Ideal solutions for 3rd semester By T.Sweta Department of chemistry

Roault’s law on ideal solutions Two-Component System – Mixture of Two Miscible Liquids Raoult’s Law for Ideal Solutions Deviation from Raoult’s Law – Non-ideal Solutions Fractional Distillation Azeotropic Mixtures

Mixture of Two Miscible Liquids 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

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

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

Partial pressure of A: PA = χAPºA 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

22.2 Raoult’s Law for Ideal Solutions (SB p.263) 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. benzene methylbenzene Propan-1-ol Propan-2-ol bromomethane iodomethane

In this example, which is the more volatile liquid (A or B)? 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

A vapour composition line is added to the graph. 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

Positive Deviation from Raoult’s Law 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)

Solutions having positive deviation 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

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. greater than that predicted by Raoult’s Law).

Negative Deviation from Raoult’s Law 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)

Solutions having negative deviation 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

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).

Boiling point-composition curve 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.

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

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

Negative Deviation from Raoult’s Law 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 the same 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.

Positive Deviation from Raoult’s Law 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 the same 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.

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