition of the vapor is given by Dalton’s Law: • P = PA + PB • Substituting in the Ideal Gas Law for each component: • (PA = nA(RT)/V) • and canceling similar terms we find that the ratio of each component to the total vapor pressure is given by: • PA/PTOTAL = nA/nTOTAL • Substituting mole fractions for number of moles we find that at 760 torr (1 atm) the vapor component for this system is given by: • XA vapor = XA liquid (PA/760) • If we substitute this XA into Clausius-Clapeyron:

  18. Distillation - Theory • We get an expression for the composition of the vapor. • Now add this relationship, graphically, of the composition of the vapor to the mole-fraction to temperature relationship we illustrated earlier, we arrive at the goal: bp of pure A Vapor Vapor composition Temperature bp of pure B Liquid composition Liquid 0.0, 1.0 0.5, 0.5 1.0, 0.0 Mole Fraction, XA, XB

  19. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation - Theory • Now for any mixture of liquids, we can determine: • The boiling point of the mixture (liquid line) • The composition of the vapor (vapor line) which shows how much enrichment in the lower boiling component occurs

  20. Distillation – Simple • What is it? • A simple distillation uses one vaporization-condensation cycle to effect a separation: What we will be discussing occurs only in this part of the apparatus The cooling jacket and vacuum adapter function only to cool the vapors to liquid efficiently and direct them into the receiver flask The distilling flask is directly attached to the distillation head Heat

  21. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Simple B. How efficient is it? Let’s use the graphical representation we derived earlier to illustrate what occurs in a simple distillation:

  22. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Simple • B. How efficient is it? • Suppose we have a 80:20 mixture of benzene and toluene and we subject it to a simple distillation technique: From our graph, we see that this mixture will boil at ~100 °C

  23. Distillation – Simple • B. How efficient is it? • The vapor that is collected from this 80:20 mixture is enriched in the lower boiling component • We can determine that the ratio of components in the vapor is now 55:45 toluene to benzene Vapor line 110 100 Liquid line Temperature °C 90 80 0 100 20 80 40 60 60 40 80 20 100 0 Mole % Toluene Mole % Benzene Composition (mole%)

  24. Distillation – Simple • C. Application • From the graphical analysis we see that a simple distillation is not 100% efficient at separating two liquids • A simple distillation should therefore be used where: • The two components have boiling points that are more than 30-40 °C apart • One of the liquids is already ~90+% pure • You are simply removing a pure solvent from a non-volatile solute - (we mentioned this as one of the most common distillation techniques, removal of a solvent from an organic reaction to obtain the product) • You don’t have enough material to bring the more efficient fractional distillation set-up to equilibrium

  25. Distillation – Fractional • A. What is it? • A fractional distillation utilizes two or more vaporization- condensation cycles, in succession, to effect a separation. • This is accomplished by what distinguishes a • fractional distillation apparatus: • the fractionating column • The fractionating column causes the • vaporization-condensation cycle to repeat • by providing multiple surfaces for the cycle • to take place • Using our graphical representation of the • benzene-toluene mixture as an example • let’s see how this works….

  26. Distillation – Fractional • A. What is it? • The fractionating column is placed between the distilling flask and the distillation head • Using our graphical representation ofthe benzene-toluene mixture as an example let’s see how this works….

  27. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional As the hot vapors leave the distilling flask, they condense on the first cold surface, completing one vaporization-condensation cycle. Vapors from the Distilling flask Suppose we distill the same 80:20 mixture of toluene to benzene we did in the simple distillation example

  28. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional This surface begins to heat from the condensed vapors which are now 55:45 toluene-benzene This benzene enriched liquid now has a boiling point of ~94 °C (lower than the incoming vapors) and it begins to boil off this higher surface Vapors from the Distilling flask

  29. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional These vapors are even further enriched in benzene (now 30:70, toluene:benzene) and condense on the next cold surface Vapors from the Distilling flask

  30. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional This condensed liquid has an even lower boiling point (86 °C) and as this surface heats it begins to boil off this next higher surface Vapors from the Distilling flask

  31. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional This vapor now condenses on the next cold surface (now 20:80, toluene:benzene) and the cycle continues Vapors from the Distilling flask

  32. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional 1:99 toluene:benzene This cycle will continue until the top of the column is reached The liquid collected after seven cycles is now 99% benzene! Vapors from the Distilling flask 80:20 toluene-benzene

  33. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional 1:99 toluene:benzene Each vapor-condensation (or mini-distillation) cycle is known as one theoretical plate The length of distillation column required to provide one theoretical plate of separation is known as the height equivalent theoretical plate (HETP) Vapors from the Distilling flask 80:20 toluene-benzene

  34. Vapor line 110 Liquid line 100 Temperature °C 90 80 Mole % Toluene Mole % Benzene 0 100 20 80 40 60 60 40 80 20 100 0 Composition (mole%) Distillation – Fractional Important – What we have discussed is only true for the first drop of distillate! As the distillation flask loses what the vapor is enriched in, The starting point for the next drop of distillate will be slightly different! In our example, there will be more and more toluene in the distillation flask – more heat will need to be applied to get the liquid to boil, and heat the distillation column More and more toluene as distillation proceeds

  35. Industrially, fractional distillation is very common and is typically run as a continuous process

  36. Distillation – Fractional • C. Applications • Because of the efficiency of the fractional distillation set-up it should be used anywhere that two volatile liquids need to be separated. • The only drawback is that each vaporization-condensation cycle requires a volume of liquid to attain equilibrium; this is called the hold-up volume or column hold-up and places a lower limit on the amount of liquid we can distill AND how much liquid will be lost in performing the distillation • For small amounts of liquid (<1 mL) chromatography (gas chromatography, high-performance liquid chromatography or column chromatography) is the separation method of choice.

  37. Distillation – Experimental Setup • YOUR EXPERIMENT – TWO GOALS • GOAL 1: In this Experiment you will compare the efficiency of simple vs. fractional distillation by you and a partner distilling a mixture of methylene chloride (CH2Cl2, bp760 40 oC) and an unknown alcohol • One partner will perform a simple distillation on half of the mixture, the other partner will perform a fractional distillation (50 mL each) • Your receiver flask will be a graduated cylinder • GOAL 2: Determine the identity of the unknown alcohol – by bp, refractive index and gas chromatography Sand bath or mantle heat source

  38. Distillation – Experimental Setup • Set-up and perform your distillation • Every time an additional 2 mL of distillate is collected in the graduated cylinder, not the temperature on the thermometer • You will construct a plot of temperature (dependent variable) vs. volume (independent variable) • This plot is NOT the vapor-liquid phase diagrams we have discussed!!! Sand bath heat source

  39. Distillation – Experimental Setup • To draw conclusions from YOUR plot remember the following: • You have a finite amount of each liquid to distill, as one runs out and gives you x volume, there is only y volume left to distill • A pure liquid (condensed hot vapor) will give a temperature within a few degrees of its accepted boiling point • A mixture of two liquids (behaving ideally) will give a temperature between their accepted boiling points • therefore – a volume of liquid collected while the temperature is intermediary is impure; volumes of liquid collected while the temperature is holding steady are probably pure Sand bath or mantle heat source

  40. After the alcohol is purified it will be analyzed the following week by: Gas Chromatography (GC) – to determine purity – we will discuss GC in the next lecture Refractive Index (RI) - to determine purity – read Mohrig for a descrption Infrared Spectroscopy (IR) – to determine identity and purity – to be discussed in the CHEM 210 Lecture

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