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Chapter 22

Chapter 22. Phase Equilibrium II: Two-Component Systems. 22.1 Two-Component System – Mixture of Two Miscible Liquids 22.2 Raoult’s Law for Ideal Solutions 22.3 Deviation from Raoult’s Law – Non-ideal Solutions 22.4 Fractional Distillation

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Chapter 22

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  1. Chapter 22 Phase Equilibrium II: Two-Component Systems 22.1 Two-Component System – Mixture of Two Miscible Liquids 22.2 Raoult’s Law for Ideal Solutions 22.3 Deviation from Raoult’s Law – Non-ideal Solutions 22.4 Fractional Distillation 22.5 Azeotropic Mixtures

  2. 22.1 Two-Component System – Mixture of Two Miscible Liquids (SB p.258) Mixture of Two Miscible Liquids vapour phase  liquid phase 2 miscible liquids

  3. Total vapour pressure Vapour pressure liquid composition 0% 100% liquid A100% 0% liquid B 22.1 Two-Component System – Mixture of Two Miscible Liquids (SB p.258) Ideal Solutions e.g. liquid A (hexane) liquid B (heptane)

  4. 22.2 Raoult’s Law for Ideal Solutions (SB p.259) Raoult’s Law for Ideal Solutions Vapour pressure Pure A PA0 Pure B PB0 PA = v.p. of A PB = v.p. of B 0% 100% liquid A100% 0% liquid B Partial pressure of A: PA = χAPºA Partial pressure of B: PB = χBPºB PT = PA + PB Raoult’s Law

  5. 22.2 Raoult’s Law for Ideal Solutions (SB p.259) Raoult’s Law Raoult’s Law states that the vapour pressure of a component in a mixture at a given temperature is directly proportional to its mole fraction in the liquid mixture, and is equal to the product of its mole fraction in the liquid mixture and the vapour pressure of the pure component at that temperature. Partial pressure of A: PA = χAPºA Partial pressure of B: PB = χBPºB

  6. 22.2 Raoult’s Law for Ideal Solutions (SB p.263) Propan-2-ol Propan-1-ol benzene methylbenzene bromomethane iodomethane Ideal Solution The tendency of the molecules of A and B in the mixture to change from liquid phase to the vapour phase is almost equal to that in pure A and pure B.

  7. 22.2 Raoult’s Law for Ideal Solutions (SB p.262) In this example, which is the more volatile liquid (A or B)? a is the liquid composition (A,B),b is the vapour composition (yA,yB). PT , b a

  8. 22.2 Raoult’s Law for Ideal Solutions (SB p.262) A vapour composition line is added to the graph. Can you read from the graph that A is the more volatile liquid? T1 , b a

  9. 22.3 Deviation from Raoult’s Law – Non-ideal solutions (SB p.264) Positive Deviation from Raoult’s Law (a max. in v.p. graph) (a min. in b.p. graph)

  10. 22.3 Deviation from Raoult’s Law – Non-ideal solutions (SB p.266) Solutions having positive deviation The average intermolecular attraction between a molecule of A and a molecule of B is weaker than the average of that between two A molecules in pure A and that between two B molecules in pure B. ethanol cyclohexane

  11. 22.3 Deviation from Raoult’s Law – Non-ideal solutions (SB p.266) The introduction of cyclohexane inhibits the formation of some of the hydrogen bonds. This leads to +ve deviation (a v.p. greaterthan that predicted by Raoult’s Law).

  12. 22.3 Deviation from Raoult’s Law – Non-ideal solutions (SB p.267) Negative Deviation from Raoult’s Law (a min. in v.p. graph) (a max. in b.p. graph)

  13. 22.3 Deviation from Raoult’s Law – Non-ideal solutions (SB p.268) Solutions having negative deviation The average intermolecular attraction between a molecule of A and a molecule of B is stronger than the average of that between two A molecules in pure A and that between two B molecules in pure B. trichloromethane propanone

  14. 22.3 Deviation from Raoult’s Law – Non-ideal solutions (SB p.268) Hydrogen bond cannot be formed in individual liquids but in a mixture of trichloromethane and propanone. This leads to -ve deviation (a v.p. lower than that predicted by the Raoult’s Law).

  15. 22.4 Fractional Distillation (SB p.269) Boiling point-composition curve Starting with a liquid mixture of composition x, after a series of consecutive distillations, B Pure A is obtained in the final distillate. A is the more volatile. Pure B is obtained in the final residue.

  16. 22.4 Fractional Distillation (SB p.270) The laboratory set-up of fractional distillation An ideal solution can be separated completely by fractional distillation.

  17. 22.4 Fractional Distillation (SB p.271) A fractionating tower

  18. 22.5 Azeotropic Mixtures (SB p.272) Negative Deviation from Raoult’s Law If original conc of liquid mixture is M (azeotropic mixture), vapour has thesame composition as liquid throughout. If original conc of liquid mixture is to the left of M (say x): fractional distillation leads to pure B in the vapour & an azeotropic mixture in the residue. If original conc of liquid mixture is to the right of M (say w): fractional distillation leads to pure A in the vapour & an azeotropic mixture in the residue.

  19. 22.5 Azeotropic Mixtures (SB p.272) Positive Deviation from Raoult’s Law If original conc of liquid mixture is M (azeotropic mixture), vapour has thesame composition as liquid throughout. If original conc of liquid mixture is to the left of M (say x): fractional distillation leads to an azeotropic mixture in the vapour & pure liquid B in the residue. If original conc of liquid mixture is to the right of M (say w): fractional distillation leads to an azeotropic mixture in the vapour & pure liquid A in the residue.

  20. The END

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