Boiling Point and Distillation

Boiling Point

Characteristic feature of a liquid and can be used for its identification (at p = ct) (like mp or Rf)

Compounds with stronger attractive forces (H-bonding/dipole) or higher molecular weights have higher boiling points (like mp)

The temperature at which the vapor pressure of the liquid equals the external pressure.

• The Boiling Point is the temperature at which the vapor pressure of a liquid slightly exceeds the pressure of the atmosphere above the liquid. During boiling, vapors form inside the liquid and the temperature remains constant. • Below bp, evaporation takes place at any temperature only from the liquid surface. (Remember the filter paper in the TLC experiment?)

Evaporation vs Boiling?

If the liquid is a pure substance or an azeotrope, boiling occurs at constant temperature regardless of the amount of heat applied to the liquid.

What does more heat do?

When the pressure above a liquid is increased, the bp goes up. Water at 1 atm. boils at 100° C, but at 218 atm, the bp = 374° C. Above this temperature (critical temperature), liquid water = saturated steam (supercritical).

If the pressure above a liquid is reduced, the bp is lowered. At higher elevations, air pressure decreases, and water boils below 100° C. In Mexico City, 2.3 km above sea level, the bp = 92.8° C. If pressure falls to 4.55 torr, boiling occurs at 0° C!

The lowest bp is that of He, -268.9 °C. The highest is tungsten, 5900° C . Literature bps refer to standard pressure unless specifically stated otherwise.

Boiling Point

What is the effect of pressure on bp?

It increases the rate.

Vapor Pressure Increases with Temperature.

BoilingPoint

AtmosphericPressure

Boiling occurs when the vapor pressure = external pressure

Ideal solutions of miscible, non-interacting liquids

78 oC

111 oC

The process of heat evaporating a liquid followed by cooling/condensation of the vapor and recovery of the liquid.

Distillation: a liquid purification technique

Purpose: Separation of a liquid/liquid or liquid/solid mixture based on the different volatilities/vapor pressures of the components. During distillation, the liquid mixture changes composition and becomes richer in the least volatile compound. Vapor temperature is constant during distillation.

Azeotropes (mix of liquids behaving as one compound) do not change composition when they boil.

DistillationThe process of purifying a liquid by successive evaporation and condensation steps

http://metalab.unc.edu/moonshine/drink/drink.htmlApparently, it all started with the goal of obtaining a stronger beverage…

When a dilute alcohol solution is boiled, alcohol makes up most of the initial vapor because it boils off faster than water (i.e. hot sake is not as strong as cold sake…;-) The vapor is trapped and re-condensed several times. The concentrated alcohol is used in whiskey production. In this Scottish distillery, the copper stills concentrate alcohol after fermentation. However, water and EtOH form an azeotrope and EtOH can not be separated by distillation in better than 95.5 % purity.

cold

hothothot

Desalinization is a process that uses distillation to remove salt from seawater. Water is heated and pumped into a tank where it is vaporized. The steam is condensed and removed as pure water. The process is repeated many times. The remaining liquid (brine), contains a large amount of salt and is processed for minerals.

Distillation: Desalinization of Seawater

Distillation Theory (2 liquids and vapors, a & b)Raoult’s Law: Partial pressures (Pa, Pb) of each component above the liquid mixture are proportional to the mole fraction of that component in the ideal liquid solution.

Pa = Pao Na

Dalton’s Law: Total pressure above the liquid mix is the sum of partial pressures

P = Pa + Pb = PXa+ PXb = PaoNa+ Pb

oNb

Xa = Pa/P = PaoNa/P = 1-Xb (i.e. Xa < Na at equil)

Pao = Vapor pressure of pure a (tabulated)

Na = Mole fraction of a in the liq mix

Xa = Mole fraction of a in the vapor phase, Xa = Na

P = Total pressure (above the liquid)

Pb = Pbo Nb

Not to be confused with Daltonism… : http://en.wikipedia.org/wiki/Color_blindness

Azeotrope (non-ideal solutions)

A mixture of liquids that distills at a constant temperature without changing composition: i.e. vapor composition = liquid composition (XA = NA). Thus, NO separation occurs, e.g. Ethanol/Water 95%.

Azeotropes are observed when liquids interact, for example by H-bonding. The mix behaves as if it was 1 pure compound.

The azeotropic composition is analogous to the eutectic point from solids melting.

The bp of the azeotrope can be lower or higher than the bp of each components.

Boiling Point Phase DiagramAzeotropes: non-ideal solutions due to molecular interactions (H-Bonding, etc)

Remember eutectic?

78.30 C

78.15 C

100 oC

Water Ethanol95.5 %

azeotrope

Can cyclohexane and toluene be separated by distillation?

BoilingPoint

AtmosphericPressure

Boiling occurs when the vapor pressure = external pressure

Ideal solutions, miscible, non-interacting liquids

78 oC

111 oC

Simple Distillation

Partial Press. Chex., PC = 0.25*1308 = 433 torr at 100°C

Partial Press. Tol, PT= 0.75*577 = 327 torr at 100°C

X1,Ch = P1/P = 433/760 = 0.57, X2,Tol = P2/P = 327/760 = 0.43 mole frs in the gas phaseN1,C = 0.25, N2,T = 0.75 mole fractions in the original liquid phase.The gas phase becomes enriched in the more volatile component (hexanes in this case)

Thus, X1gas > N1

liq and cyclohexane can be separated

• Consider a 75/25 toluene/cyclohexane mixture to be distilled (bptol = 111 oC, bpchex = 78 oC). Like mp, bp depends on composition. Can this mix boil at 100 C? • From tables or the previous graph, at 100 C, Po

tol = 577 torr and Po

chex = 1308 torr. The mix will boil at 100 oC if the total pressure equals 760 torr.

P = 433 + 327 = 760 torr (yes it will boil)

Ideal Liquid-Vapor Phase Diagram

75/25

43/57

15/85

Simple distillation A-B

A-F Fractionaldistillation

5/95

dew-point curve; below it, the mix condenses

bubble-point curve; above it, the mixture boils

Vapor A+B

Liquid A+B

X1

N243/57

N1X2

N1 = X1

But

N2 = X1

Nk = Xk-1

toluene

chexane

temp is constant

Fractional Distillation

Re-distillation (fractional)

Therefore, the components may be separated

After condensation, N2 = X1 But upon next distillation X2 < N2

However, for azeotropes, N1= X1 and no separation is possible.

The liquid phase becomes poorer while the vapor phase becomes richer in the more volatile component.

Hottestplatecondenses the highest bpfraction

Fractional Distillation (water/EtOH)A large number of distillation setups (plates) connected in series. For example, a H2O/EtOH 90/10 solution drips down through a column onto a series of perforated plates.

The vapor rises up and bubbles through the liquid at each plate where some water vapor condenses and some liquid alcohol evaporates. The interaction at each plate is a redistillation.

By building a column with a sufficient number of plates, 95% alcohol can be obtained in a single operation.

Separation ability for fractional distillation is the composition at the top of the column compared to that in the original mixture.see also petroleum (oil) refinement

hot vapor

in

Collect fractions with various bps

Coldestplate condenses the lowest bpfraction

This process (rectification) is common in industrial separations: alcohol/water, O2/N2 (liquid air), complex mixtures of coal tar and petroleum for gasoline). The fractionating column most often used is the bubble tower.

Fractional Distillation

The disadvantage is that a large fraction (~0.5) of the condensed distillate must be returned to the tower top and re-boiled. However, in continuous fractionation the outgoing distillate preheats the incoming feed and allows great heat economy.

When the mixture consists of many components, various fractions are drawn off at different points along the tower. Industrial distillation towers for petroleum often have over 100 plates. Towers with >500 plates were used for the separation of isotopes.

http://www.youtube.com/watch?v=26AN1LfbUPc

Experimental Setup Simple Distillation movie-microscale

http://www.youtube.com/watch?v=3JlIPnyrZMw (10 mins)

http://www.youtube.com/watch?v=kMSwA93ap-M

Distillation Apparatus

Fractional Distillation+movies

multipledistillationsoccur

http://www.youtube.com/watch?v=0x2-8dedmE4

In the Lab

5.4 A and 5.5 A (5th ed)

Compare and Contrast Simple and Fractional Distillation of Toluene/Cyclohexane Mixtures

5.6

Distillation of an Unknown Mixture

Distillation Lab simple distillation, book, p 99-104 5th ed.First lab: (5.4A and 5.5A in the Lab. text)

The purpose is to compare and contrast the simple (5.4A) vs. fractional (5.5A) distillation of a 50/50 mixture of cyclohexane/toluene. Students will work in pairs, one student doing part 5.4A and the other part 5.5A.

Work in the hoods. The apparatus used for the simple and fractional distillations should be set up as pictured in Figs. 5.10 (p. 94) and 5.11 (p. 96) of the text, respectively. Conduct the distillations on a slightly smaller scale (30 mL) so the mixtures should be distilled from 50-mL round bottom flasks. Support the graduated cylinder by clamping it to the ring stand. Be sure: 1) the apparatus is properly cramped, 2) there is a boiling chip in the distillation flask, 3) all connections are tight, 4) a slow flow of water is running through the condenser, 5) the thermometer is positioned properly, and 6) that your T A has checked the apparatus before beginning the distillation. NEVER heat a closed system. Using the Variac, adjust the heating rate so that the mixture distills at a rate of c.a. 1 drop/sec. DO NOT plug the heating mantle into line voltage. Collect the distillate directly into a graduated cylinder, record the volume collected vs. the observed temperature (e.g. every 1.0 mL, have a table prepared in your notebook to record your data). Discontinue heating when only a small amount of liquid (~ 5 mL) remains in the flask; do not distill to dryness. Exchange data and observations with your partner before leaving the lab. Place the distillate collected in the graduated cylinder in the recovered cyclohexane/toluene container. For your report, compare and contrast the two techniques and prepare graphs of volume of distillate (mL) vs. temperature (C) for each technique.

Distillation of unknownsSecond lab: (5.6A in the book)

Work alone. Clean your apparatus from part one by pouring any liquids remaining into the non-halogenated organic waste, use a portion of acetone to complete the transfer and rinse the glassware. Blow the glassware dry with a stream of air in the hood. Obtain an unknown liquid mixture from your TA; record the unknown number in your notebook. The mixture will contain two compounds from Table 5.2 whose boiling points differ by > 20 °C. Set up a fractional distillation apparatus as described above. Carefully distill the whole mixture (i.e. place the whole mixture in the distillation flask but do not distill to dryness), this time collecting fractions based on boiling ranges in round bottom flasks.

Record the boiling ranges and measure the volumes of the fractions you collect. Some of the high boiling unknowns also have a high heat of vaporization; thus you may need to insulate your column with glass wool to get the 2nd component to distill. For your report identify the two major fractions of the mixture based on your observed boiling ranges and plot a distillation curve as above. If unsure of the identity of the major fraction redistill it by simple distillation using the appropriate size flask. When done, place all liquids in the non-halogenated wasted container.

Distillation Curve

You’ll do it in the lab

Toluene + cyclohexanes

But water/EtOH is not the same story…

HomeworkChpt. 5 (5th, 6th ed): Distillation Q. 5, 6, 7, 12.

To Read Boiling point: 4th Ch 4, p. 72 – 76; 5th Ch 3, 54-59; 6th 55-60.Distillation: 4th Ch 5, p. 80 – 97; 5th Ch. 5, 88-104. 6th Ch 5 p 86-101

Next classSteam Distillation: 4th Ch. 6, p 98-109, 5th 105-134, 6th 102-130.Vacuum distillation and Sublimation 4th Ch 7. p 110 - 125

5th Ch 6 p 105-134 .

If you get a chance, check out the huskython 2012 link I got from one of your colleagues!http://www.youtube.com/watch?v=g_Ng8pl-A14

extra slides

                                                                                                                                  

Destructive distillation is used to convert raw materials (wood by-products), into useful chemicals. Desalinization only performs a physical separation of the constituents. Destructive distillation is a chemical change; the end products (methane, charcoal, and coal tar) cannot be turned back into wood.

Destructive Distillation

Theoretical Plates

HETP = height equivalent to a theoretical plate; the smaller the better; the taller the column, the better

1 plate = 1 evaporation + 1 condensation step

The more efficient column will have a larger number of theoretical plates, or the smaller height equivalent to a theoretical plate.

condenser and fractional column

Apparatus-Vacuum Distillation