1. Distillation... A Separation Method

2. 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.

3. 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... Press = 760 torr B Pt = 78 o C Press = 600 torr B Pt = 73 o C Press = 10 torr B Pt = -2 o C

4. 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.

5. 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.

6. Background – Raoult’s Law For an ideal solution with a volatile solvent and a non-volatile solute (e.g. water and salt) … VP solution = X solvent × VP solvent As you increase the concentration of the non-volatile solute, you decrease the vapor pressure of the solution and increase the boiling point. (Colligative property)

7. 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 – simple distillation.

8. 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 or more solvents. P total = P a + P b +... For a solution of liquid ‘a’ and liquid ‘b’ VP total = X a (VP a ) + X b (VP b )

9. Deviation’s from Dalton’s Law For non-ideal solutions the combined vapor pressure may be higher (positive deviation) or lower (negative deviation) than predicted by Dalton’s Law.

10. Deviation’s 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. Non-ideal solutions may distill with a constant boiling point as if it were one pure substance = Azeotrope.

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

12. 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.

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

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

15. Terms and Definitions Theoretical Plate – one of the horizontal lines in the previous graph or 1 simple equilibration between liquid and vapor phase

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

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

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

19. 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.