Distillation ... A Separation Method

Background Concepts - Definitions Vapor Pressure – Gas pressure created by the molecules of a liquid, which have acquired sufficient Kinetic Energy to escape to the vapor phase. As Temperature increases, the average Kinetic Energy and the Vapor Pressure increase until the boiling point is reached.

Background Concepts (definitions) Boiling Point – The temperature at which the vapor pressure of a liquid equals the pressure applied by the surroundings to the liquid. Boiling points for ethanol ... Pressure = 760 torr B Pt = 78 oC Pressure = 600 torr B Pt = 73 oC Pressure = 10 torr B Pt = -2 oC

Background Concepts (Relationships) For two pure liquids, which have different boiling points… .... the vapor pressure at a given temperature will be higher for the liquid of lower boiling point.

Background Concepts The temperature of the vapors above a boiling liquid will remain constant at the boiling point. At the boiling point for a pure liquid, the liquid and vapor (gas) phases are in equilibrium. The boiling point (at a specified pressure) is a characteristic property of a pure liquid.

Background – Raoult’s Law For an ideal solution with a volatile solvent and a non-volatile solute (e.g. water and salt) … VPsolution = Xsolvent x VPsolvent As you increase the concentration of the non-volatile solute, you decrease the vapor pressure of the solution and increase the boiling point. … hence “Boiling Point Elevation”

So What ?? For an ideal solution with a volatile solvent and a non-volatile solute (e.g. water and salt) … …separation of the two components is as simple as heating the mixture and collecting and condensing the vapor.

Background – Dalton’s Law For an ideal solution made of two volatile solvents, the total vapor pressure is the sum of the vapor pressures of the two solvents. For a solution of liquid ‘a’ and liquid ‘b’ VPtotal = Xa (VPa) + Xb (VPb)

Deviations from Dalton’s Law For non-ideal solutions, the combined vapor pressure may be either higher (positive deviation) or lower (negative deviation) than predicted by Dalton’s Law. When VP is higher, means BP is lower… When VP is lower, means BP is higher… …than predicted by Dalton’s Law.

Deviations from Dalton’s Law For non-ideal solutions, there is either strong forces of attraction (negative deviation) or repulsion (positive deviation) between the molecules of the different components of the solution. Attractive forces = components held together more strongly = lower VP = requires more heat = higher BP Repulsive forces = components not held together = higher VP = requires less heat = lower BP A non-ideal solution may distill with a constant boiling point as if it were one pure substance = Azeotrope.

Positive Deviation Classic example: ethanol and water Pure ethanol – B. Pt of 78 oC Pure water – B. Pt of 100 oC Mixture of ethanol (95 %) and water (5 %) distills with a constant B. Pt. of about 75 oC.

Background Concepts For an ideal solution of two miscible liquids of different boiling points, the composition of the liquid and vapor phases is not the same. The vapor will contain more of the liquid with the higher vapor pressure or lower boiling point.

(X’s and O’s represent percentage of each component at each level.)

Phase Diagram (Liquids A and B) You would have to perform multiple simple distillations in order to fully separate two volatile substances from each other. (Later, we will see how fractional distillation fixes this problem, by allowing a 1-pot purification of two volatile substances.) http://www.uwlas.edu/faculty/koster/Distillation305.htm

Terms and Definitions Theoretical Plate – one of the horizontal lines in the previous graph or 1 simple equilibration between the liquid and vapor phase. As the solution to be purified is heated, its vapors rise to the fractionating column. As it rises, it cools, condensing on the condenser walls and the surfaces of the packing material. Here, the condensate continues to be heated by the rising hot vapors; it vaporizes once more. However, the composition of the fresh vapors are determined once again by Raoult's law. Each vaporization-condensation cycle (called a theoretical plate) will yield a purer solution of the more volatile component. In reality, each cycle at a given temperature does not occur at exactly the same position in the fractionating column; theoretical plate is thus a concept rather than an accurate description. --wikipedia page

Simple Distillation (Use a 10-mL graduated cylinder to collect fractions) http://www.uwlas.edu/faculty/koster/Distillation305.htm

Simple Distillation http://www.uwlas.edu/faculty/koster/Distillation305.htm

Fractional Distillation Increasing the surface area that the vapors contact between the liquid and the condenser, increases the number of theoretical plates. http://www.uwlas.edu/faculty/koster/Distillation305.htm

Phase Diagram (Liquids A and B) http://www.uwlas.edu/faculty/koster/Distillation305.htm

Fractional Distillation Greater plates = greater purity of distillate = sharper transition in distillation plot The number of Theoretical Plates or the height equivalent to a theoretical plate (HETP) is a measure of the efficiency of a column to separate components.

Preparing your NB… It would be helpful to read pgs. 55-57 (“Simple Distillation, Miniscale Apparatus”) and pgs. 131-132 (in addition to Sec. 4.1 and 4.2) Figure 2.37‒(a) on pg. 56! 2 chemicals to include in Table of Properties: ethyl acetate & butyl acetate No chemical reactions Procedure – you can print the procedural steps that are in the handout (on Moodle).

Results After collecting the temperature data, prepare a graph of “Temperature (y-axis) vs. Volume (mL) of Distillate Collected (x-axis)” and put it in the Calculations section of your NB. (see slide #17 for a sample of what a good simple distillation looks like) Remember – a graph must be drawn using either graph paper or a computer – It can not simply be “guesstimated” on the notebook page.

Discussion Continue writing discussion as outlined in the syllabus. Results = Graph of “Temperature vs. Volume (mL) of Distillate collected” How similar does your graph look to the one that’s on slide #17? Ideally, you’d want the temperature transition (from low temp. to higher temp.) to be as sharp as possible in order to minimize collection of a mixture of ethyl and butyl acetates. Continue writing discussion as outlined in the syllabus.